Body fluids and circulation | ISC Class 11 Biology Notes

This note covers blood and its components, blood groups, clotting, tissue fluid and lymph, blood vessels, heart structure, the heartbeat and cardiac cycle, cardiac output, electrical recording of heart activity, double circulation, blood pressure and circulatory disorders.

What are the components of blood and the functions of plasma?

Blood is a special connective tissue, a tissue whose components include cells within a surrounding material called a matrix. Its fluid matrix is plasma. The suspended cells and cell fragments are called formed elements. Blood carries essential substances towards cells and waste substances away from them.

What does plasma contain?

Plasma is a straw-coloured, viscous fluid, meaning that it resists flowing freely. It constitutes nearly 55 per cent of blood. Water makes up 90 to 92 per cent of plasma, while proteins contribute 6 to 8 per cent.

Plasma transports glucose, amino acids and lipids, which are sugars, protein-building units and fats or related substances respectively. It also contains small quantities of mineral ions, or electrically charged particles, including sodium, calcium, magnesium, bicarbonate and chloride ions.

Plasma proteins have different functions. Fibrinogen participates in coagulation, the process of blood clotting. Globulins are primarily involved in defence. Albumins help maintain osmotic balance, the balance of water movement associated with dissolved substances.

How does serum differ from plasma?

Clotting factors occur in plasma in inactive forms. Serum is plasma without the clotting factors. Thus, serum and plasma are related fluids, but the terms cannot be substituted for each other when describing clot formation.

Note: The nearly 55 per cent figure refers to plasma as a proportion of blood. The 90 to 92 per cent figure refers to water as a proportion of plasma. These percentages describe different wholes.

How do the formed elements of blood differ?

Formed elements comprise erythrocytes, leucocytes and platelets, together constituting nearly 45 per cent of blood. Erythrocytes are red blood cells, abbreviated RBCs; leucocytes are white blood cells, abbreviated WBCs. Platelets, also called thrombocytes, are cell fragments involved in clotting.

What distinguishes red blood cells?

RBCs are the most abundant blood cells. A healthy adult man has, on average, 5 million to 5.5 million per cubic millimetre of blood. A cubic millimetre, written mm³, is a unit of volume. Adult RBCs are formed in red bone marrow, the blood-forming tissue inside bones.

RBCs are biconcave, meaning depressed on both faces, and lack a nucleus in most mammals. Their haemoglobin is a red, iron-containing protein that transports respiratory gases. A healthy individual has 12 to 16 grams of haemoglobin per 100 millilitres of blood.

The abbreviations g and mL mean gram and millilitre respectively. RBCs have an average lifespan of 120 days and are then destroyed in the spleen, an organ involved in blood filtration and blood-cell removal.

What do white blood cells and platelets do?

WBCs contain nuclei but lack haemoglobin. Their average count is 6,000 to 8,000 per mm³ of blood, and they are generally short-lived. Granulocytes and agranulocytes are the two categories, distinguished by the presence or absence of conspicuous granules in the cell contents.

An immune response is the body's defensive reaction to a recognised foreign substance.

Phagocytosis is the engulfing and destruction of foreign organisms by cells. Neutrophils and monocytes carry out this process. Inflammatory reactions are defensive responses to tissue injury or harmful agents; allergic reactions are excessive immune responses to certain substances.

Platelets arise from megakaryocytes, special bone-marrow cells. Blood normally contains 150,000 to 350,000 platelets per mm³. Most substances released by platelets participate in coagulation. A reduced platelet count can cause clotting disorders and excessive blood loss.

The coloured diagram separately labels an RBC, platelet fragments, eosinophil, basophil, neutrophil, monocyte, T lymphocyte and B lymphocyte. The platelet fragments are drawn as a small cluster beside the larger cells.

See Fig. 15.1 in your NCERT textbook

Why are ABO and Rh blood groups important?

Blood grouping classifies blood according to particular features of RBC surfaces. An antigen is a substance that can induce an immune response. An antibody is a protein produced in response to an antigen. ABO grouping depends on the presence or absence of antigens A and B.

How does ABO compatibility work?

A donor supplies blood, and a recipient receives it during a transfusion, the transfer of blood into the circulation. Incompatible blood can cause clumping and destruction of RBCs. The recipient's plasma antibodies must therefore be considered alongside the donor's RBC antigens.

