Circulatory System | ICSE Class 10 Biology Notes

This note covers the ICSE Class 10 Biology topic Circulatory System: the composition of blood, red blood cells, white blood cells and platelets, terms for abnormal blood cell counts, blood clotting, tissue fluid, lymph and the lymphatic organs, the structure and working of the heart, systole and diastole, double circulation, blood vessels, pulse and blood pressure, the main vessels of the heart, lungs, liver and kidneys, the hepatic portal system, and the ABO and Rh blood groups.

What is blood, and what is it made of?

All living cells need nutrients, oxygen and other essential substances, and their wastes must be removed continuously. In humans, the main fluid that does this transport is blood; another body fluid, lymph, also carries certain substances.

Blood is a special connective tissue made of a fluid matrix, the plasma, in which the formed elements are suspended. It counts as a connective tissue because it connects the parts of the body, carrying nutrients, gases and hormones between them.

Plasma

Plasma is a straw-coloured, viscous fluid that makes up nearly 55 per cent of the blood. About 90 to 92 per cent of plasma is water, and proteins make up 6 to 8 per cent of it.

• Fibrinogen is needed for the clotting (coagulation) of blood.
• Globulins are mainly involved in the defence mechanisms of the body.
• Albumins help in osmotic balance.

Plasma also contains small amounts of minerals such as Na⁺, Ca²⁺, Mg²⁺, HCO₃⁻ and Cl⁻, and glucose, amino acids and lipids, which are always in transit in the body. It carries digested food, carbon dioxide and nitrogenous wastes in dissolved form.

The clotting factors are also present in plasma, in an inactive form. Plasma without the clotting factors is called serum.

Formed elements

The erythrocytes (red blood cells), leucocytes (white blood cells) and platelets together make up nearly 45 per cent of the blood.

What are red blood cells, and why do mammalian RBCs lack certain organelles?

Erythrocytes, or red blood cells (RBCs), are the most abundant cells in blood. A healthy adult man has, on average, 5 million to 5.5 million RBCs per mm³ of blood. In adults, RBCs are formed in the red bone marrow.

In most mammals, RBCs have no nucleus and are biconcave. They contain haemoglobin, a red, iron-containing complex protein that gives them their colour and name. A healthy person has 12 to 16 g of haemoglobin in every 100 mL of blood.

RBCs live for an average of 120 days and are then destroyed in the spleen, the graveyard of RBCs.

How RBCs carry gases

• About 97 per cent of the oxygen in blood is carried by RBCs; the remaining 3 per cent is dissolved in plasma.
• Oxygen binds reversibly with haemoglobin to form oxyhaemoglobin in the lungs and is released in the tissues. Each haemoglobin molecule can carry at most four molecules of oxygen.
• About 20 to 25 per cent of carbon dioxide is carried by haemoglobin as carbamino-haemoglobin. RBCs also contain the enzyme carbonic anhydrase, which helps most carbon dioxide travel as bicarbonate.

Why the absence of certain organelles makes them more efficient

• No nucleus: this provides more space for haemoglobin, so each cell can carry more oxygen. It also allows the cell to take its biconcave shape.
• Biconcave shape: a thin, disc-like cell has a large surface area for its volume, so oxygen diffuses in and out quickly, and the flexible cell can squeeze through narrow capillaries.
• No mitochondria: the RBC respires anaerobically, so it does not use up the oxygen it is carrying for the tissues.
• No endoplasmic reticulum: this too leaves more room for haemoglobin.

Note: There is a cost. Without a nucleus, a mature RBC cannot repair itself or divide, so it lives only about 120 days and must be replaced regularly.

What are white blood cells and platelets?

Leucocytes, or white blood cells (WBCs), are colourless because they lack haemoglobin. They are nucleated, generally short-lived and fewer in number, averaging 6,000 to 8,000 per mm³ of blood. Like all blood cells, they are produced in the bone marrow.

There are two main categories: granulocytes (neutrophils, eosinophils and basophils) and agranulocytes (lymphocytes and monocytes).

