CHAPTER 18 – BODY FLUIDS AND CIRCULATION

CHAPTER 18

BODY FLUIDS AND CIRCULATION

  • Simple organisms like sponges and coelenterates circulate water from their surroundings through their body cavities to facilitate the cells to exchange these substances.
  • More complex organisms use special fluids within their bodies to transport such materials like Blood and lymph (tissue fluid).

BLOOD

Blood is a special connective tissue consisting of a fluid matrix, plasma, and formed elements.

Plasma

  • Plasma is a straw coloured, viscous fluid constituting nearly 55 per cent of the blood.
  • 90-92 per cent of plasma is water and proteins contribute 6-8 per cent of it.
  • Fibrinogen, globulins and albumins are the major proteins
  • Fibrinogens are needed for clotting or coagulation of blood.
  • Globulins involved in defense mechanisms of the body and the albumins help in osmotic balance.
  • Plasma also contains small amounts of minerals like Na+, Ca++, Mg++, HCO3, Cl, etc.
  • Glucose, amino acids, lipids, etc., are also present in the plasma as they are always in transit in the body.
  • Factors for coagulation or clotting of blood are also present in the plasma in an inactive form.
  • Plasma without the clotting factors is called serum.

Formed Elements

  • Erythrocytes, leucocytes and platelets are collectively called formed elements and they constitute nearly 45 per cent of the blood.

Erythrocytes or red blood cells (RBC)

  • Most abundant of all the cells in blood.
  • Count – (In a healthy adult man) 5 – 5.5 millions of RBCs mm-3 of blood.
  • Formed in – Red bone marrow in the adults.
  • Nucleus – absent in most of the mammals
  • Shape – biconcave
  • Haemoglobin – red coloured, iron containing complex protein. hence the colour and name of cells.
  • Haemoglobin count – 12-16 gms of haemoglobin in every 100 ml of blood. These molecules play a significant role in transport of respiratory gases.
  • Average life span – 120 days.
  • Destroyed in – spleen (graveyard of RBCs).

Leucocytes or white blood cells (WBC)

  • Colourless due to the lack of haemoglobin.
  • Nucleus – present
  • Count (TLC – total leucocyte count) – 6000-8000 mm-3 of blood.
  • Leucocytes are generally short lived.
  • We have two main categories of WBCs – granulocytes and agranulocytes.
  • Neutrophils, eosinophils and basophils are different types of granulocytes, while lymphocytes and monocytes are the agranulocytes.
  • Neutrophils – most abundant cells (60-65 per cent), phagocytic.
  • Basophils – least abundant (0.5-1 per cent), secrete histamine, serotonin, heparin, etc., and are involved in inflammatory reactions.
  • Monocytes – (6-8 per cent), phagocytic cells.
  • Eosinophils – (2-3 per cent), resist infections and are also associated with allergic reactions.
  • Lymphocytes – (20-25 per cent) are of two major types – ‘B’ and ‘T’ forms. Both B and T lymphocytes are responsible for immune responses of the body.

Platelets or thrombocytes

  • Cell fragments produced from megakaryocytes (special cells in the bone marrow).
  • Count – 1,500,00-3,500,00 platelets mm-3 of blood.
  • Role – release a variety of substances most of which are involved in the coagulation or clotting of blood.
  • A reduction in their number can lead to clotting disorders which will lead to excessive loss of blood from the body.

Screenshot (91)

 BLOOD GROUPS

  • Various types of grouping of blood has been done.
  • Two such groupings – the ABO and Rh – are widely used all over the world.

ABO grouping

  • ABO grouping is based on the presence or absence of two surface antigens (chemicals that can induce immune response) on the RBCs namely A and B.
  • Similarly, the plasma of different individuals contain two natural antibodies (proteins produced in response to antigens).
  • During blood transfusion, any blood cannot be used; the blood of a donor has to be carefully matched with the blood of a recipient before any blood transfusion to avoid severe problems of clumping (destruction of RBC).

Blood Groups and Donor Compatibility

Blood Group Antigens on RBCs Antibodies in Plasma Donor’s Group
A A anti-B A, O
B B anti-A B, O
AB A, B nil AB, A, B, O
O nil anti-A, B O
  • Group ‘O’ blood can be donated to persons with any other blood group and hence ‘O’ group individuals are called ‘universal donors’.
  • ‘AB’ group can accept blood from persons with AB as well as the other groups of blood. Therefore, such persons are called ‘universal recipients’.

Rh grouping

  • Another antigen, the Rh antigen similar to one present in Rhesus monkeys (hence Rh), is also observed on the surface of RBCs of majority (nearly 80 per cent) of humans. Such individuals are called Rh positive (Rh+ve) and those in whom this antigen is absent are called Rh negative (Rh-ve).
  • An Rh-ve person, if exposed to Rh+ve blood, will form specific antibodies against the Rh antigens. Therefore, Rh group should also be matched before transfusions.

Erythroblastosis foetalis –

  • A special case of Rh incompatibility (mismatching) observed between the Rh-ve blood of a pregnant mother with Rh+ve blood of the foetus.
  • Rh antigens of the foetus do not get exposed to the Rh-ve blood of the mother in the first pregnancy as the two bloods are well separated by the placenta.
  • However, during the delivery of the first child, there is a possibility of exposure of the maternal blood to small amounts of the Rh+ve blood from the foetus.
  • In such cases, the mother starts preparing antibodies against Rh in her blood.
  • in case of her subsequent pregnancies, the Rh antibodies from the mother (Rh-ve) can leak into the blood of the foetus (Rh+ve) and destroy the foetal RBCs.
  • This could be fatal to the foetus or could cause severe anaemia and jaundice to the baby.
  • This condition is called erythroblastosis foetalis.
  • This can be avoided by administering anti-Rh antibodies to the mother immediately after the delivery of the first child.