Here, “nil” means absent; “anti-A, B” means antibodies against A and B. Within this ABO scheme, O is called the universal donor group and AB the universal recipient group. These descriptions do not remove the need to match blood carefully, including its Rh group.

Why does Rh incompatibility matter in pregnancy?

Rh antigen, named for its similarity to an antigen in Rhesus monkeys, occurs on RBCs in nearly 80 per cent of humans. Rh positive means the antigen is present; Rh negative means it is absent. An Rh-negative person exposed to Rh-positive blood will form antibodies against the Rh antigen, called anti-Rh antibodies.

Consider an Rh-negative mother carrying an Rh-positive foetus, the developing unborn child. The placenta, the organ connecting maternal and foetal exchange systems, separates their blood. During delivery, there is a possibility that small amounts of foetal blood expose the mother to Rh antigen.

In such cases, maternal antibodies can form. In a subsequent Rh-positive pregnancy, these antibodies can enter the foetal blood and destroy RBCs. The condition, erythroblastosis foetalis, could be fatal or could cause severe anaemia and jaundice, meaning deficient oxygen-carrying capacity and yellowing of tissues respectively.

Administration of anti-Rh antibodies to the mother immediately after delivery of the first child can prevent this condition. The sequence depends on exposure and antibody formation; it should not be described as an inevitable outcome of every Rh-incompatible pregnancy.

How does blood clot after an injury?

Coagulation helps prevent excessive blood loss after injury. A clot forms mainly from a network of fibrin threads that traps dead and damaged formed elements. Fibrin is the thread-forming material produced from soluble plasma fibrinogen during clotting.

What is the sequence of clot formation?

1. An injury stimulates platelets to release factors that activate coagulation. Factors released by tissues at the injured site can also initiate the process.
2. A series of linked enzyme reactions, called a cascade, produces the enzyme complex thrombokinase. Enzymes are substances that speed up chemical reactions in the body.
3. Thrombokinase enables conversion of prothrombin, an inactive plasma substance, into the active enzyme thrombin. Calcium ions have an important role in clotting.
4. Thrombin converts inactive fibrinogen into fibrin. The fibrin network traps formed elements and produces the clot at the injured site.

The order distinguishes the substances clearly: prothrombin is the precursor of thrombin, whereas fibrinogen is the precursor of fibrin. A precursor is a substance from which another substance is formed. Confusing the two pairs reverses the enzyme and its target.

Platelets help activate this process; they are not themselves the fibrin threads. Plasma supplies inactive clotting factors, and injury triggers their linked activation. This explains why reduced platelet numbers can interfere with effective clot formation even though platelets make up only one category of formed elements.

How are tissue fluid and lymph formed and used?

Capillaries are the smallest blood vessels, with walls one cell thick. As blood passes through tissue capillaries, some water and many small water-soluble substances enter the spaces between cells. Larger proteins and most formed elements remain in the blood vessels.

The fluid outside these vessels is interstitial fluid, also called tissue fluid. It has the same mineral distribution as plasma. Nutrients and gases pass between blood and tissue cells through this fluid, so it forms the immediate exchange medium around cells.

How does fluid return to the blood?

1. Water and small dissolved substances leave blood capillaries and enter the spaces between tissue cells, forming tissue fluid.
2. Tissue fluid enters lymphatic capillaries, the small collecting vessels of the lymphatic system. Fluid within this system is called lymph.
3. Lymphatic capillaries join to form larger lymph vessels. Lymph is colourless, contains less protein than plasma and carries specialised lymphocytes.
4. The larger lymph vessels drain into major veins, returning excess fluid from the spaces around cells to the bloodstream.

What roles do lymph and lymph nodes perform?

Lymph nodes are small solid structures at different points along lymph vessels. They trap microorganisms and other antigens entering lymph and tissue fluid. Trapped antigens activate lymphocytes in the nodes, producing immune responses.

Lymph also transports nutrients and hormones, chemical messengers carried between parts of the body. Lacteals are lymphatic vessels in intestinal villi, the small projections of the intestinal lining. They absorb fats, which lymph then carries away from the intestine.