Platelets

Platelets, or thrombocytes, are cell fragments produced from megakaryocytes, special cells in the bone marrow. Blood normally contains 1,50,000 to 3,50,000 platelets per mm³.

Platelets release substances, most of which are involved in clotting, and so plug leaks at points of injury. A fall in their number can lead to clotting disorders and excessive loss of blood.

A row of labelled drawings: the RBC as a small, plain red disc with no nucleus; the platelets as a cluster of tiny fragments; the eosinophil, with reddish granules; and the basophil, densely covered in dark blue-purple granules.

Then come the neutrophil, with a lobed nucleus; the monocyte, the largest cell, with a large indented nucleus; and the T and B lymphocytes, each with a large nucleus filling most of the cell.

See Fig. 15.1 in your NCERT textbook

Terms for abnormal blood cell counts

How does blood clot?

Blood shows coagulation, or clotting, in response to an injury. This prevents excessive loss of blood, and also the loss of pressure that would reduce the efficiency of the heart's pumping.

The dark reddish-brown scum that forms at a cut is a clot, or coagulum. It is made mainly of a network of fibrin threads in which dead and damaged formed elements of blood are trapped.

1. An injury stimulates the platelets to release certain factors. Factors released by the damaged tissues at the site can also start clotting.
2. These factors set off a series of linked enzyme reactions (a cascade) among clotting factors present in the plasma in an inactive state, forming the enzyme complex thrombokinase (thromboplastin).
3. Thrombokinase, with calcium ions, converts the inactive plasma protein prothrombin into the active enzyme thrombin.
4. Thrombin converts soluble fibrinogen into insoluble threads of fibrin.
5. The fibrin threads form a network over the wound, trapping blood cells to form the clot that seals it.
6. The clot then shrinks, and a pale yellow fluid, the serum, oozes out of it.

In short: prothrombin → thrombin (needs thrombokinase and Ca²⁺); fibrinogen → fibrin (needs thrombin); fibrin + blood cells → clot.

Why blood does not clot inside the vessels

The clotting factors circulate in an inactive form and are activated by an injury. Heparin, secreted by basophils, acts as an anticoagulant.

Prothrombin and fibrinogen are made in the liver, and vitamin K is needed to make prothrombin, so a shortage of vitamin K slows clotting.

What are tissue fluid and lymph, and what do the spleen and tonsils do?

As blood passes through the capillaries, some water and many small, water-soluble substances move out into the spaces between the cells. The larger proteins and most of the formed elements stay in the vessels.

This fluid is called interstitial fluid or tissue fluid. It has the same mineral distribution as plasma; all exchange of nutrients and gases between blood and cells takes place through it.

Lymph and the lymphatic system

1. Excess tissue fluid drains into thin lymphatic capillaries; the fluid inside them is called lymph.
2. The lymphatic capillaries join to form larger lymph vessels, whose valves keep lymph moving one way, towards the heart.
3. On the way, lymph passes through lymph nodes, small solid structures that trap micro-organisms and other antigens and activate the lymphocytes there.
4. The large lymph vessels, the main one being the thoracic duct, finally open into large veins near the neck, returning the fluid to the blood.

Lymph is colourless and similar to plasma but contains less protein. It contains specialised lymphocytes responsible for immune responses, and carries nutrients and hormones. Fats are absorbed into lymph through the lacteals in the intestinal villi.

Lymphatic organs: spleen and tonsils

Lymphoid organs are where lymphocytes originate, mature or multiply. The bone marrow and thymus are the primary ones. The spleen, lymph nodes, tonsils, Peyer's patches of the small intestine and the appendix are secondary lymphoid organs, where lymphocytes meet antigens and multiply.

The spleen is a large, bean-shaped organ in the upper left part of the abdomen, behind the stomach. It mainly contains lymphocytes and phagocytes. It filters the blood by trapping micro-organisms carried in it, destroys old RBCs, and holds a large reservoir of erythrocytes.

The tonsils are masses of lymphoid tissue at the back of the throat, one on each side. They are sites where lymphocytes meet microbes entering through the mouth and nose. Infection of the tonsils causes tonsillitis, and their surgical removal is a tonsillectomy.

