Body Fluids and Circulation

Every living cell needs nutrients, O₂ and other essential substances brought to it, and needs its waste and harmful substances taken away. In a single cell or a thin animal, simple diffusion across the surface is enough. In a large animal the inner cells are far from the surface, so the body uses fluids that move through it and act as a medium of transport. This is the whole point of the chapter and NCERT states it in the first paragraph: complex organisms use special fluids within their bodies to transport such materials.

Two body fluids are commonly used by higher organisms. Blood is the most commonly used, and lymph (tissue fluid) helps in the transport of certain substances. Sponges and coelenterates do without either; they circulate water from their surroundings through their body cavities to move nutrients and gases. Everything that follows is organised around these two fluids and the pump that moves blood.

Read the chapter in NCERT order, because school papers follow the same order: first what blood is made of (plasma, formed elements, blood groups, clotting), then lymph, then the two patterns of circulation in animals and how the vertebrate heart evolved, then the human heart in detail (structure, nodal tissue, cardiac cycle, ECG, double circulation, regulation), and finally the disorders. The examiner rewards the exact NCERT terms and numbers, so keep a list of every count and value as you go; the formula sheet at the end of this guide collects them.

Blood is a special connective tissue consisting of a fluid matrix, plasma, and formed elements. It is called a connective tissue because, like every connective tissue, it has cells lying in a matrix; in blood the matrix happens to be fluid and has no fibres except during clotting. When a question asks 'why is blood considered a connective tissue', write exactly that: it is mesodermal in origin, its cells (formed elements) are scattered in a non-living matrix (plasma), and it connects different parts of the body by transporting materials between them.

Plasma is a straw-coloured, viscous fluid constituting nearly 55 per cent of the blood. It is 90 to 92 per cent water and 6 to 8 per cent proteins. The plasma proteins are of three kinds: fibrinogen, globulins and albumins. Fibrinogens are needed for clotting or coagulation of blood. Globulins are involved primarily in defence mechanisms of the body (antibodies are gamma globulins). Albumins help in osmotic balance, holding water inside the vessels. Small amounts of minerals like Na⁺, Ca²⁺, Mg²⁺, HCO₃⁻ and Cl⁻ are also present in plasma, along with glucose, amino acids and lipids, which are always in transit in the body.

One more sentence from NCERT matters: 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. So the 2-mark difference between plasma and serum is simply this: plasma is the whole fluid part of blood and contains fibrinogen and the other clotting factors; serum is what is left after blood has clotted, so it lacks fibrinogen and the clotting factors but still has albumins, globulins, minerals and nutrients.

Erythrocytes, leucocytes and platelets are collectively called formed elements and they constitute nearly 45 per cent of the blood. The counts, sites of formation and functions in this section are the most heavily examined facts of the chapter.

Erythrocytes (RBCs) are the most abundant of all the cells in blood. A healthy adult man has, on an average, 5 million to 5.5 million RBCs per mm³ of blood. RBCs are formed in the red bone marrow in adults. In most mammals they are devoid of nucleus and are biconcave in shape; the lost nucleus leaves the whole cell volume for haemoglobin, and the biconcave shape increases the surface for gas exchange. They have a red-coloured, iron-containing complex protein called haemoglobin, and a healthy person has 12 to 16 g of haemoglobin in every 100 mL of blood. Haemoglobin plays a significant role in the transport of respiratory gases. RBCs have an average life span of 120 days, after which they are destroyed in the spleen, which is therefore called the graveyard of RBCs.

Leucocytes (WBCs) are colourless due to the lack of haemoglobin, and they are nucleated. They are relatively few, 6000 to 8000 per mm³, and are generally short-lived. There are two main categories. Granulocytes are neutrophils, eosinophils and basophils. Agranulocytes are lymphocytes and monocytes. Learn the percentages: neutrophils 60 to 65 per cent (the most abundant), lymphocytes 20 to 25 per cent, monocytes 6 to 8 per cent, eosinophils 2 to 3 per cent, basophils 0.5 to 1 per cent (the least abundant). Neutrophils and monocytes are phagocytic cells that destroy foreign organisms entering the body. Basophils secrete histamine, serotonin and heparin and are involved in inflammatory reactions. Eosinophils resist infections and are also associated with allergic reactions. Lymphocytes are of two major types, B and T forms, and both are responsible for immune responses of the body.