The lymphatic system therefore combines fluid return, transport and defence. Tissue fluid and lymph are closely related, but their names identify different locations: tissue fluid surrounds cells, whereas lymph occupies the lymphatic vessels.

How do circulatory systems and blood vessels differ?

In an open circulatory system, the heart pumps blood through large vessels into sinuses, which are open spaces or body cavities. In a closed circulatory system, blood remains within a closed network of vessels. Closed circulation allows more precise regulation of flow.

Annelids, the segmented worms, and chordates, the animal group that includes vertebrates, possess closed circulation. Vertebrates are animals with a backbone. Humans have a closed blood vascular system comprising the heart, blood and branching vessels.

What is inside an artery or vein?

An artery carries blood away from the heart; a vein brings it back. These names describe direction of flow. Arteries have thick, elastic walls to withstand blood emerging under high pressure. Veins have valves that help maintain flow towards the heart.

Both arteries and veins have three basic layers. The tunica intima is the inner lining of squamous endothelium, a layer of flattened lining cells. The tunica media contains smooth muscle, which contracts involuntarily, and elastic fibres that permit stretching and recoil.

The outer tunica externa contains fibrous connective tissue and collagen fibres, strong supporting fibres. The tunica media is comparatively thin in veins. The central space through which blood flows is the lumen.

How do capillaries support exchange?

Arteries divide into smaller vessels called arterioles and then capillaries. A capillary's one-cell-thick wall permits exchange with surrounding tissues. Capillaries lead into venules, small vessels that join veins. The thin exchange wall contrasts with the layered walls of arteries and veins.

How is the human heart organised externally and internally?

The heart is a muscular pumping organ about the size of a clenched fist. It lies in the thoracic cavity, the chest space, between the lungs and is slightly tilted to the left. Its pointed lower end is called the apex.

The pericardium is the double-walled membranous bag protecting the heart. It encloses pericardial fluid. The large vessels connected to the heart provide its routes for receiving blood and pumping blood onwards.

What separates the chambers?

The four chambers are two relatively small upper atria and two larger lower ventricles. “Auricular” refers here to the atria. A septum is a separating partition: the thin muscular interatrial septum separates the atria, and the thick interventricular septum separates the ventricles.

Thick fibrous atrioventricular tissue separates the atrium and ventricle on each side, with an opening connecting them. Cardiac muscle, the specialised muscle of the heart, forms its muscular walls. Ventricular walls are much thicker than atrial walls.

Which valves control blood flow?

Pulmonary means related to the lungs. The pulmonary artery carries blood from the right ventricle towards the lungs. The aorta is the main artery leaving the left ventricle. Pulmonary veins return blood from the lungs, while the venae cavae are the major veins returning blood from the body.

The coloured section labels the atria, ventricles, vena cava, aorta, pulmonary artery and pulmonary veins. The interventricular septum divides the lower chambers. Yellow conducting tissue is shown within the heart, and the pointed lower apex is labelled.

See Fig. 15.2 in your NCERT textbook

How is the heartbeat initiated and regulated?

Nodal tissue is specialised cardiac muscle that initiates and conducts electrical signals. An action potential is an electrical excitation that travels through excitable tissue. Nodal tissue is autoexcitable, meaning it can generate action potentials without external stimulation.

How does excitation spread through the heart?

1. The sinoatrial node, abbreviated SAN, lies in the upper right corner of the right atrium. It initiates an action potential that stimulates both atria.
2. The atrioventricular node, abbreviated AVN, lies in the lower left region of the right atrium near the atrioventricular septum and conducts excitation towards the ventricles.
3. The atrioventricular bundle, also called the bundle of His, continues from the AVN through the atrioventricular partition to the top of the interventricular septum.
4. The bundle divides into right and left branches. Minute Purkinje fibres distribute excitation through the ventricular muscle on the respective sides.

The SAN generates the highest frequency of action potentials, 70 to 75 per minute. It is therefore the pacemaker, the tissue setting the rhythm of the heartbeat. The normal heartbeat is 70 to 75 times per minute, averaging 72 beats per minute.

How do nerves and hormones modify this rhythm?

The heart is myogenic: its normal rhythm originates in its own specialised muscle. A centre in the medulla oblongata, a part of the brain, can modify cardiac function through the autonomic nervous system, which controls involuntary functions.

