Body Fluids and Circulation: NEET notes

1. Blood and plasma

NCERT §15 (opening text) and §15.1–§15.1.1

• Every living cell needs a steady supply of nutrients, O₂ and other essentials, and its wastes must be carried away. Animals solve this in different ways: sponges and coelenterates pass water from their surroundings through body cavities, while more complex animals move special body fluids.
• Most higher animals, humans included, use blood for this transport; a second fluid, lymph, carries certain substances too.
• Blood is a special connective tissue: a fluid matrix, plasma, with formed elements suspended in it.
• Plasma is a viscous, straw-coloured fluid making up nearly 55 per cent of blood. Water forms 90–92 per cent of plasma and proteins 6–8 per cent.
• The major plasma proteins and their jobs: fibrinogen for clotting (coagulation); globulins mainly for the body's defence; albumins for osmotic balance.
• Plasma also carries small amounts of minerals such as Na⁺, Ca⁺⁺, Mg⁺⁺, HCO₃⁻ and Cl⁻, and glucose, amino acids and lipids that are always in transit through the body.
• Clotting factors are present in plasma in an inactive form. Plasma from which the clotting factors have been removed is serum.

2. Formed elements: RBCs, WBCs and platelets

NCERT §15.1.2

• Erythrocytes, leucocytes and platelets together are the formed elements, making up nearly 45 per cent of blood.
• RBCs are the most numerous blood cells: on average 5 to 5.5 million per mm³ of blood in a healthy adult man. In adults they are made in the red bone marrow.
• In most mammals RBCs lack a nucleus and are biconcave. Their red colour and name come from haemoglobin, an iron-containing complex protein; a healthy person has 12–16 g of haemoglobin per 100 mL of blood, and it is central to carrying the respiratory gases.
• An RBC lives on average 120 days and is then destroyed in the spleen, which is therefore called the graveyard of RBCs.
• WBCs (leucocytes) are colourless because they lack haemoglobin, have a nucleus, are fewer (on average 6000–8000 per mm³) and are generally short-lived.
• Granulocytes are neutrophils, eosinophils and basophils; agranulocytes are lymphocytes and monocytes.
• Neutrophils are the most abundant WBCs (60–65 per cent) and basophils the least (0.5–1 per cent). Neutrophils and monocytes (6–8 per cent) are phagocytes that destroy invading organisms.
• Basophils release histamine, serotonin and heparin and are involved in inflammation. Eosinophils make up 2–3 per cent; they help the body fight infection and are associated with allergy. Lymphocytes (20–25 per cent) are of two main kinds, B and T, and both are responsible for immune responses.
• Platelets (thrombocytes) are cell fragments budded off from megakaryocytes, special cells of the bone marrow. Blood normally holds 1,50,000–3,50,000 platelets per mm³.
• Platelets release a range of substances, most of them involved in clotting. Too few platelets lead to clotting disorders and excessive blood loss.

3. Blood groups

NCERT §15.1.3

• Human blood looks alike but differs in certain respects; the two groupings used worldwide are ABO and Rh.
• ABO grouping depends on whether two surface antigens, A and B, are present on the RBCs. Antigens are chemicals that can trigger an immune response. Plasma carries natural antibodies, proteins made in response to antigens.
• Group A: antigen A on RBCs, anti-B in plasma, can receive from A and O. Group B: antigen B, anti-A, can receive from B and O.
• Group AB: both antigens, no antibodies, can receive from AB, A, B and O. Group O: no antigens, both anti-A and anti-B, can receive only from O.
• Donor and recipient blood must be matched before a transfusion; a mismatch causes clumping, which destroys RBCs.
• Group O can be given to people of any group, so O individuals are universal donors. Group AB can receive from every group, so AB individuals are universal recipients.
• Rh grouping: an antigen similar to one in Rhesus monkeys, the Rh antigen, is found on the RBCs of nearly 80 per cent of humans (Rh positive); people without it are Rh negative.
• An Rh negative person exposed to Rh positive blood makes antibodies against the Rh antigen, so Rh group must also be matched before a transfusion.
• Rh incompatibility in pregnancy: in the first pregnancy the placenta keeps the blood of an Rh negative mother apart from that of an Rh positive foetus, but at delivery a little foetal blood may reach the mother, who then starts making anti-Rh antibodies.
• In later pregnancies these antibodies can leak into the blood of an Rh positive foetus and destroy its RBCs, which may be fatal or cause severe anaemia and jaundice in the baby: erythroblastosis foetalis. It can be prevented by giving the mother anti-Rh antibodies immediately after the first delivery.