An outline of the human body from the head down to the thighs, covered with a branching network labelled lymphatic vessels. Dots marking lymph nodes lie along the vessels in the neck, around the shoulders and armpits, and near the groin and upper thighs, and a small bar in the centre of the chest is labelled thymus.

See Fig. 7.5 in your NCERT textbook

What is the structure of the human heart?

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

Chambers and septa

The two relatively small upper chambers, the atria, receive blood. The two larger lower chambers, the ventricles, pump it out.

• A thin, muscular inter-atrial septum separates the right and left atria.
• A thick-walled inter-ventricular septum separates the left and right ventricles.
• A thick, fibrous atrio-ventricular septum separates the atrium and ventricle of each side, with an opening on each side connecting the atrium to the ventricle below it.

The whole heart is made of cardiac muscle. The ventricle walls are much thicker than the atrial walls, and the left ventricle has the thickest wall, as it pumps blood round the whole body.

Valves

The tricuspid valve, of three muscular flaps or cusps, guards the opening from the right atrium to the right ventricle. The bicuspid or mitral valve guards the opening from the left atrium to the left ventricle. Semilunar valves guard the openings of the right ventricle into the pulmonary artery and of the left ventricle into the aorta.

The valves let blood flow one way only and prevent backward flow. Fine cords, the chordae tendineae, hold the flaps of the tricuspid and bicuspid valves to the ventricle walls.

The heart muscle itself is supplied by a special coronary system of blood vessels.

A cut-open heart with the apex at the bottom. At the top are the blue vena cava on the left, the red aorta arching in the centre, the pulmonary artery beside it and the pulmonary veins on the right.

The four chambers are labelled, with the interventricular septum between the ventricles and chordae tendinae in the right ventricle. A yellow sino-atrial node sits in the wall of the right atrium, with the atrio-ventricular node lower down, and yellow bands labelled bundle of His run down the septum and along the ventricle walls.

See Fig. 15.2 in your NCERT textbook

How does the heart work: the pacemaker, systole and diastole

Nodal tissue and the pacemaker

A specialised cardiac musculature, the nodal tissue, can generate impulses (action potentials) without any outside stimulus; it is autoexcitable.

• 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, close to the atrio-ventricular septum.
• The bundle of His (AV bundle) runs from the AVN, divides into right and left bundles, and gives rise to fine Purkinje fibres throughout the ventricle walls.

The SAN generates the most impulses, 70 to 75 per minute, and starts and maintains the rhythm of the heart, so it is called the pacemaker. The heart normally beats 70 to 75 times a minute, on average 72.

Because the beat starts in the heart muscle itself, the heart is myogenic. A centre in the medulla oblongata adjusts it through the autonomic nervous system: sympathetic nerves speed it up and parasympathetic nerves slow it down.

Systole, diastole and the cardiac cycle

Systole is the contraction of a heart chamber, which forces blood out. Diastole is its relaxation, during which it fills with blood. The events of one heartbeat, repeated in a cycle, form the cardiac cycle.

1. Joint diastole: all four chambers are relaxed. The tricuspid and bicuspid valves are open, so blood flows through the atria into the ventricles; the semilunar valves are closed.
2. Atrial systole: the SAN fires and both atria contract, increasing the flow of blood into the ventricles by about 30 per cent.
3. Ventricular systole: the impulse passes through the AVN and bundle of His, and the ventricles contract while the atria relax. The rising pressure closes the tricuspid and bicuspid valves, then forces the semilunar valves open, sending blood into the pulmonary artery and aorta.
4. Ventricular diastole: the ventricles relax and their pressure falls, so the semilunar valves close and prevent backflow.
5. As the pressure falls further, the tricuspid and bicuspid valves are pushed open by blood in the atria. The heart returns to joint diastole, and the SAN starts the next cycle.

Heart sounds and cardiac output

The two heart sounds, "lub" and "dub", are heard through a stethoscope. At 72 beats a minute, one cardiac cycle lasts about 0.8 seconds. Each ventricle pumps out about 70 mL of blood per cycle, the stroke volume. Stroke volume multiplied by heart rate gives the cardiac output, which averages about 5 litres a minute (70 mL × 72 = 5,040 mL).