Coagulation of Blood

  • Blood exhibits coagulation or clotting in response to an injury or trauma. This is a mechanism to prevent excessive loss of blood from the body.
  • a dark reddish brown scum formed at the site of a cut or an injury over a period of time is a clot or coagulam, formed mainly of a network of threads called fibrins in which dead and damaged formed elements of blood are trapped.
  • Fibrins are formed by the conversion of inactive fibrinogens in the plasma by the enzyme thrombin.
  • Thrombins, in turn are formed from another inactive substance present in the plasma called prothrombin.
  • An enzyme complex, thrombokinase, is required for the above reaction.
  • This complex is formed by a series of linked enzymic reactions (cascade process) involving a number of factors present in the plasma in an inactive state.
  • An injury or a trauma stimulates the platelets in the blood to release certain factors which activate the mechanism of coagulation.
  • Certain factors released by the tissues at the site of injury also can initiate coagulation.
  • Calcium ions play a very important role in clotting.

LYMPH (TISSUE FLUID)

  • As the blood passes through the capillaries in tissues, some water along with many small water soluble substances move out into the spaces between the cells of tissues leaving the larger proteins and most of the formed elements in the blood vessels.
  • This fluid released out is called the interstitial fluid or tissue fluid.
  • It has the same mineral distribution as that in plasma.
  • Exchange of nutrients, gases, etc., between the blood and the cells always occur through this fluid.
  • An elaborate network of vessels called the lymphatic system collects this fluid and drains it back to the major veins. The fluid present in the lymphatic system is called the lymph.
  • Lymph is a colourless fluid containing specialised lymphocytes which are responsible for the immune responses of the body.
  • Lymph is also an important carrier for nutrients, hormones, etc. Fats are absorbed through lymph in the lacteals present in the intestinal villi.

 CIRCULATORY PATHWAYS

  • The circulatory patterns are of two types – open or closed.
  • Open circulatory system is present in arthropods and molluscs in which blood pumped by the heart passes through large vessels into open spaces or body cavities called sinuses.
  • Annelids and chordates have a closed circulatory system in which the blood pumped by the heart is always circulated through a closed network of blood vessels. This pattern is considered to be more advantageous as the flow of fluid can be more precisely regulated.
  • All vertebrates possess a muscular chambered heart.
  • Fishes have a 2-chambered heart with an atrium and a ventricle. Amphibians and the reptiles (except crocodiles) have a 3-chambered heart with two atria and a single ventricle, whereas crocodiles, birds and mammals possess a 4-chambered heart with two atria and two ventricles.
  • In fishes the heart pumps out deoxygenated blood which is oxygenated by the gills and supplied to the body parts from where deoxygenated blood is returned to the heart (single circulation).
  • In amphibians and reptiles, the left atrium receives oxygenated blood from the gills/lungs/skin and the right atrium gets the deoxygenated blood from other body parts. However, they get mixed up in the single ventricle which pumps out mixed blood (incomplete double circulation).
  • In birds and mammals, oxygenated and deoxygenated blood received by the left and right atria respectively passes on to the ventricles of the same sides. The ventricles pump it out without any mixing up, i.e., two separate circulatory pathways are present in these organisms, hence, these animals have double circulation.

 HUMAN CIRCULATORY SYSTEM

  • Human circulatory system, also called the blood vascular system consists of a muscular chambered heart, a network of closed branching blood vessels and blood, the fluid which is circulated.

HEART

  • Heart, the mesodermally derived organ, is situated in the thoracic cavity, in between the two lungs, slightly tilted to the left.
  • It has the size of a clenched fist.
  • It is protected by a double walled membranous bag, pericardium, enclosing the pericardial fluid.
  • Our heart has four chambers, two relatively small upper chambers called atria and two larger lower chambers called
  • A thin, muscular wall called the inter­atrial septum separates the right and the left atria, whereas a thick-walled, the inter-ventricular septum, separates the left and the right ventricles.
  • The atrium and the ventricle of the same side are also separated by a thick fibrous tissue called the atrio-ventricular septum.
  • However, each of these septa are provided with an opening through which the two chambers of the same side are connected.
  • The opening between the right atrium and the right ventricle is guarded by a valve formed of three muscular flaps or cusps, the tricuspid valve, whereas a bicuspid or mitral valve guards the opening between the left atrium and the left ventricle.
  • The openings of the right and the left ventricles into the pulmonary artery and the aorta respectively are provided with the semilunar valves. The valves in the heart allows the flow of blood only in one direction, i.e., from the atria to the ventricles and from the ventricles to the pulmonary artery or aorta. These valves prevent any backward flow.
  • The entire heart is made of cardiac muscles.
  • The walls of ventricles are much thicker than that of the atria.

Screenshot (92)

  • A specialised cardiac musculature called the nodal tissue is also distributed in the heart.
  • A patch of this tissue is present in the right upper corner of the right atrium called the sino-atrial node (SAN).
  • Another mass of this tissue is seen in the lower left corner of the right atrium close to the atrio-ventricular septum called the atrio-ventricular node (AVN).
  • A bundle of nodal fibres, atrio­ventricular bundle (AV bundle) continues from the AVN which passes through the atrio-ventricular septa to emerge on the top of the inter­ventricular septum and immediately divides into a right and left bundle.
  • These branches give rise to minute fibres throughout the ventricular musculature of the respective sides and are called purkinje fibres.
  • These fibres alongwith right and left bundles are known as bundle of HIS.
  • The nodal musculature has the ability to generate action potentials without any external stimuli, i.e., it is autoexcitable.
  • However, the number of action potentials that could be generated in a minute vary at different parts of the nodal system.
  • The SAN can generate the maximum number of action potentials, i.e., 70-75 min-1, and is responsible for initiating and maintaining the rhythmic contractile activity of the heart. Therefore, it is called the pacemaker.
  • Our heart normally beats 70-75 times in a minute (average 72 beats min-1).