Sympathetic nerves, one division of this system, can increase heart rate, ventricular contraction strength and the amount pumped per minute. Parasympathetic signals, from the other division, decrease heart rate, conduction speed and the amount pumped per minute.

Hormones from the adrenal medulla, the inner region of an adrenal gland, can also increase the amount of blood pumped per minute. Neural and hormonal regulation modify the heart's intrinsic activity; an external nerve signal is not required to initiate each normal beat.

What happens during each phase of the cardiac cycle?

The cardiac cycle is the repeating sequence of contraction and relaxation of the heart chambers. Systole means contraction, and diastole means relaxation. Pressure changes open and close valves, directing blood through the chambers and into the arteries.

Joint diastole

In joint diastole, all four chambers are relaxed. The tricuspid and bicuspid valves are open, while the semilunar valves are closed. Blood enters the right atrium from the venae cavae and the left atrium from the pulmonary veins, then flows into the corresponding ventricles.

Auricular systole

The SAN stimulates both atria to contract together. This auricular systole, also called atrial systole, increases blood flow into the ventricles by about 30 per cent. The atrioventricular valves remain open during this transfer.

Auricular diastole

The atria relax as the ventricles contract. During auricular diastole, blood returning through veins enters the relaxed atria. Atrial relaxation and ventricular contraction overlap, so the named phases should not be imagined as five entirely separate events.

Ventricular systole

Excitation reaching the ventricular muscle produces ventricular systole. Rising ventricular pressure and attempted backward flow close the tricuspid and bicuspid valves. As pressure increases further, the semilunar valves open. The right ventricle ejects blood into the pulmonary artery, and the left into the aorta.

Ventricular diastole

During ventricular diastole, the ventricles relax and their pressure falls. The semilunar valves close, preventing blood from returning from the arteries. When ventricular pressure falls further, blood pressure in the atria pushes the atrioventricular valves open.

Blood again flows into the ventricles, restoring joint diastole. The next SAN action potential begins another cycle. At the average rate of 72 cycles per minute, the cycle lasts about 0.8 seconds.

The first heart sound, lub, is associated with closure of the tricuspid and bicuspid valves. The second, dub, accompanies semilunar valve closure. A stethoscope, an instrument for listening to internal body sounds, allows these sounds to be heard.

What do cardiac output and an electrocardiogram measure?

Stroke volume is the volume of blood pumped out by each ventricle during one cardiac cycle. It is approximately 70 mL. Cardiac output is the volume pumped by each ventricle in one minute, averaging 5,000 mL, or 5 litres, in a healthy individual.

How is cardiac output calculated?

Cardiac output = stroke volume × heart rate. Heart rate means beats per minute, and × means multiplication. Multiplying millilitres per beat by beats per minute gives millilitres per minute. The body can alter either factor and thereby change cardiac output.

Using a stroke volume of 70 mL per beat and a heart rate of 72 beats per minute gives 70 × 72 = 5,040 mL per minute. This calculated value is consistent with the approximate average of 5 litres per minute.

What does an ECG record?

An electrocardiogram, abbreviated ECG, is a graphical record of the heart's electrical activity during a cardiac cycle. The recording instrument is an electrocardiograph. Electrical leads, the connections used to detect this activity, link the body to the instrument.

Depolarisation means electrical excitation. Repolarisation is the return from the excited to the normal electrical state. P, Q, R, S and T are identifying letters for parts of the ECG trace, not separate heart chambers.

Counting QRS complexes over a known time gives heart rate. ECG traces from different people have roughly the same shape for a given lead configuration. A deviation indicates a possible abnormality or disease, rather than establishing a particular diagnosis by itself.

A green trace shows a small P wave, downward Q deflection, tall R peak, downward S deflection and broader T wave. The letters identify electrical events; the drawing has no numbered time or voltage scale.

See Fig. 15.3 in your NCERT textbook

How does double circulation carry blood through the lungs and body?

Double circulation comprises two linked pathways: pulmonary circulation between heart and lungs, and systemic circulation between heart and body tissues. Oxygenated blood is blood enriched with oxygen; deoxygenated blood has given up oxygen to tissues and returns for renewed gas exchange.