What is double circulation?

In humans, blood passes through the heart twice in each complete round of the body, through two separate circuits. This is double circulation.

• Pulmonary circulation: from the right ventricle to the lungs and back to the left atrium.
• Systemic circulation: from the left ventricle to the body tissues and back to the right atrium.

The path of blood

1. Deoxygenated blood from the body enters the right atrium through the venae cavae and passes through the tricuspid valve into the right ventricle.
2. The right ventricle pumps it through a semilunar valve into the pulmonary artery, to the lungs, where it takes up oxygen and gives up carbon dioxide.
3. Oxygenated blood returns in the pulmonary veins to the left atrium, and passes through the bicuspid valve into the left ventricle.
4. The left ventricle pumps it through a semilunar valve into the aorta, and arteries, arterioles and capillaries carry it to the tissues.
5. In the tissue capillaries the blood gives up oxygen and nutrients and collects carbon dioxide and wastes; venules, veins and the venae cavae return it to the right atrium.

Why double circulation matters

Separating the right and left sides keeps oxygenated and deoxygenated blood from mixing, allowing a highly efficient supply of oxygen. This suits birds and mammals, which have high energy needs and constantly use energy to keep their body temperature steady.

Fishes have a two-chambered heart and single circulation: blood goes through the heart only once per round. Amphibians and reptiles (except crocodiles) have three chambers with a single ventricle, where the two bloods mix (incomplete double circulation). Crocodiles, birds and mammals have four chambers.

The heart sits in the middle, with lung capillaries in a loop at the top and capillaries in body organs apart from the lungs in a loop at the bottom.

The pulmonary artery to the lungs leaves from the right side of the heart and the pulmonary vein from the lungs returns to the left side. The aorta carries blood to the body and the vena cava brings it back. The vessels of the heart's right side (on the left of the drawing) are blue, and those of its left side are red.

See Fig. 5.11 in your NCERT textbook

How do arteries, veins and capillaries differ?

Arteries and veins have three layers in their walls: the tunica intima, an inner lining of squamous endothelium; the tunica media, of smooth muscle and elastic fibres; and the tunica externa, of fibrous connective tissue with collagen fibres. The tunica media is comparatively thin in veins.

Arteries carry blood away from the heart. Blood leaves the heart under high pressure, so arteries have thick, elastic walls. In an organ, an artery divides into arterioles and finally into capillaries, whose walls are one cell thick; materials are exchanged with the cells across this thin wall.

Capillaries join to form venules and veins, which carry blood back to the heart. Veins do not need thick walls, as their blood is no longer under high pressure. Instead they have valves that keep blood flowing one way.

The heart (RA, RV, LA, LV) is in the centre. A purple pulmonary artery runs up to the lungs and a red pulmonary vein returns; a red dorsal aorta runs down to the body parts, and a purple vena cava (great veins) returns.

Insets show the vessels: the vein's cross-section has a thin layer of smooth muscle and a wide lumen, the artery's a thicker ring of smooth muscle and a small lumen, and the capillary is a thin tube of a single layer of cells.

See Fig. 15.4 in your NCERT textbook

Note: Arteries and veins are named by the direction of flow, not by the kind of blood. The pulmonary artery carries deoxygenated blood, and the pulmonary veins carry oxygenated blood.

What are pulse and blood pressure?

Each time the left ventricle contracts, it forces blood into the aorta, and a wave of expansion passes along the elastic walls of the arteries. This rhythmic throb, felt where an artery runs close to the surface, is the pulse.

The pulse is usually felt at the wrist, on the thumb side, over the radial artery, or at the side of the neck. Each heartbeat produces one pulse wave, so pulse rate equals heart rate, about 72 per minute at rest.

Blood pressure is the force that blood exerts against the wall of a vessel. It is much greater in arteries than in veins, and is measured with a sphygmomanometer.

• Systolic pressure is the pressure in an artery during ventricular systole; normally about 120 mm Hg.
• Diastolic pressure is the pressure in an artery during ventricular diastole; normally about 80 mm Hg.