CARDIAC CYCLE

  • To begin with, all the four chambers of heart are in a relaxed state, i.e., they are in joint diastole.
  • As the tricuspid and bicuspid valves are open, blood from the pulmonary veins and vena cava flows into the left and the right ventricle respectively through the left and right atria. The semilunar valves are closed at this stage.
  • The SAN now generates an action potential which stimulates both the atria to undergo a simultaneous contraction – the atrial systole. This increases the flow of blood into the ventricles by about 30 per cent.
  • The action potential is conducted to the ventricular side by the AVN and AV bundle from where the bundle of HIS transmits it through the entire ventricular musculature.
  • This causes the ventricular muscles to contract, (ventricular systole), the atria undergo relaxation (diastole), coinciding with the ventricular systole.
  • Ventricular systole increases the ventricular pressure causing the closure of tricuspid and bicuspid valves due to attempted backflow of blood into the atria.
  • As the ventricular pressure increases further, the semilunar valves guarding the pulmonary artery (right side) and the aorta (left side) are forced open, allowing the blood in the ventricles to flow through these vessels into the circulatory pathways.
  • The ventricles now relax (ventricular diastole) and the ventricular pressure falls causing the closure of semilunar valves which prevents the backflow of blood into the ventricles.
  • As the ventricular pressure declines further, the tricuspid and bicuspid valves are pushed open by the pressure in the atria exerted by the blood which was being emptied into them by the veins. The blood now once again moves freely to the ventricles.
  • The ventricles and atria are now again in a relaxed (joint diastole) state, as earlier. Soon the SAN generates a new action potential and the events described above are repeated in that sequence and the process continues.
  • This sequential event in the heart which is cyclically repeated is called the cardiac cycle and it consists of systole and diastole of both the atria and ventricles.
  • The heart beats 72 times per minute, i.e., that many cardiac cycles are performed per minute.
  • From this it could be deduced that the duration of a cardiac cycle is 0.8 seconds.
  • During a cardiac cycle, each ventricle pumps out approximately 70 mL of blood which is called the stroke volume.
  • The stroke volume multiplied by the heart rate (no. of beats per min.) gives the cardiac output.
  • Therefore, the cardiac output can be defined as the volume of blood pumped out by each ventricle per minute and averages 5000 mL or 5 litres in a healthy individual.
  • The body has the ability to alter the stroke volume as well as the heart rate and thereby the cardiac output. For example, the cardiac output of an athlete will be much higher than that of an ordinary man.
  • During each cardiac cycle two prominent sounds are produced which can be easily heard through a stethoscope.
  • The first heart sound (lub) is associated with the closure of the tricuspid and bicuspid valves whereas the second heart sound (dub) is associated with the closure of the semilunar valves. These sounds are of clinical diagnostic significance.

 ELECTROCARDIOGRAPH (ECG)

  • ECG is a graphical representation of the electrical activity of the heart during a cardiac cycle.
  • To obtain a standard ECG, a patient is connected to the machine with three electrical leads (one to each wrist and to the left ankle) that continuously monitor the heart activity.
  • For a detailed evaluation of the heart’s function, multiple leads are attached to the chest region.
  • Each peak in the ECG is identified with a letter from P to T that corresponds to a specific electrical activity of the heart.
  • The P-wave represents the electrical excitation (or depolarisation) of the atria, which leads to the contraction of both the atria.
  • The QRS complex represents the depolarisation of the ventricles, which initiates the ventricular contraction. The contraction starts shortly after Q and marks the beginning of the systole.
  • The T-wave represents the return of the ventricles from excited to normal state (repolarisation). The end of the T-wave marks the end of systole.
  • By counting the number of QRS complexes that occur in a given time period, one can determine the heart beat rate of an individual.
  • Since the ECGs obtained from different individuals have roughly the same shape for a given lead configuration, any deviation from this shape indicates a possible abnormality or disease. Hence, it is of a great clinical significance.

 Screenshot (93)

 DOUBLE CIRCULATION

  • The blood pumped by the right ventricle enters the pulmonary artery, whereas the left ventricle pumps blood into the aorta.
  • The deoxygenated blood pumped into the pulmonary artery is passed on to the lungs from where the oxygenated blood is carried by the pulmonary veins into the left atrium. This pathway constitutes the pulmonary circulation.
  • The oxygenated blood entering the aorta is carried by a network of arteries, arterioles and capillaries to the tissues from where the deoxygenated blood is collected by a system of venules, veins and vena cava and emptied into the right atrium. This is the systemic circulation.
  • The systemic circulation provides nutrients, O2 and other essential substances to the tissues and takes CO2 and other harmful substances away for elimination.
  • A unique vascular connection exists between the digestive tract and liver called hepatic portal system. The hepatic portal vein carries blood from intestine to the liver before it is delivered to the systemic circulation.
  • A special coronary system of blood vessels is present in our body exclusively for the circulation of blood to and from the cardiac musculature.

Screenshot (94)

REGULATION OF CARDIAC ACTIVITY

  • Normal activities of the heart are regulated intrinsically, i.e., auto regulated by specialised muscles (nodal tissue), hence the heart is called myogenic.
  • A special neural centre in the medulla oblangata can moderate the cardiac function through autonomic nervous system (ANS).
  • Neural signals through the sympathetic nerves (part of ANS) can increase the rate of heart beat, the strength of ventricular contraction and thereby the cardiac output.
  • On the other hand, parasympathetic neural signals (another component of ANS) decrease the rate of heart beat, speed of conduction of action potential and thereby the cardiac output.
  • Adrenal medullary hormones can also increase the cardiac output.