4. Coagulation of blood

NCERT §15.1.4

• Blood clots in response to injury or trauma; this stops excessive loss of blood.
• The dark reddish-brown scum over a cut is a clot (coagulum): a mesh of threads called fibrin in which dead and damaged formed elements are trapped.
• Fibrin is made from inactive fibrinogen in plasma by the enzyme thrombin.
• Thrombin itself is made from another inactive plasma substance, prothrombin. This step needs an enzyme complex, thrombokinase.
• Thrombokinase forms through a cascade: a chain of linked enzyme reactions involving several factors present in plasma in an inactive state.
• Injury stimulates platelets to release factors that set the cascade going; factors released by damaged tissue at the site can also start it.
• Calcium ions play a very important role in clotting.

5. Lymph (tissue fluid)

NCERT §15.2

• As blood flows through capillaries in the tissues, water together with small dissolved molecules seeps out into the gaps around the cells. The larger proteins and most formed elements stay inside the vessels.
• The fluid that leaves is interstitial fluid or tissue fluid. Its mineral distribution is the same as that of plasma.
• All exchange of nutrients, gases and so on between blood and cells happens through this fluid.
• The lymphatic system, a network of vessels, collects this fluid and drains it back into the major veins. The fluid inside the lymphatic system is called lymph.
• Lymph is colourless and contains specialised lymphocytes responsible for immune responses. It also carries nutrients, hormones and other substances.
• Fats are absorbed into lymph through the lacteals in the intestinal villi.
• Lymph resembles blood except for its protein content and formed elements.

6. Circulatory pathways in animals

NCERT §15.3

• Circulation is either open or closed.
• Open circulation (arthropods, molluscs): blood leaves the heart through large vessels and pours into open cavities, the sinuses, where it bathes the tissues directly.
• Closed circulation (annelids, chordates): blood pumped by the heart always travels through a closed network of vessels. It is considered more advantageous because the flow can be regulated more precisely.
• All vertebrates have a muscular, chambered heart. Fishes have 2 chambers (one atrium, one ventricle); amphibians and reptiles other than crocodiles have 3 (two atria, one ventricle); crocodiles, birds and mammals have 4 (two atria, two ventricles).
• Single circulation (fishes): the heart pumps deoxygenated blood to the gills, where it is oxygenated and sent to the body; deoxygenated blood returns to the heart.
• Incomplete double circulation (amphibians, reptiles): the left atrium receives oxygenated blood from gills, lungs or skin and the right atrium deoxygenated blood from the body, but the two mix in the single ventricle, which pumps out mixed blood.
• Double circulation (birds, mammals): oxygenated and deoxygenated blood enter the left and right atria respectively, pass to the ventricles of the same side and are pumped out without mixing, through two separate pathways.

7. The human heart

NCERT §15.3.1

• The human circulatory (blood vascular) system has a muscular chambered heart, a closed network of branching blood vessels, and blood, the fluid that circulates.
• The heart develops from mesoderm and sits in the thoracic cavity between the two lungs, tilted slightly to the left. It is about the size of a clenched fist and is enclosed by a double-walled membranous bag, the pericardium, containing pericardial fluid.
• It has four chambers: two smaller upper atria and two larger lower ventricles. A thin muscular inter-atrial septum separates the atria; a thick-walled inter-ventricular septum separates the ventricles; a thick fibrous atrio-ventricular septum separates each atrium from the ventricle below it, with an opening connecting the two on each side.
• The right atrio-ventricular opening is guarded by the tricuspid valve (three muscular flaps or cusps); the left by the bicuspid or mitral valve. The openings of the right and left ventricles into the pulmonary artery and aorta carry semilunar valves.
• The valves let blood flow only one way, atria to ventricles and ventricles to the pulmonary artery or aorta, and prevent backflow.
• The whole heart is cardiac muscle, and the ventricle walls are much thicker than the atrial walls.
• Nodal tissue, a specialised cardiac musculature, is spread through the heart. The sino-atrial node (SAN) lies in the right upper corner of the right atrium; the atrio-ventricular node (AVN) lies in the lower left corner of the right atrium, near the atrio-ventricular septum.
• From the AVN, the atrio-ventricular bundle (AV bundle) passes through the atrio-ventricular septa, emerges on top of the inter-ventricular septum and divides into right and left bundles, which give rise to fine Purkinje fibres through the ventricular muscle of each side.
• Nodal musculature is autoexcitable: it produces action potentials without any outside stimulus. The SAN produces the most, 70–75 per minute, so it starts and maintains the heart's rhythm and is called the pacemaker.
• The heart normally beats 70–75 times a minute, averaging 72 beats per minute.