Normal blood pressure is written as 120/80 mm Hg (millimetres of mercury). Hypertension is blood pressure higher than normal; repeated readings of 140/90 or higher show hypertension.

It is caused by the constriction of arterioles, which increases resistance to blood flow. It can lead to the rupture of an artery and internal bleeding, leads to heart diseases, and affects vital organs such as the brain and kidneys.

Which main blood vessels enter and leave the heart, lungs, liver and kidneys?

Most arteries branch from the aorta, and the veins of the lower body drain into the inferior vena cava.

Draw the dorsal aorta down the middle of the body with the inferior vena cava beside it. Show the hepatic artery from the aorta to the liver, the hepatic portal vein from the intestine to the liver, and the hepatic vein from the liver to the inferior vena cava.

Lower down, draw a renal artery from the aorta into each kidney and a renal vein from each kidney into the inferior vena cava. Label every vessel and add arrows for the direction of flow.

What is the hepatic portal system, and why is it significant?

A portal vein begins in capillaries in one organ and ends in capillaries in another. The hepatic portal system is a unique vascular connection 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.

1. Digested food, such as glucose and amino acids, is absorbed into the capillaries in the wall of the intestine.
2. These capillaries join to form the hepatic portal vein.
3. The hepatic portal vein enters the liver and breaks up again into capillaries among the liver cells, which take up, store or change the absorbed substances.
4. The blood leaves the liver in the hepatic vein, which joins the inferior vena cava and returns to the right atrium.

Significance

• Absorbed food passes through the liver before reaching the rest of the body, so the liver can control how much enters the general circulation.
• Excess glucose is converted to glycogen and stored in the liver, helping to keep the blood sugar level steady.
• Excess amino acids are broken down in the liver, and their nitrogen is turned into urea for excretion.
• Harmful substances absorbed from the gut can be detoxified by the liver before they reach other organs.

What are the ABO and Rh blood groups?

Two groupings, ABO and Rh, are widely used all over the world.

ABO grouping

ABO grouping is based on whether two surface antigens, A and B, are present on the RBCs. Antigens are chemicals that can induce an immune response. The plasma contains natural antibodies, proteins produced in response to antigens, depending on the blood group.

Before a blood transfusion, the donor's blood must be matched with the recipient's, or severe clumping, in which the RBCs are destroyed, can occur. Group O blood can be given to any group, so group O people are universal donors. Group AB people can accept blood from all groups and are universal recipients.

Rh grouping and pregnancy

The Rh antigen, similar to one found in Rhesus monkeys, is present on the RBCs of nearly 80 per cent of humans, who are Rh positive (Rh+ve); those without it are Rh negative (Rh−ve). An Rh−ve person exposed to Rh+ve blood forms antibodies against the Rh antigen, so the Rh group must also be matched before a transfusion.

1. An Rh−ve mother carries an Rh+ve foetus. In the first pregnancy, the foetus's Rh antigens do not reach her blood, because the placenta keeps the two bloods well apart.
2. During the delivery of the first child, small amounts of the baby's Rh+ve blood may reach the mother's blood.
3. The mother then starts making antibodies against the Rh antigen.
4. In a later pregnancy with an Rh+ve foetus, these antibodies can leak into the foetal blood and destroy its RBCs, which can be fatal or cause severe anaemia and jaundice. This is erythroblastosis foetalis.

It can be avoided by giving anti-Rh antibodies to the mother immediately after the delivery of the first child.