DISORDERS OF CIRCULATORY SYSTEM

High Blood Pressure (Hypertension):

  • Hypertension is the term for blood pressure that is higher than normal (120/80).
  • In this measurement 120 mm Hg (millimetres of mercury pressure) is the systolic, or pumping, pressure and 80 mm Hg is the diastolic, or resting, pressure.
  • If repeated checks of blood pressure of an individual is 140/90 (140 over 90) or higher, it shows hypertension.
  • High blood pressure leads to heart diseases and also affects vital organs like brain and kidney.

Coronary Artery Disease (CAD):

  • Coronary Artery Disease, often referred to as atherosclerosis, affects the vessels that supply blood to the heart muscle.
  • It is caused by deposits of calcium, fat, cholesterol and fibrous tissues, which makes the lumen of arteries narrower.

Angina:

  • It is also called ‘angina pectoris’.
  • A symptom of acute chest pain appears when no enough oxygen is reaching the heart muscle.
  • Angina can occur in men and women of any age but it is more common among the middle-aged and elderly.
  • It occurs due to conditions that affect the blood flow.

Heart Failure:

  • Heart failure means the state of heart when it is not pumping blood effectively enough to meet the needs of the body.
  • It is sometimes called congestive heart failure because congestion of the lungs is one of the main symptoms of this disease.
  • Heart failure is not the same as cardiac arrest (when the heart stops beating) or a heart attack (when the heart muscle is suddenly damaged by an inadequate blood supply).

 

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CHAPTER 18 – CIRCULATORY SYSTEM

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CHAPTER 17 – BREATHING AND EXCHANGE OF GASES

CHAPTER 17

BREATHING AND EXCHANGE OF GASES

  • Oxygen (O2) is utilised by the organisms to indirectly break down nutrient molecules like glucose and to derive energy for performing various activities.
  • Carbon dioxide (CO2) which is harmful is also released during the above catabolic reactions.
  • This process of exchange of O2 from the atmosphere with CO2 produced by the cells is called breathing, commonly known as  respiration.

Respiratory Organs

  • Lower invertebrates like sponges, coelenterates, flatworms, etc., exchange O2 with CO2 by simple diffusion over their entire body surface.
  • Earthworms use their moist cuticle and insects have a network of tubes (tracheal tubes) to transport atmospheric air within the body.
  • Special vascularised structures called gills are used by most of the aquatic arthropods and molluscs whereas vascularised bags called lungs are used by the terrestrial forms for the exchange of gases.
  • Among vertebrates, fishes use gills whereas reptiles, birds and mammals respire through lungs.
  • Amphibians like frogs can respire through their moist skin also.
  • Mammals have a well developed respiratory system.

Human Respiratory System

  • We have a pair of external nostrils opening out above the upper lips. It leads to a nasal chamber through the nasal passage.
  • The nasal chamber opens into nasopharynx, which is a portion of pharynx, the common passage for food and air.
  • Nasopharynx opens through glottis of the larynx region into the trachea.
  • Larynx is a cartilaginous box which helps in sound production and hence called the sound box.
  • During swallowing glottis can be covered by a thin elastic cartilaginous flap called epiglottis to prevent the entry of food into the larynx.
  • Trachea is a straight tube extending up to the mid-thoracic cavity, which divides at the level of 5th thoracic vertebra into a right and left primary bronchi.
  • Each bronchi undergoes repeated divisions to form the secondary and tertiary bronchi and bronchioles ending up in very thin terminal bronchioles.
  • The tracheae, primary, secondary and tertiary bronchi, and initial bronchioles are supported by incomplete cartilaginous rings.
  • Each terminal bronchiole gives rise to a number of very thin, irregular- walled and vascularised bag-like structures called alveoli.

Screenshot (84)

  • The branching network of bronchi, bronchioles and alveoli comprise the lungs.
  • We have two lungs which are covered by a double layered pleura, with pleural fluid between them. It reduces friction on the lung- surface.
  • The outer pleural membrane is in close contact with the thoracic lining whereas the inner pleural membrane is in contact with the lung surface.
  • The part starting with the external nostrils up to the terminal bronchioles constitute the conducting part whereas the alveoli and their ducts form the respiratory or exchange part of the respiratory system.
  • The conducting part transports the atmospheric air to the alveoli, clears it from foreign particles, humidifies and also brings the air to body temperature.
  • Exchange part is the site of actual diffusion of O2 and CO2 between blood and atmospheric air.
  • The lungs are situated in the thoracic chamber which is anatomically an air-tight chamber.
  • The thoracic chamber is formed dorsally by the vertebral column, ventrally by the sternum, laterally by the ribs and on the lower side by the dome-shaped diaphragm.
  • The anatomical setup of lungs in thorax is such that any change in the volume of the thoracic cavity will be reflected in the lung (pulmonary) cavity. Such an arrangement is essential for breathing, as we cannot directly alter the pulmonary volume.
  • Respiration involves the following steps:
    • Breathing or pulmonary ventilation by which atmospheric air is drawn in and CO2 rich alveolar air is released out.
    • Diffusion of gases (O2 and CO2) across alveolar membrane.
    • Transport of gases by the blood.
    • Diffusion of O2 and CO2 between blood and tissues.
    • Utilisation of O2 by the cells for catabolic reactions and resultant release of CO2 (cellular respiration)