Platelets, also called thrombocytes, are cell fragments produced from megakaryocytes (special cells in the bone marrow). Blood normally contains 1.5 lakh to 3.5 lakh platelets per mm³. Platelets 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 and excessive loss of blood from the body; this is why a dengue report is watched for its platelet count.

The blood of all humans looks alike but differs in certain aspects, and two groupings are widely used: the ABO system and the Rh system. Both matter because transfusing the wrong blood makes the recipient's antibodies clump (agglutinate) and destroy the donor's RBCs.

ABO grouping is based on the presence or absence of two surface antigens (chemicals that can induce an immune response) on the RBCs, namely antigen A and antigen B. Similarly, the plasma of different individuals contains two natural antibodies (proteins produced in response to antigens). The distribution is Table 15.1 of NCERT and you must reproduce it from memory. Group A: antigen A on RBCs, antibody anti-B in plasma, can receive from A and O. Group B: antigen B, antibody anti-A, can receive from B and O. Group AB: antigens A and B, no antibodies, can receive from AB, A, B and O. Group O: no antigen, antibodies anti-A and anti-B, can receive from O only. Because group O has no antigens, its blood can be donated to persons with any other blood group, so O is the universal donor. Because group AB has no antibodies, persons with AB can accept blood from any group, so AB is the universal recipient. The logic to write in the exam: the donor's RBC antigen must not meet a matching antibody in the recipient's plasma.

Rh grouping: another antigen, the Rh antigen, similar to one present in Rhesus monkeys, is 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 Rh negative (Rh−ve). An Rh−ve person, if exposed to Rh+ve blood, forms specific antibodies against the Rh antigen, so Rh group should also be matched before transfusion.

The examiner's favourite case is erythroblastosis foetalis: a special case of Rh incompatibility between the Rh−ve mother and the Rh+ve foetus. In the first pregnancy the Rh antigens of the foetus do not get exposed to the mother's blood, because the two bloods are well separated by the placenta. During the delivery of the first child, however, there is a possibility of exposure of the maternal blood to small amounts of Rh+ve blood from the foetus. The mother then starts preparing antibodies against the Rh antigen. In a subsequent pregnancy with an Rh+ve foetus, these antibodies can leak into the blood of the foetus and destroy its RBCs, causing severe anaemia and jaundice in the baby; the condition can be fatal. It can be avoided by administering anti-Rh antibodies to the mother immediately after the delivery of the first child, which mop up the foetal Rh antigens before the mother's immune system is sensitised.

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 forms at the site of a cut, and that is the clot.

NCERT describes the clot precisely, and the wording earns marks: the clot or coagulum is 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, prothrombin. An enzyme complex, thrombokinase, is required for this conversion. This complex is formed by a series of linked enzymic reactions, a cascade process, involving a number of factors present in the plasma in an inactive state. An injury or 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 can also initiate coagulation. Calcium ions play a very important role in clotting.

The three-step cascade to write, in order, as a flow chart:

1. Injury: platelets and damaged tissue release factors → inactive plasma factors are activated one after another → the enzyme complex thrombokinase is formed.
2. Thrombokinase, with Ca²⁺, converts inactive prothrombin into active thrombin.
3. Thrombin converts soluble fibrinogen into insoluble fibrin threads; the network traps formed elements and forms the clot.

Two related facts are often asked from outside the strict NCERT text but from its logic. Blood does not clot inside an intact vessel because thrombin does not exist there (only inactive prothrombin), the endothelium is smooth, and heparin from basophils acts as an anticoagulant. In a blood bank, blood is kept from clotting by adding sodium citrate or oxalate, which removes Ca²⁺; EDTA works the same way. A deficiency of vitamin K delays clotting because prothrombin is synthesised in the liver with vitamin K.