What is the pulmonary route?

1. The right ventricle pumps deoxygenated blood into the pulmonary artery, beginning the pulmonary pathway.
2. Blood reaches the lungs, where it becomes oxygenated through gas exchange.
3. Pulmonary veins carry oxygenated blood back from the lungs to the left atrium.
4. Blood passes through the bicuspid valve into the left ventricle, ready to enter the systemic pathway.

What is the systemic route?

The left ventricle pumps oxygenated blood into the aorta. Arteries, arterioles and capillaries distribute it to the tissues. Venules, veins and the venae cavae collect deoxygenated blood and return it to the right atrium, from which it enters the right ventricle.

Systemic circulation delivers nutrients, oxygen and other essential substances while carrying away carbon dioxide and other substances for elimination. The separate right and left sides of the heart prevent mixing of oxygenated and deoxygenated blood within its chambers.

The pulmonary artery therefore carries deoxygenated blood, while pulmonary veins carry oxygenated blood. Vessel names depend on direction relative to the heart, so oxygen content alone cannot identify a vessel as an artery or vein.

Which special vascular connections are present?

The hepatic portal system links the digestive tract to the liver. Its hepatic portal vein carries blood from the intestine to the liver before that blood enters systemic circulation. The coronary circulation is the special vessel system supplying and draining the cardiac muscle.

The schematic places lungs above the heart and body parts below it, with arrows around two loops. RA, RV, LA and LV mean right atrium, right ventricle, left atrium and left ventricle. Artery and vein sections label smooth muscle and lumen; a capillary is shown below.

See Fig. 15.4 in your NCERT textbook

What are arterial blood pressure and the main circulatory disorders?

Blood pressure is the pressure exerted by flowing blood against a vessel wall. Arterial pressure is described using systolic and diastolic values. The unit mm Hg means millimetres of mercury pressure.

How are systolic and diastolic pressures distinguished?

Systolic pressure is arterial pressure during ventricular contraction; diastolic pressure is arterial pressure during ventricular relaxation. The normal reference is about 120/80 mm Hg: 120 is systolic, and 80 is diastolic. The slash separates the two pressure readings.

Hypertension means blood pressure higher than normal. Repeated readings of 140/90 mm Hg or higher indicate hypertension. High blood pressure leads to heart diseases and also affects vital organs such as the brain and kidneys.

How do the disorders differ?

Coronary artery disease, often referred to as atherosclerosis, affects the vessels supplying the heart. Cholesterol is a lipid substance found among the deposits. Narrowing the lumen restricts the space available for blood flow.

Angina can occur in men and women of any age, but is more common among middle-aged and elderly people. It occurs in conditions affecting blood flow. It describes the chest-pain symptom associated with inadequate oxygen reaching cardiac muscle.

Heart failure is sometimes called congestive heart failure because congestion of the lungs, involving fluid accumulation, is one of its main symptoms. It differs from cardiac arrest, when the heart stops beating, and a heart attack, when inadequate blood supply suddenly damages heart muscle.

Glossary

• Plasma — Straw-coloured fluid matrix of blood containing water, proteins, mineral ions and transported nutrients.
• Serum — Plasma without the clotting factors that participate in blood coagulation.
• Erythrocyte — A red blood cell containing haemoglobin and involved in respiratory gas transport.
• Leucocyte — A nucleated white blood cell participating in the body's defence mechanisms.
• Platelet — A cell fragment derived from a megakaryocyte that releases substances involved in clotting.
• Antigen — A substance capable of inducing an immune response in the body.
• Antibody — A protein produced in response to an antigen during an immune response.
• Lymph — Colourless fluid within lymphatic vessels that carries lymphocytes and transported substances.
• Septum — A partition separating chambers, such as the wall between the two ventricles.
• Pacemaker — The rhythm-setting tissue of the heart, normally represented by the sinoatrial node.
• Cardiac cycle — The repeating sequence of systole and diastole of the atria and ventricles.
• Stroke volume — Volume of blood pumped out by each ventricle during one cardiac cycle.
• Cardiac output — Volume of blood pumped out by each ventricle in one minute.
• Electrocardiogram — A graphical recording of the electrical activity of the heart during a cardiac cycle.
• Double circulation — Circulation through distinct pulmonary and systemic pathways connected through the heart.