Mechanism of Breathing

  • Breathing involves two stages: inspiration during which atmospheric air is drawn in and expiration by which the alveolar air is released out.
  • The movement of air into and out of the lungs is carried out by creating a pressure gradient between the lungs and the atmosphere.
  • Inspiration can occur if the pressure within the lungs (intra-pulmonary pressure) is less than the atmospheric pressure, i.e., there is a negative pressure in the lungs with respect to atmospheric pressure.
  • Similarly, expiration takes place when the intra-pulmonary pressure is higher than the atmospheric pressure.
  • The diaphragm and a specialised set of muscles – external and internal intercostals between the ribs, help in generation of such gradients.
  • Inspiration is initiated by the contraction of diaphragm which increases the volume of thoracic chamber in the antero-posterior axis. The contraction of external inter-costal muscles lifts up the ribs and the sternum causing an increase in the volume of the thoracic chamber in the dorso-ventral axis.
  • The overall increase in the thoracic volume causes a similar increase in pulmonary volume.
  • An increase in pulmonary volume decreases the intra-pulmonary pressure to less than the atmospheric pressure which forces the air from outside to move into the lungs, i.e., inspiration.
  • Relaxation of the diaphragm and the inter-costal muscles returns the diaphragm and sternum to their normal positions and reduce the thoracic volume and thereby the pulmonary volume. This leads to an increase in intra-pulmonary pressure to slightly above the atmospheric pressure causing the expulsion of air from the lungs, i.e., expiration.
  • We have the ability to increase the strength of inspiration and expiration with the help of additional muscles in the abdomen.
  • On an average, a healthy human breathes 12-16 times/minute.
  • The volume of air involved in breathing movements can be estimated by using a spirometer which helps in clinical assessment of pulmonary functions.

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Respiratory Volumes and Capacities

Tidal Volume (TV):

Volume of air inspired or expired during a normal respiration. It is approx. 500 mL., i.e., a healthy man can inspire or expire approximately 6000 to 8000 mL of air per minute.

Inspiratory Reserve Volume (IRV):

Additional volume of air, a person can inspire by a forcible inspiration. This averages 2500 mL to 3000 mL.

Expiratory Reserve Volume (ERV):

Additional volume of air, a person can expire by a forcible expiration. This averages 1000 mL to 1100 mL.

Residual Volume (RV):

Volume of air remaining in the lungs even after a forcible expiration. This averages 1100 mL to 1200 mL.

By adding up a few respiratory volumes described above, one can derive various pulmonary capacities, which can be used in clinical diagnosis.

Inspiratory Capacity (IC):

Total volume of air a person can inspire after a normal expiration. This includes tidal volume and inspiratory reserve volume (TV+IRV).

Expiratory Capacity (EC):

Total volume of air a person can expire after a normal inspiration. This includes tidal volume and expiratory reserve volume (TV+ERV).

Functional Residual Capacity (FRC):

Volume of air that will remain in the lungs after a normal expiration. This includes ERV+RV.

Vital Capacity (VC):

The maximum volume of air a person can breathe in after a forced expiration. This includes ERV, TV and IRV or the maximum volume of air a person can breathe out after a forced inspiration.

Total Lung Capacity:

Total volume of air accommodated in the lungs at the end of a forced inspiration. This includes RV, ERV, TV and IRV or vital capacity + residual volume.

Exchange of Gases

  • Alveoli are the primary sites of exchange of gases. Exchange of gases also occur between blood and tissues.
  • O2 and CO2 are exchanged in these sites by simple diffusion mainly based on pressure/concentration gradient.
  • Solubility of the gases as well as the thickness of the membranes involved in diffusion are also some important factors that can affect the rate of diffusion.
  • Pressure contributed by an individual gas in a mixture of gases is called partial pressure and is represented as pO2 for oxygen and pCO2 for carbon dioxide.

 

Respiratory Gas Atmospheric Air Alveoli Blood (Deoxygenated) Blood (Oxygenated) Tissues
O2 159 104 40 95 40
CO2 0.3 40 45 40 45

The data given in the table clearly indicates a concentration gradient for oxygen from alveoli to blood and blood to tissues. Similarly, a gradient is present for CO2 in the opposite direction, i.e., from tissues to blood and blood to alveoli.

  • As the solubility of CO2 is 20-25 times higher than that of O2, the amount of CO2 that can diffuse through the diffusion membrane per unit difference in partial pressure is much higher compared to that of O2.
  • The diffusion membrane is made up of three major layers namely, the thin squamous epithelium of alveoli, the endothelium of alveolar capillaries and the basement substance in between them. However, its total thickness is much less than a millimetre.

Screenshot (87)

  • Therefore, all the factors in our body are favourable for diffusion of O2 from alveoli to tissues and that of CO2 from tissues to alveoli.

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Transport of Gases

  • Blood is the medium of transport for O2 and CO2. About 97 per cent of O2 is transported by RBCs in the blood.
  • The remaining 3 per cent of O2 is carried in a dissolved state through the plasma.
  • Nearly 20-25 per cent of CO2 is transported by RBCs whereas 70 per cent of it is carried as bicarbonate. About 7 per cent of CO2 is carried in a dissolved state through plasma.

Transport of Oxygen

  • Haemoglobin is a red coloured iron containing pigment present in the RBCs. O2 can bind with haemoglobin in a reversible manner to form
  • Each haemoglobin molecule can carry a maximum of four molecules of O2.
  • Binding of oxygen with haemoglobin is primarily related to partial pressure of O2.
  • Partial pressure of CO2, hydrogen ion concentration and temperature are the other factors which can interfere with this binding.
  • A sigmoid curve is obtained when percentage saturation of haemoglobin with O2 is plotted against the pO2. This curve is called the Oxygen dissociation curve and is highly useful in studying the effect of factors like pCO2, H+ concentration, etc., on binding of O2 with haemoglobin.
  • In the alveoli, where there is high pO2, low pCO2, lesser H+ concentration and lower temperature, the factors are all favourable for the formation of oxyhaemoglobin, whereas in the tissues, where low pO2, high pCO2, high H+ concentration and higher temperature exist, the conditions are favourable for dissociation of oxygen from the oxyhaemoglobin.
  • This clearly indicates that O2 gets bound to haemoglobin in the lung surface and gets dissociated at the tissues.
  • Every 100 ml of oxygenated blood can deliver around 5 ml of O2 to the tissues under normal physiological conditions.