As blood passes through the capillaries in the tissues, some water along with many small water-soluble substances moves 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, and it is the medium through which exchange of nutrients, gases and other substances between the blood and the cells actually takes place.

An elaborate network of vessels, the lymphatic system, collects this fluid and drains it back to the major veins. The fluid present in the lymphatic system is called 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 and so on. Fats are absorbed through lymph in the lacteals present in the intestinal villi, which is why lymph draining from the intestine after a fatty meal looks milky (chyle).

The 2-mark and 3-mark question on this section is the comparison of blood and lymph. Blood is red because of haemoglobin in RBCs; lymph is colourless because it has no RBCs. Blood has RBCs, all types of WBCs and platelets; lymph has only leucocytes, mainly lymphocytes, and no platelets. Blood plasma has more proteins, including fibrinogen; lymph has less protein and clots slowly. Blood flows in a closed loop from the heart and back; lymph flows only in one direction, from the tissues towards the heart, driven by muscle movement and valves, and joins the venous blood. Blood carries O₂, CO₂, nutrients and wastes throughout the body; lymph returns tissue fluid and proteins to the blood, carries absorbed fats and provides immunity.

The circulatory patterns are of two types, open or closed. An open circulatory system is present in arthropods and molluscs: blood pumped by the heart passes through large vessels into open spaces or body cavities called sinuses, bathing the organs directly. 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 more advantageous because the flow of fluid can be more precisely regulated: blood can be directed to the organs that need it, at a controlled pressure and speed.

All vertebrates possess a muscular chambered heart, and the chapter traces its evolution. Fishes have a 2-chambered heart with one atrium and one ventricle. Amphibians and reptiles (except crocodiles) have a 3-chambered heart with two atria and a single ventricle. 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. Blood passes through the heart only once per circuit: single circulation. In amphibians and reptiles, the left atrium receives oxygenated blood from the gills, lungs or 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 pass on to the ventricles of the same sides; the ventricles pump it out without any mixing, so two separate circulatory pathways are present: double circulation.

When the exercise asks you to describe the evolutionary change in the pattern of heart among vertebrates, write these three stages with the number of chambers, the example animals, and the consequence for mixing of blood. The examiner is looking for the trend: from a single circuit with a two-chambered heart, through partial separation with three chambers, to complete separation with four chambers and no mixing, which supports the high metabolic rate of endothermic birds and mammals.

The 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 heart is mesodermally derived, situated in the thoracic cavity, in between the two lungs, slightly tilted to the left, and is about the size of a clenched fist. It is protected by a double-walled membranous bag, the pericardium, enclosing the pericardial fluid, which reduces friction as the heart beats.

The heart has four chambers: two relatively small upper chambers, the atria, and two larger lower chambers, the ventricles. A thin muscular wall, the inter-atrial septum, separates the right and left atria; a thick-walled inter-ventricular septum separates the left and right ventricles. The atrium and ventricle of the same side are separated by a thick fibrous tissue, the atrio-ventricular septum, with an opening on each side through which the two chambers of that side are connected. The opening between the right atrium and right ventricle is guarded by the tricuspid valve, formed of three muscular flaps or cusps; the opening between the left atrium and left ventricle is guarded by the bicuspid or mitral valve. The openings of the right and left ventricles into the pulmonary artery and the aorta respectively are provided with semilunar valves. All these valves allow the flow of blood in one direction only, from the atria to the ventricles and from the ventricles to the pulmonary artery or aorta, and prevent any backward flow. The left ventricle has the thickest wall because it pumps blood to the whole body.

The entire heart is made of cardiac muscles. The walls of the ventricles are much thicker than those of the atria. 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, the sino-atrial node (SAN); another mass is in the lower left corner of the right atrium close to the atrio-ventricular septum, the atrio-ventricular node (AVN). A bundle of nodal fibres, the atrio-ventricular bundle (AV bundle, bundle of His), continues from the AVN, passes through the atrio-ventricular septa to emerge on the top of the inter-ventricular septum, and immediately divides into a right and a left bundle. These branches give rise to minute fibres throughout the ventricular musculature of the respective sides, the Purkinje fibres.