Glossary

• Plasma — The straw-coloured, viscous fluid part of blood, nearly 55 per cent of it, mostly water with proteins, minerals and nutrients.
• Serum — Plasma from which the clotting factors have been removed, so it cannot clot.
• Haemoglobin — The red, iron-containing protein in red blood cells that binds oxygen reversibly and carries it to the tissues.
• Fibrin — Insoluble protein threads formed from fibrinogen by thrombin; their network traps blood cells to form a clot.
• Tissue fluid — Fluid that leaks out of blood capillaries into the spaces between cells; all exchange between blood and cells passes through it.
• Lymph — The colourless fluid inside lymphatic vessels, similar to plasma but with less protein and many lymphocytes.
• Spleen — A large, bean-shaped lymphatic organ that filters blood, destroys old red blood cells and stores a reservoir of them.
• Pacemaker — The sino-atrial node in the right atrium, which generates 70 to 75 impulses a minute and sets the heart's rhythm.
• Systole — The contraction of a chamber of the heart, which forces blood out of it.
• Diastole — The relaxation of a chamber of the heart, during which it fills with blood.
• Double circulation — Circulation in which blood passes through the heart twice in each complete round of the body, through pulmonary and systemic circuits.
• Hepatic portal vein — The vein that carries blood from the intestine to the liver before it enters the general circulation.

Common errors and misconceptions

• Misconception: Arteries always carry oxygenated blood. Correct: Vessels are named by direction. The pulmonary artery carries deoxygenated blood, and the pulmonary veins carry oxygenated blood.
• Misconception: Lymph is the same as plasma. Correct: Lymph is similar to plasma but contains less protein, and it flows in its own vessels before draining into the veins.
• Misconception: The "lub" sound is made by the heart muscle contracting. Correct: "Lub" comes with the closure of the tricuspid and bicuspid valves, and "dub" with the closure of the semilunar valves.
• Misconception: Group AB is the universal donor. Correct: Group O is the universal donor, as its RBCs carry no A or B antigen; group AB is the universal recipient.
• Misconception: The heartbeat is started by the brain. Correct: The heart is myogenic; the SA node starts each beat, and a centre in the medulla oblongata moderates the rate through the autonomic nervous system.
• Misconception: Rh incompatibility harms the first Rh+ve baby. Correct: In the first pregnancy the placenta keeps the bloods apart; the danger is to later Rh+ve babies.
• Misconception: The hepatic portal vein carries blood from the liver to the heart. Correct: It carries blood from the intestine to the liver; the hepatic vein carries blood from the liver towards the heart.

Exam-style questions with model answers

Q1. Name the blood vessel that (a) brings oxygenated blood to the left atrium, (b) carries blood from the intestine to the liver. [2 marks]

1. (a) The pulmonary veins bring oxygenated blood from the lungs to the left atrium.
2. (b) The hepatic portal vein carries blood from the intestine to the liver.

Q2. Why is the sino-atrial node called the pacemaker of the heart? [2 marks]

1. The sino-atrial node is a patch of autoexcitable nodal tissue in the right upper corner of the right atrium.
2. It generates the most impulses, 70 to 75 a minute, so it starts and maintains the rhythmic contraction of the heart and sets its pace.

Q3. Mammalian red blood cells lack certain organelles. Explain how this makes them more efficient. [3 marks]

1. No nucleus: more space is available for haemoglobin, so each cell carries more oxygen, and the cell can take a biconcave shape with a large surface area for quick diffusion of gases.
2. No mitochondria: the RBC respires anaerobically and does not use up the oxygen it is carrying to the tissues.
3. No endoplasmic reticulum: this also leaves more room for haemoglobin.

Q4. Give three differences between an artery and a vein. [3 marks]

1. An artery carries blood away from the heart, while a vein carries blood towards the heart.
2. An artery has a thick, muscular and elastic wall with a narrow lumen; a vein has a thinner wall, with a thin tunica media, and a wide lumen.
3. An artery has no valves along its length, while a vein has valves that keep blood flowing one way, because the blood in it is under low pressure.

Q5. Describe the process of blood clotting. [4 marks]

1. An injury stimulates the platelets, and the damaged tissues, to release factors that set off a cascade of enzyme reactions among inactive clotting factors in the plasma, forming thrombokinase.
2. Thrombokinase, with calcium ions, converts inactive prothrombin into active thrombin.
3. Thrombin converts soluble fibrinogen into insoluble fibrin threads.
4. The fibrin threads form a network over the wound in which blood cells are trapped, forming a clot that stops the loss of blood.