 Screenshot (88)

Transport of Carbon dioxide

  • CO2 is carried by haemoglobin as carbamino-haemoglobin (about 20-25 per cent).
  • This binding is related to the partial pressure of CO2. pO2 is a major factor which could affect this binding.
  • When pCO2 is high and pO2 is low as in the tissues, more binding of carbon dioxide occurs whereas, when the pCO2 is low and pO2 is high as in the alveoli, dissociation of CO2 from carbamino-haemoglobin takes place, i.e., CO2 which is bound to haemoglobin from the tissues is delivered at the alveoli.
  • RBCs contain a very high concentration of the enzyme, carbonic anhydrase and minute quantities of the same is present in the plasma too.
  • This enzyme facilitates the following reaction in both directions.

Screenshot (90)

  • At the tissue site where partial pressure of CO2 is high due to catabolism, CO2 diffuses into blood (RBCs and plasma) and forms HCO3 and H+.
  • At the alveolar site where pCO2 is low, the reaction proceeds in the opposite direction leading to the formation of CO2 and H2
  • Thus, CO2 trapped as bicarbonate at the tissue level and transported to the alveoli is released out as CO2.
  • Every 100 ml of deoxygenated blood delivers approximately 4 ml of CO2 to the alveoli.

 Regulation of Respiration

  • Human beings have a significant ability to maintain and moderate the respiratory rhythm to suit the demands of the body tissues. This is done by the neural system.
  • A specialised centre present in the medulla region of the brain called respiratory rhythm centre is primarily responsible for this regulation.
  • Another centre present in the pons region of the brain called pneumotaxic centre can moderate the functions of the respiratory rhythm centre.
  • Neural signal from this centre can reduce the duration of inspiration and thereby alter the respiratory rate.
  • A chemosensitive area is situated adjacent to the rhythm centre which is highly sensitive to CO2 and hydrogen ions.
  • Increase in these substances can activate this centre, which in turn can signal the rhythm centre to make necessary adjustments in the respiratory process by which these substances can be eliminated.
  • Receptors associated with aortic arch and carotid artery also can recognise changes in CO2 and H+ concentration and send necessary signals to the rhythm centre for remedial actions.
  • The role of oxygen in the regulation of respiratory rhythm is quite insignificant.

 Disorders of Respiratory System

  • Asthma is a difficulty in breathing causing wheezing due to inflammation of bronchi and bronchioles.
  • Emphysema is a chronic disorder in which alveolar walls are damaged due to which respiratory surface is decreased. One of the major causes of this is cigarette smoking.
  • Occupational Respiratory Disorders: In certain industries, especially those involving grinding or stone-breaking, so much dust is produced that the defense mechanism of the body cannot fully cope with the situation. Long exposure can give rise to inflammation leading to fibrosis (proliferation of fibrous tissues) and thus causing serious lung damage. Workers in such industries should wear protective masks.

 

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CHAPTER 17 – BREATHING AND EXCHANGE OF GASES

CHAPTER 16 – DIGESTION AND ABSORPTION

Chapter 16

Digestion and Absorption

  • The major components of our food are carbohydrates, proteins and fats. Vitamins and minerals are also required in small quantities.
  • Food provides energy and organic materials for growth and repair of tissues.
  • The water we take in, plays an important role in metabolic processes and also prevents dehydration of the body.
  • Biomacromolecules in food cannot be utilised by our body in their original form. They have to be broken down and converted into simple substances in the digestive system. This process of conversion of complex food substances to simple absorbable forms is called digestion and is carried out by our digestive system by mechanical and biochemical methods.

 DIGESTIVE SYSTEM

The human digestive system consists of the alimentary canal and the associated glands.

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 Alimentary Canal

  • The alimentary canal begins with an anterior opening – the mouth, and it opens out posteriorly through the anus.
  • The mouth leads to the buccal cavity or oral cavity.
  • The oral cavity has a number of teeth and a muscular tongue.
  • Each tooth is embedded in a socket of jaw bone. This type of attachment is called
  • Majority of mammals including human being forms two sets of teeth during their life, a set of temporary milk or deciduous teeth replaced by a set of permanent or adult teeth. This type of dentition is called
  • An adult human has 32 permanent teeth which are of four different types (Heterodont dentition), namely, incisors (I), canine (C), premolars (PM) and molars (M).
  • Arrangement of teeth in each half of the upper and lower jaw in the order I, C, PM, M is represented by a dental formula which in human is 2123/2123.

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  • The hard chewing surface of the teeth, made up of enamel, helpsin the mastication of food.
  • The tongue is a freely movable muscular organ attached to the floor of the oral cavity by the frenulum.
  • The upper surface of the tongue has small projections called papillae, some of which bear taste buds.
  • The oral cavity leads into a short pharynx which serves as a common passage for food and air. The oesophagus and the trachea (wind pipe) open into the pharynx.
  • A cartilaginous flap called epiglottis prevents the entry of food into the glottis – opening of the wind pipe – during swallowing.
  • The oesophagus is a thin, long tube which extends posteriorly passing through the neck, thorax and diaphragm and leads to a ‘J’ shaped bag like structure called stomach.
  • A muscular sphincter (gastro-oesophageal) regulates the opening of oesophagus into the stomach.
  • The stomach, located in the upper left portion of the abdominal cavity, has three major parts – a cardiac portion into which the oesophagus opens, a fundic region and a pyloric portion which opens into the first part of small intestine.