The nodal musculature has the ability to generate action potentials without any external stimuli: it is autoexcitable. The number of action potentials that could be generated in a minute varies at different parts of the nodal system. The SAN can generate the maximum, 70 to 75 per minute, and is responsible for initiating and maintaining the rhythmic contractile activity of the heart; so it is called the pacemaker. Our heart normally beats 70 to 75 times in a minute, average 72 beats per minute. Because the impulse originates in the muscle itself and not in a nerve, the heart is called myogenic; the heart of a cockroach or an annelid, whose beat is started by nerves, is neurogenic.

NCERT explains the cycle by starting from the moment when all four chambers are relaxed, the joint diastole. At this time the tricuspid and bicuspid valves are open, so blood from the pulmonary veins and the venae cavae flows into the left and right atria respectively and passes straight on into the ventricles; the semilunar valves are closed.

The SAN now generates an action potential which stimulates both atria to undergo simultaneous contraction, the atrial systole. This increases the flow of blood into the ventricles by about 30 per cent; that is, most ventricular filling happens passively during diastole, and the atrial kick tops it up. The action potential is conducted to the ventricular side through the AVN and AV bundle and then to the ventricular musculature by the Purkinje fibres, so the ventricles contract: ventricular systole. The atria meanwhile relax (atrial diastole), coinciding with ventricular systole. Ventricular systole increases the ventricular pressure, causing the closure of the tricuspid and bicuspid valves due to the 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 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 the semilunar valves, which prevents the backflow of blood into the ventricles. As the ventricular pressure declines further, the tricuspid and bicuspid valves open and blood from the atria, which is already being filled from the veins, flows into the ventricles. The ventricles and atria are now again in a relaxed (joint diastole) state.

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, so that many cardiac cycles are performed per minute, and the duration of one cardiac cycle is 0.8 second. Teachers usually split it as atrial systole 0.1 s, ventricular systole 0.3 s and joint diastole 0.4 s; NCERT gives only the total, so quote the total and add the split as a bonus.

During a cardiac cycle each ventricle pumps out approximately 70 mL of blood, called the stroke volume. The stroke volume multiplied by the heart rate (number of beats per minute) gives the cardiac output: 70 mL × 72 = about 5000 mL or 5 litres per minute in a normal adult. The body has the ability to alter the stroke volume as well as the heart rate and thereby the cardiac output; the cardiac output of an athlete is 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 at the start of ventricular systole; the second, dub, is associated with the closure of the semilunar valves at the start of ventricular diastole. These sounds are of clinical diagnostic significance: a murmur points to a leaking or narrowed valve.

An electrocardiograph is the machine, and the electrocardiogram (ECG) is the graphical representation of the electrical activity of the heart during a cardiac cycle that it records. To obtain a standard ECG, a patient is connected to the machine with three electrical leads (one to each wrist and one 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. The chapter asks you to understand only the standard ECG, which is Fig. 15.4.

Each peak in the ECG is identified with a letter from P to T, corresponding to a specific electrical activity of the heart. The P wave represents the electrical excitation (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 the excited to the normal state (repolarisation); the end of the T wave marks the end of systole. Atrial repolarisation also happens, but it is masked by the large QRS complex and does not appear as a separate wave.

Two practical points are asked. First, 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 has great clinical significance. Second, by counting the number of QRS complexes that occur in a given time period, one can determine the heart beat rate of an individual. If a strip shows 6 QRS complexes in 5 seconds, the heart rate is 6 × 12 = 72 per minute.

When you draw the ECG, plot voltage against time as a baseline with a small rounded P bump, a sharp downward Q, a tall narrow upward R, a small downward S and then a broad rounded T. Label the five letters and write against them 'atrial depolarisation', 'ventricular depolarisation' and 'ventricular repolarisation'. Mark the P-Q interval as the time taken for the impulse to travel from the atria through the AVN to the ventricles, and the S-T segment as the plateau when the ventricles are fully depolarised.

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 venae cavae 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.