Common errors and misconceptions

• Misconception: Plasma and serum are identical. Correct: Serum is plasma without clotting factors; plasma contains factors needed for coagulation.
• Misconception: Every formed element is a complete cell. Correct: RBCs and WBCs are cells, while platelets are fragments produced from megakaryocytes.
• Misconception: “Universal donor” means blood needs no matching. Correct: The ABO description of group O does not remove the requirement for careful matching, including Rh compatibility.
• Misconception: Every artery contains oxygenated blood. Correct: Arteries carry blood away from the heart; the pulmonary artery carries deoxygenated blood towards the lungs.
• Misconception: Nerves initiate every normal heartbeat. Correct: The heart is myogenic; nodal tissue initiates its rhythm, while nerves can modify cardiac activity.
• Misconception: All heart valves open together. Correct: Atrioventricular valves allow ventricular filling; semilunar valves open later when ventricular pressure rises sufficiently during contraction.
• Misconception: An ECG directly records the volume of blood pumped. Correct: It records electrical activity; cardiac output is calculated from stroke volume and heart rate.
• Misconception: Heart failure, cardiac arrest and heart attack are interchangeable. Correct: They mean inadequate pumping, cessation of beating and sudden heart-muscle damage from inadequate blood supply respectively.

Exam-style questions with model answers

Q1. State one function each of the plasma proteins fibrinogen and albumin. [2 marks]

1. Fibrinogen participates in blood coagulation by providing the precursor from which fibrin threads form.
2. Albumin helps maintain osmotic balance, supporting the balance of water movement associated with dissolved substances.

Q2. A recipient has group A blood with anti-B antibodies but no anti-A antibodies. Available donor RBCs are group A bearing A antigen, group B bearing B antigen and group O bearing neither antigen. Considering ABO compatibility alone, assess each donor group. [3 marks]

1. Group A is compatible within this ABO scheme because its RBCs carry A antigen, against which the recipient has no anti-A antibodies.
2. Group B is incompatible because its B antigen encounters the recipient's anti-B antibodies, risking clumping and destruction of the donor RBCs.
3. Group O is compatible within the stated scheme because its RBCs lack both A and B antigens, including the B antigen targeted by the recipient.

Q3. Describe blood coagulation from the release of injury-related factors to formation of a fibrin clot. Give four sequential points. [4 marks]

1. Injury stimulates platelets to release factors activating coagulation; substances released by damaged tissues can also initiate the process.
2. Linked enzyme reactions produce thrombokinase, the enzyme complex required for conversion of inactive prothrombin into thrombin; calcium ions support clotting.
3. Thrombin acts on the plasma protein fibrinogen and converts it into fibrin threads.
4. The fibrin threads form a network trapping dead and damaged formed elements, producing a clot that helps prevent excessive blood loss.

Q4. Starting with joint diastole, explain one cardiac cycle in six ordered points, including chamber activity, blood movement and valve changes. [6 marks]

1. During joint diastole, all chambers relax. Open tricuspid and bicuspid valves allow blood arriving through veins to pass through the atria into the ventricles; semilunar valves are closed.
2. The sinoatrial node initiates excitation of both atria. Auricular systole pushes additional blood into the ventricles through the open atrioventricular valves.
3. Excitation passes through the atrioventricular conducting tissue to ventricular muscle. Ventricular systole begins while the atria enter auricular diastole.
4. Rising ventricular pressure closes the tricuspid and bicuspid valves. Further pressure rise opens the semilunar valves, ejecting blood into the pulmonary artery and aorta.
5. During ventricular diastole, ventricular pressure falls. The semilunar valves close, preventing blood from flowing backwards from the arteries into the ventricles.
6. Further pressure decline allows atrial pressure to reopen the atrioventricular valves. Blood enters the ventricles, and joint diastole is restored before the next cycle.

Q5. Each ventricle pumps 70 mL per beat at a heart rate of 72 beats per minute. Calculate cardiac output per ventricle, showing the relationship and substitution. [2 marks]

1. Cardiac output is stroke volume multiplied by heart rate, giving the volume pumped by each ventricle per minute.
2. Substitution gives 70 mL per beat × 72 beats per minute = 5,040 mL per minute.