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  • Small intestine is distinguishable into three regions, a ‘U’ shaped duodenum, a long coiled middle portion jejunum and a highly coiled ileum.
  • The opening of the stomach into the duodenum is guarded by the pyloric sphincter.
  • Ileum opens into the large intestine. It consists of caecum, colon and rectum.
  • Caecum is a small blind sac which hosts some symbiotic micro-organisms.
  • A narrow finger-like tubular projection, the vermiform appendix which is a vestigial organ, arises from the caecum.
  • The caecum opens into the colon. The colon is divided into three parts – an ascending, a transverse and a descending part.
  • The descending part opens into the rectum which opens out through the anus.
  • The wall of alimentary canal from oesophagus to rectum possesses four layers namely serosa, muscularis, sub-mucosa and mucosa.
  • Serosa is the outermost layer and is made up of a thin mesothelium (epithelium of visceral organs) with some connective tissues.
  • Muscularis is formed by smooth muscles usually arranged into an inner circular and an outer longitudinal layer. An oblique muscle layer may be present in some regions(Stomach).

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  • The sub­mucosal layer is formed of loose connective tissues containing nerves, blood and lymph vessels. In duodenum, glands are also present in sub-mucosa.
  • The innermost layer lining the lumen of the alimentary canal is the mucosa. This layer forms irregular folds (rugae) in the stomach and small finger-like foldings called villi in the small intestine.
  • The cells lining the villi produce numerous microscopic projections called microvilli giving a brush border appearance. These modifications increase the surface area enormously.
  • Villi are supplied with a network of capillaries and a large lymph vessel called the lacteal.

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  • Mucosal epithelium has goblet cells which secrete mucus that help in lubrication.
  • Mucosa also forms glands in the stomach (gastric glands) and crypts in between the bases of villi in the intestine (crypts of Lieberkuhn).
  • All the four layers show modifications in different parts of the alimentary canal.

 Digestive Glands

  • The digestive glands associated with the alimentary canal include the salivary glands, the liver and the pancreas.
  • Saliva is mainly produced by three pairs of salivary glands, the parotids (cheek), the sub-maxillary/sub-mandibular (lower jaw) and the sub-lingual (below the tongue).
  • These glands situated just outside the buccal cavity secrete salivary juice into the buccal cavity.
  • Liver is the largest gland of the body weighing about 1.2 to 1.5 kg in an adult human.
  • It is situated in the abdominal cavity, just below the diaphragm and has two lobes.
  • The hepatic lobules are the structural and functional units of liver containing hepatic cells arranged in the form of cords.
  • Each lobule is covered by a thin connective tissue sheath called the Glisson’s capsule.
  • The bile secreted by the hepatic cells passes through the hepatic ducts and is stored and concentrated in a thin muscular sac called the gall bladder.
  • The duct of gall bladder (cystic duct) along with the hepatic duct from the liver forms the common bile duct.
  • The bile duct and the pancreatic duct open together into the duodenum as the common hepato-pancreatic duct which is guarded by a sphincter called the sphincter of Oddi.
  • The pancreas is a compound (both exocrine and endocrine) elongated organ situated between the limbs of the ‘U’ shaped duodenum.
  • The exocrine portion secretes an alkaline pancreatic juice containing enzymes and the endocrine portion secretes hormones, insulin and glucagon.

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DIGESTION OF FOOD

  • The process of digestion is accomplished by mechanical and chemical processes.
  • The buccal cavity performs two major functions, mastication of food and facilitation of swallowing. The teeth and the tongue with the help ofsaliva masticate and mix up the food thoroughly.
  • Mucus in saliva helps in lubricating and adhering the masticated food particles into a BOLUS.
  • The bolus is then conveyed into the pharynx and then into the oesophagus by swallowing or DEGLUTITION.
  • The bolus further passes down through the oesophagus by successive waves of muscular contractions called peristalsis.
  • The gastro-oesophageal sphincter controls the passage of food into the stomach.
  • The saliva secreted into the oral cavity contains electrolytes (Na+, K+, Cl, HCO) and enzymes, salivary amylase and lysozyme.
  • The chemical process of digestion is initiated in the oral cavity by the hydrolytic action of the carbohydrate splitting enzyme, the salivary amylase.

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  • About 30 per cent of starch is hydrolysed here by this enzyme (optimum pH 6.8) into a disaccharide – maltose.
  • Lysozyme present in saliva acts as an antibacterial agent that prevents infections.
  • The mucosa of stomach has gastric glands. Gastric glands have three major types of cells namely –
    • mucus neck cells which secrete mucus;
    • peptic or chief cells which secrete the proenzyme pepsinogen; and
    • parietal or oxyntic cells which secrete HCl and intrinsic factor (factor essential for absorption of vitamin B12).
  • The stomach stores the food for 4-5 hours.
  • The food mixes thoroughly with the acidic gastric juice of the stomach by the churning movements of its muscular wall and is called the CHYME.
  • The proenzyme pepsinogen, on exposure to hydrochloric acid gets converted into the active enzyme pepsin, the proteolytic enzyme of the stomach.
  • Pepsin converts proteins into proteoses and peptones (peptides).
  • The mucus and bicarbonates present in the gastric juice play an important role in lubrication and protection of the mucosal epithelium from excoriation by the highly concentrated hydrochloric acid. HCl provides the acidic pH (pH 1.8) optimal for pepsins.
  • Rennin is a proteolytic enzyme found in gastric juice of infants which helps in the digestion of milk proteins. Small amounts of lipases are also secreted by gastric glands.
  • Various types of movements are generated by the muscularis layer of the small intestine.
  • These movements help in a thorough mixing up of the food with various secretions in the intestine and thereby facilitate digestion.
  • The bile, pancreatic juice and the intestinal juice are the secretions released into the small intestine.
  • Pancreatic juice and bile are released through the hepato-pancreatic duct.
  • The pancreatic juice contains inactive enzymes – trypsinogen, chymotrypsinogen, procarboxypeptidases, amylases, lipases and nucleases.
  • Trypsinogen is activated by an enzyme, enterokinase, secreted by the intestinal mucosa into active trypsin, which in turn activates the other enzymes in the pancreatic juice.
  • The bile released into the duodenum contains bile pigments (bilirubin and bili-verdin), bile salts, cholesterol and phospholipids but no enzymes.
  • Bile helps in emulsification of fats, i.e., breaking down of the fats into very small micelles. Bile also activates lipases.
  • The intestinal mucosal epithelium has goblet cells which secrete mucus.
  • The secretions of the brush border cells of the mucosa alongwith the secretions of the goblet cells constitute the intestinal juice or succus entericus.
  • This juice contains a variety of enzymes like disaccharidases (e.g., maltase), dipeptidases, lipases, nucleosidases, etc.
  • The mucus alongwith the bicarbonates from the pancreas protects the intestinal mucosa from acid as well as provide an alkaline medium (pH 7.8) for enzymatic activities.
  • Sub-mucosal glands (Brunner’s glands) also help in this.
  • Proteins, proteoses and peptones (partially hydrolysed proteins) in the chyme reaching the intestine are acted upon by the proteolytic enzymes of pancreatic juice as given below:

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  • Carbohydrates in the chyme are hydrolysed by pancreatic amylase into disaccharides.

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  • Fats are broken down by lipases with the help of bile into di-and monoglycerides.

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  • Nucleases in the pancreatic juice acts on nucleic acids to form nucleotides and nucleosides

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  • The enzymes in the succus entericus act on the end products of the above reactions to form the respective simple absorbable forms. These final steps in digestion occur very close to the mucosal epithelial cells of the intestine.

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  • The breakdown of biomacromolecules mentioned above occurs in the duodenum region of the small intestine.
  • The simple substances thus formed are absorbed in the jejunum and ileum regions of the small intestine.
  • The undigested and unabsorbed substances are passed on to the large intestine.
  • No significant digestive activity occurs in the large intestine. The functions of large intestine are:
    • absorption of some water, minerals and certain drugs;
    • secretion of mucus which helps in adhering the waste (undigested) particles together and lubricating it for an easy passage.
  • The undigested, unabsorbed substances called faeces enters into the caecum of the large intestine through ileo-caecal valve, which prevents the back flow of the faecal matter.
  • It is temporarily stored in the rectum till defaecation.
  • The activities of the gastro-intestinal tract are under neural and hormonal control for proper coordination of different parts.
  • The sight, smell and/or the presence of food in the oral cavity can stimulate the secretion of saliva.
  • Gastric and intestinal secretions are also, similarly, stimulated by neural signals.
  • The muscular activities of different parts of the alimentary canal can also be moderated by neural mechanisms, both local and through CNS.
  • Hormonal control of the secretion of digestive juices is carried out by the local hormones produced by the gastric and intestinal mucosa.

ABSORPTION OF DIGESTED PRODUCTS

  • Absorption is the process by which the end products of digestion pass through the intestinal mucosa into the blood or lymph.
  • It is carried out by passive, active or facilitated transport mechanisms.
  • Small amounts of monosacharides like glucose, amino acids and some of electrolytes like chloride ions are generally absorbed by simple diffusion.
  • The passage of these substances into the blood depends upon the concentration gradients. However, some of the substances like fructose and some amino acids are absorbed with the help of the carrier ions like Na+. This mechanism is called the facilitated transport.
  • Transport of water depends upon the osmotic gradient.
  • Active transport occurs against the concentration gradient and hence requires energy.
  • Various nutrients like amino acids, monosacharides like glucose, electrolytes like Na+ are absorbed into the blood by this mechanism.
  • Fatty acids and glycerol being insoluble, cannot be absorbed into the blood.
  • They are first incorporated into small droplets called micelles which move into the intestinal mucosa.
  • They are re-formed into very small protein coated fat globules called the chylomicrons which are transported into the lymph vessels (lacteals) in the villi.
  • These lymph vessels ultimately release the absorbed substances into the blood stream.
  • Absorption of substances takes place in different parts of the alimentary canal, like mouth, stomach, small intestine and large intestine.
  • However, maximum absorption occurs in the small intestine.

Table : The Summary of Absorption in Different Parts of Digestive System

Mouth Stomach Small Intestine Large Intestine
Certain drugs coming in contact with the mucosa of mouth and lower side of the tongue are absorbed into the blood capillaries lining them. Absorption of water, simple sugars, and alcohol etc. takes place. Principal organ for absorption of nutrients. The digestion is completed here and the final products of digestion such as glucose, fructose, fatty acids, glycerol and amino acids are absorbed through the mucosa into the blood stream and lymph. Absorption of water, some minerals and drugs takes place.
  • The absorbed substances finally reach the tissues which utilise them for their activities. This process is called assimilation.
  • The digestive wastes, solidified into coherent faeces in the rectum initiate a neural reflex causing an urge or desire for its removal. The egestion of faeces to the outside through the anal opening (defaecation) is a voluntary process and is carried out by a mass peristaltic movement.

DISORDERS OF DIGESTIVE SYSTEM

  • The inflammation of the intestinal tract is the most common ailment due to bacterial or viral infections. The infections are also caused by the parasites of the intestine like tape worm, round worm, thread worm, hook worm, pin worm, etc.
  • Jaundice: The liver is affected, skin and eyes turn yellow due to the deposit of bile pigments.
  • Vomiting: It is the ejection of stomach contents through the mouth. This reflex action is controlled by the vomit centre in the medulla. A feeling of nausea precedes vomiting.
  • Diarrhoea: The abnormal frequency of bowel movement and increased liquidity of the faecal discharge is known as diarrhoea. It reduces the absorption of food.
  • Constipation: In constipation, the faeces are retained within the rectum as the bowel movements occur irregularly.
  • Indigestion: In this condition, the food is not properly digested leading to a feeling of fullness. The causes of indigestion are inadequate enzyme secretion, anxiety, food poisoning, over eating, and spicy food.

 

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CHAPTER 16 – DIGESTION AND ABSORPTION