Because the blood goes through the heart twice in one complete circuit, once through the right side on its way to the lungs and once through the left side on its way to the body, the circulation is called double circulation. Its significance, which is a standard 2-mark question: oxygenated and deoxygenated blood are kept completely separate, so the tissues receive fully oxygenated blood at high pressure, which supports the high metabolic rate and constant body temperature of birds and mammals; and the lungs receive blood at a lower pressure suited to the delicate alveolar capillaries.

Two special circuits are named in NCERT. A unique vascular connection exists between the digestive tract and the liver, the hepatic portal system: the hepatic portal vein carries blood from the intestine to the liver before it is delivered to the systemic circulation, so absorbed nutrients are processed and toxins removed first. A special coronary system of blood vessels is present in our body exclusively for the circulation of blood to and from the cardiac musculature; blockage of a coronary artery is what causes a heart attack.

The schematic plan of Fig. 15.5 should be written as one flow: body tissues → superior and inferior venae cavae → right atrium → tricuspid valve → right ventricle → pulmonary artery → lungs → pulmonary veins → left atrium → bicuspid valve → left ventricle → aorta → body tissues. Note the two counter-intuitive vessel names: the pulmonary artery is the only artery carrying deoxygenated blood, and the pulmonary veins are the only veins carrying oxygenated blood.

Normal activities of the heart are regulated intrinsically, that is, auto-regulated by specialised muscles (nodal tissue); hence the heart is called myogenic. The SAN sets the basic rhythm without any input from the nervous system, which is why an isolated heart kept in a suitable solution keeps beating. The nervous and endocrine systems only adjust this rhythm up or down.

A special neural centre in the medulla oblongata can moderate the cardiac function through the autonomic nervous system (ANS). Neural signals through the sympathetic nerves (part of the ANS) can increase the rate of heart beat, the strength of ventricular contraction and thereby the cardiac output; this is the fight-or-flight adjustment during exercise, fear or blood loss. On the other hand, parasympathetic neural signals (another component of the ANS, carried by the vagus nerve) decrease the rate of heart beat, the speed of conduction of the action potential and thereby the cardiac output; this is the rest-and-digest setting. Adrenal medullary hormones, adrenaline and noradrenaline, can also increase the cardiac output, which is why the heart races before an exam.

Put the three influences in one table when you answer: nodal tissue sets the rhythm (myogenic); sympathetic nerves and adrenal medullary hormones speed it up and strengthen it; parasympathetic nerves slow it down. A common 2-mark question is 'why is the heart called myogenic even though it is innervated'. The answer: the rhythm originates in the heart's own nodal tissue, which is autoexcitable, and the nerves only modify the rate; a denervated or transplanted heart still beats.

High blood pressure (hypertension) is the term for blood pressure that is higher than normal. Normal blood pressure is 120/80 mm Hg: 120 is the systolic pressure, the pressure in the arteries when the ventricles contract, and 80 is the diastolic pressure, the pressure when the ventricles relax. If repeated checks of blood pressure of an individual are 140/90 mm Hg or higher, it shows hypertension. High blood pressure leads to heart diseases and also affects vital organs like the brain and kidney.

Coronary artery disease (CAD), 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 in the coronary arteries, which make the lumen of the arteries narrower and reduce the blood supply to the cardiac muscle.

Angina pectoris, or simply angina, is a symptom of acute chest pain that appears when not enough oxygen is reaching the heart muscle. It can occur in men and women of any age but is more common among the middle-aged and elderly. It occurs because blood flow to the heart is affected, typically by narrowed coronary arteries during exertion.

Heart failure is the state of the 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 its main symptoms. The examiner likes the three-way distinction from the NCERT paragraph: 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). Write all three as one-line definitions in a 3-mark question and you will get full marks.

For the case-study style question, connect the four: fatty deposits cause atherosclerosis (CAD); the narrowed lumen reduces O₂ to the myocardium, felt as angina; a complete block kills part of the muscle, a heart attack; a weakened heart that cannot meet demand is in heart failure; and hypertension accelerates all of this by damaging the artery walls.