8. The cardiac cycle

NCERT §15.3.2

• Start: all four chambers are relaxed (joint diastole). The tricuspid and bicuspid valves are open, so blood from the pulmonary veins and vena cava flows through the left and right atria into the left and right ventricles. The semilunar valves are closed.
• The SAN fires an action potential that makes both atria contract together: atrial systole. Their squeeze pushes roughly 30 per cent more blood down into the ventricles.
• The impulse is carried to the ventricles by the AVN and AV bundle, and the bundle of His spreads it through the ventricular muscle. The ventricles contract (ventricular systole) while the atria relax (atrial diastole).
• Rising ventricular pressure closes the tricuspid and bicuspid valves as blood tries to flow back into the atria. As pressure rises further, the semilunar valves of the pulmonary artery (right) and aorta (left) are forced open and blood leaves the ventricles.
• The ventricles then relax (ventricular diastole); falling pressure closes the semilunar valves, preventing backflow. As pressure falls further, blood collected in the atria from the veins pushes the tricuspid and bicuspid valves open, blood flows freely into the ventricles again, and the heart is back in joint diastole until the SAN fires once more.
• This cyclically repeated sequence of systole and diastole of atria and ventricles is the cardiac cycle. With 72 beats per minute, one cycle lasts 0.8 seconds.
• Each ventricle pumps out about 70 mL of blood per cycle: the stroke volume. Stroke volume × heart rate = cardiac output, the volume pumped by each ventricle per minute, averaging 5000 mL (5 litres) in a healthy person.
• The body can change both stroke volume and heart rate and so the cardiac output; an athlete's cardiac output is much higher than an ordinary person's.
• Two prominent heart sounds are heard through a stethoscope each cycle: the first, lub, from closure of the tricuspid and bicuspid valves; the second, dub, from closure of the semilunar valves. They are of clinical diagnostic value.

9. Electrocardiogram (ECG)

NCERT §15.3.3

• An ECG is a trace, drawn by a machine called an electrocardiograph, of the electrical events of the heart through each cardiac cycle.
• For a standard ECG the patient is connected with three leads, one to each wrist and one to the left ankle, which monitor heart activity continuously. Detailed studies use multiple leads on the chest.
• Each peak is named by a letter from P to T, and each corresponds to a particular electrical event.
• P-wave: electrical excitation (depolarisation) of the atria, which leads to contraction of both atria.
• QRS complex: depolarisation of the ventricles, which starts ventricular contraction. Contraction begins shortly after Q and marks the start of systole.
• T-wave: the ventricles recover their resting electrical state (repolarisation). Where the T-wave finishes, systole finishes.
• Counting QRS complexes over a known time gives the heart rate.
• ECGs from different people have roughly the same shape for a given lead arrangement, so any deviation hints at an abnormality or disease; hence their clinical importance.

10. Double circulation

NCERT §15.4

• Blood travels by a fixed route through arteries and veins. Each has three layers: tunica intima, an inner lining of squamous endothelium; tunica media, a middle layer of smooth muscle and elastic fibres; tunica externa, an outer coat of fibrous connective tissue rich in collagen. The middle coat is comparatively thin in veins.
• The right ventricle pumps blood into the pulmonary artery; the left ventricle pumps into the aorta.
• Pulmonary circulation: deoxygenated blood goes from the right ventricle through the pulmonary artery to the lungs, and oxygenated blood returns through the pulmonary veins to the left atrium.
• Systemic circulation: oxygenated blood leaves the left ventricle through the aorta and passes through arteries, arterioles and capillaries to the tissues; deoxygenated blood is collected by venules, veins and the vena cava and returned to the right atrium.
• The systemic circulation supplies nutrients, O₂ and other essentials to the tissues and removes CO₂ and other harmful substances for elimination.
• Hepatic portal system: a special vascular link between the digestive tract and the liver. The hepatic portal vein carries blood from the intestine to the liver before it enters the systemic circulation.
• The coronary system is a special set of vessels that carries blood exclusively to and from the heart muscle.
• Humans have complete double circulation: two separate pathways, pulmonary and systemic.