Q6. Explain the electrical event and relationship to contraction represented by the P wave, QRS complex and T wave of an ECG. [3 marks]

1. The P wave represents depolarisation, or electrical excitation, of the atria. This electrical event leads to contraction of both atria.
2. The QRS complex represents depolarisation of the ventricles. Ventricular contraction begins shortly after Q, marking the beginning of ventricular systole.
3. The T wave represents ventricular repolarisation, the return from excitation to the normal electrical state. Its end marks the end of systole.

Q7. Trace the pulmonary and systemic pathways of human double circulation and state why the separation of the two sides of the heart matters. [5 marks]

1. The right ventricle pumps deoxygenated blood into the pulmonary artery, which carries it to the lungs for oxygenation. This begins pulmonary circulation.
2. Pulmonary veins return oxygenated blood from the lungs to the left atrium. Blood then enters the left ventricle through the bicuspid valve.
3. The left ventricle pumps blood into the aorta. Arteries, arterioles and capillaries distribute oxygenated blood to body tissues as part of systemic circulation.
4. Venules and veins collect deoxygenated blood from tissues. The venae cavae return it to the right atrium, from which it enters the right ventricle.
5. The separate right and left chambers prevent mixing of oxygenated and deoxygenated blood in the heart, maintaining distinct pulmonary and systemic pathways.

Q8. Explain how exposure of an Rh-negative mother to her Rh-positive baby's blood at the first delivery can endanger a subsequent Rh-positive foetus, and how sensitisation can be prevented. [4 marks]

1. Exposure to foetal Rh antigen can stimulate the mother to produce her own anti-Rh antibodies, sensitising her to this antigen.
2. In a subsequent Rh-positive pregnancy, maternal anti-Rh antibodies can enter the foetal blood and destroy its red blood cells.
3. This causes erythroblastosis foetalis, which can be fatal to the foetus or cause severe anaemia and jaundice in the baby.
4. Giving anti-Rh antibodies immediately after the first delivery, before the mother becomes sensitised, can prevent her own antibody response; this prophylaxis does not reverse established sensitisation.

Key takeaways

• Blood consists of plasma and formed elements; plasma proteins contribute to coagulation, defence and osmotic balance.
• RBCs transport respiratory gases, WBCs participate in defence, and platelet fragments release substances involved in coagulation.
• ABO and Rh matching matter because incompatible antigens and antibodies can lead to destruction of red blood cells.
• Tissue fluid mediates exchange around cells; lymph returns excess fluid, transports absorbed fats and participates in immune responses.
• The four-chambered heart uses valves and pressure changes to maintain forward flow through pulmonary and systemic pathways.
• The sinoatrial node sets the rhythm; autonomic nerves and adrenal medullary hormones can modify cardiac activity.
• Cardiac output equals stroke volume multiplied by heart rate, whereas an ECG records the heart's electrical activity.
• Hypertension, coronary artery disease, angina and heart failure describe different problems affecting pressure, blood supply or pumping.

Test yourself

Which plasma proteins primarily support defence and osmotic balance?

Globulins primarily participate in defence mechanisms, while albumins help maintain osmotic balance.

Which two white blood cell types are phagocytic?

Neutrophils and monocytes engulf and destroy foreign organisms that enter the body.

What distinguishes tissue fluid from lymph?

Tissue fluid occupies spaces between tissue cells. Once this fluid enters the lymphatic vessels, it is called lymph.

Why is the sinoatrial node called the pacemaker?

It generates the highest frequency of action potentials in the nodal system, initiating and maintaining the heart's rhythmic contractions.

What causes the first and second heart sounds?

The first sound accompanies tricuspid and bicuspid valve closure; the second accompanies semilunar valve closure.

What distinguishes systolic from diastolic arterial pressure?

Systolic pressure occurs during ventricular contraction, whereas diastolic pressure occurs during ventricular relaxation.

Where does the hepatic portal vein carry blood?

It carries blood from the intestine to the liver before delivery to systemic circulation.

Why is heart failure different from cardiac arrest?

In heart failure, the heart pumps inadequately for the body's needs; in cardiac arrest, the heart stops beating.