Q6. Explain how Rh incompatibility can harm a baby, and how it can be prevented. [4 marks]

1. When an Rh−ve mother carries an Rh+ve foetus, the first pregnancy is usually safe because the placenta keeps the two bloods apart.
2. During the first delivery, a little of the baby's Rh+ve blood may reach the mother, who then makes antibodies against the Rh antigen.
3. In a later pregnancy with an Rh+ve foetus, these antibodies can leak into the foetal blood and destroy its RBCs, causing severe anaemia and jaundice or death. This is erythroblastosis foetalis.
4. It is prevented by giving anti-Rh antibodies to the mother immediately after the first delivery.

Q7. What is meant by double circulation? Trace the path of blood from the right atrium back to the right atrium, and state one advantage of double circulation. [5 marks]

1. Double circulation means that blood passes through the heart twice in each complete round of the body, through a pulmonary and a systemic circuit.
2. Pulmonary circuit: right atrium → tricuspid valve → right ventricle → semilunar valve → pulmonary artery → lungs, where blood is oxygenated → pulmonary veins → left atrium.
3. Systemic circuit: left atrium → bicuspid valve → left ventricle → semilunar valve → aorta → arteries, arterioles and capillaries in the tissues → venules, veins and venae cavae → right atrium.
4. Advantage: the right and left sides are separate, so oxygenated and deoxygenated blood do not mix, giving a highly efficient supply of oxygen to the body.

Q8. Describe the events of one cardiac cycle, mentioning the valves and the heart sounds. [5 marks]

1. In joint diastole all four chambers are relaxed; the tricuspid and bicuspid valves are open and blood flows through the atria into the ventricles, while the semilunar valves are closed.
2. The SA node fires and both atria contract (atrial systole), increasing the flow of blood into the ventricles by about 30 per cent.
3. The impulse passes through the AV node and bundle of His, and the ventricles contract (ventricular systole) while the atria relax. The tricuspid and bicuspid valves close, giving the first sound, "lub", and the semilunar valves open so blood enters the pulmonary artery and aorta.
4. The ventricles relax (ventricular diastole); the semilunar valves close, giving the second sound, "dub".
5. The tricuspid and bicuspid valves open again and the heart returns to joint diastole.

Key takeaways

• Blood is a fluid connective tissue: about 55 per cent plasma and 45 per cent formed elements, namely red blood cells, white blood cells and platelets.
• Mammalian RBCs are biconcave and lack a nucleus and mitochondria, leaving more room for haemoglobin and saving the oxygen they carry.
• Platelets, thrombokinase, calcium ions, thrombin and fibrinogen act in sequence to form a fibrin clot that stops bleeding at a wound.
• Tissue fluid leaks from capillaries and returns to the blood as lymph through the lymphatic system, whose organs include the spleen and tonsils.
• The heart has two atria and two ventricles, with tricuspid, bicuspid and semilunar valves that keep blood flowing one way.
• The SA node is the pacemaker; each cardiac cycle of systole and diastole lasts about 0.8 seconds at 72 beats a minute.
• In the double circulation of birds and mammals, blood passes through the heart twice per round, and oxygenated and deoxygenated blood do not mix.
• Normal blood pressure is about 120/80 mm Hg; the hepatic portal vein takes absorbed food from the intestine to the liver first.

Test yourself

Which plasma protein is needed for blood clotting?

Fibrinogen, which thrombin converts into insoluble fibrin threads during clotting.

Where are RBCs formed in adults, and where are they destroyed?

They are formed in the red bone marrow and destroyed in the spleen after an average life of 120 days.

What is the stroke volume, and how is cardiac output calculated?

The stroke volume is the blood pumped by each ventricle in one cycle, about 70 mL. Cardiac output is stroke volume multiplied by heart rate, about 5 litres a minute.

Which blood vessel carries blood away from the kidney?

The renal vein, which carries blood from the kidney into the inferior vena cava.

What does leucocytosis mean?

A rise in the number of white blood cells above normal, as often happens during an infection.

Why can a person of group A not receive group B blood?

Group A plasma contains anti-B antibodies, which would react with the B antigens on the donor's RBCs and cause clumping.