11. Regulation of cardiac activity and disorders

NCERT §15.5–§15.6

• The heart's normal activity is regulated from within by nodal tissue, so the heart is called myogenic.
• A special neural centre in the medulla oblongata can moderate heart function through the autonomic nervous system (ANS).
• Sympathetic nerve signals increase the heart rate, the strength of ventricular contraction and so the cardiac output. Parasympathetic signals decrease the heart rate, the speed of conduction of the action potential and so the cardiac output. Adrenal medullary hormones can also raise the cardiac output.
• Hypertension is blood pressure above normal, 120/80: 120 mm Hg is the systolic (pumping) pressure and 80 mm Hg the diastolic (resting) pressure. Repeated readings of 140/90 or higher indicate hypertension, which leads to heart disease and harms vital organs such as brain and kidney.
• Coronary artery disease (CAD), often called atherosclerosis, affects the vessels supplying the heart muscle; deposits of calcium, fat, cholesterol and fibrous tissue narrow the lumen of the arteries.
• Angina (angina pectoris) is acute chest pain when not enough oxygen reaches the heart muscle. It can occur at any age in men and women but is commoner in the middle-aged and elderly, and results from conditions that affect blood flow.
• Heart failure is when the heart does not pump blood effectively enough to meet the body's needs. Because fluid backs up and congests the lungs, a key sign, it is also known as congestive heart failure.
• Heart failure is not cardiac arrest, in which the heart stops beating, nor a heart attack, in which a sudden shortfall of blood supply injures part of the heart muscle.

Must-know facts

1. Plasma ≈ 55% of blood (90–92% water, 6–8% proteins); formed elements ≈ 45%.
2. Fibrinogen: clotting. Globulins: defence. Albumins: osmotic balance. Serum = plasma without clotting factors.
3. RBC: 5–5.5 million per mm³; haemoglobin 12–16 g per 100 mL; life 120 days; destroyed in spleen.
4. WBC: 6000–8000 per mm³. Neutrophils 60–65% (most), lymphocytes 20–25%, monocytes 6–8%, eosinophils 2–3%, basophils 0.5–1% (least).
5. Platelets: 1,50,000–3,50,000 per mm³, from megakaryocytes.
6. Group O: universal donor. Group AB: universal recipient. Rh positive: nearly 80% of humans.
7. Erythroblastosis foetalis: Rh negative mother, Rh positive foetus, second pregnancy; prevented by anti-Rh antibodies after first delivery.
8. Clotting chain: thrombokinase converts prothrombin → thrombin; thrombin converts fibrinogen → fibrin. Ca²⁺ is essential.
9. Open circulation: arthropods, molluscs. Closed: annelids, chordates.
10. Hearts: fish 2 chambers; amphibians and reptiles 3 (crocodiles 4); birds and mammals 4.
11. SAN: pacemaker, 70–75 action potentials per minute; heart averages 72 beats per minute.
12. Atrial systole raises ventricular filling by about 30%.
13. Cardiac cycle 0.8 s; stroke volume about 70 mL; cardiac output about 5000 mL (5 L) per minute.
14. Lub: closure of tricuspid and bicuspid valves. Dub: closure of semilunar valves.
15. ECG: P = atrial depolarisation; QRS = ventricular depolarisation; T = ventricular repolarisation (end of T = end of systole).
16. Hepatic portal vein: intestine to liver. Coronary vessels: heart muscle only.
17. Heart is myogenic; sympathetic raises, parasympathetic lowers heart rate; medulla oblongata moderates.
18. Normal BP 120/80 mm Hg; repeated 140/90 or higher = hypertension.

Common traps

Key terms

Plasma

The straw-coloured fluid matrix of blood, about 55 per cent of its volume.

Serum

Plasma without its clotting factors.

Formed elements

RBCs, WBCs and platelets, about 45 per cent of blood.

Haemoglobin

The red, iron-containing protein of RBCs that carries respiratory gases.

Megakaryocyte

A special bone marrow cell whose fragments are platelets.

Antigen

A chemical that can induce an immune response, such as A, B or Rh on RBCs.

Antibody

A protein made in response to an antigen, such as anti-A or anti-B in plasma.

Universal donor

A person of group O, whose blood can be given to any ABO group.

Universal recipient

A person of group AB, who can receive blood of any ABO group.

Erythroblastosis foetalis

Destruction of an Rh positive foetus's RBCs by anti-Rh antibodies from an Rh negative mother.

Thrombokinase

The enzyme complex, formed by a cascade, that converts prothrombin to thrombin.

Fibrin

Threads formed from fibrinogen by thrombin; the mesh of a clot.

Tissue fluid

Fluid that leaks from capillaries into spaces between cells; the medium of exchange.

Lymph

Tissue fluid inside the lymphatic system, colourless and carrying lymphocytes.

Lacteal

A lymph vessel in an intestinal villus that absorbs fats.

Pacemaker

The SAN, which generates the most action potentials and sets the heart's rhythm.

Systole

Contraction of a heart chamber.

Diastole

Relaxation of a heart chamber.

Stroke volume

Blood pumped by each ventricle in one cardiac cycle, about 70 mL.

Cardiac output

Blood pumped by each ventricle per minute: stroke volume × heart rate, about 5 litres.

Myogenic heart

A heart whose beat is started from within, by nodal muscle tissue.

Hepatic portal system

Vessels carrying blood from the intestine to the liver before it joins systemic circulation.