Body Fluids and Circulation | CBSE Class 11 Biology Notes

This note covers blood plasma and the formed elements of blood, the ABO and Rh blood groups, blood clotting, tissue fluid and lymph, open and closed circulation, vertebrate hearts, the human heart and its nodal tissue, the cardiac cycle, heart sounds, the ECG, double circulation, the regulation of cardiac activity, and circulatory disorders.

Why do animals need body fluids and circulation?

All living cells have to be provided with nutrients, O₂ and other essential substances. The waste or harmful substances that cells produce also have to be removed continuously for the healthy functioning of tissues.

It is therefore essential to have efficient mechanisms for moving these substances to the cells and from the cells. Different groups of animals have evolved different methods for this transport.

• Simple organisms like sponges and coelenterates circulate water from their surroundings through their body cavities, so that their cells can exchange these substances.
• More complex organisms use special fluids within their bodies to transport such materials.
• Blood is the body fluid most commonly used for this purpose by most of the higher organisms, including humans.
• Another body fluid, lymph, also helps in the transport of certain substances.

Blood as a fluid connective tissue

Blood is a special connective tissue consisting of a fluid matrix, plasma, and formed elements. Vertebrates circulate blood, a fluid connective tissue, in their body to transport essential substances to the cells and to carry waste substances away from them.

Blood is treated as a connective tissue because it is made of cells and cell fragments, the formed elements, lying in a matrix; in blood the matrix is a fluid, the plasma.

What does blood plasma contain?

Plasma is a straw coloured, viscous fluid constituting nearly 55 per cent of the blood. About 90 to 92 per cent of plasma is water, and proteins contribute 6 to 8 per cent of it.

Fibrinogen, globulins and albumins are the major plasma proteins. Their roles, and the other components of plasma, are set out below.

Plasma and serum

Factors for the coagulation or clotting of blood are present in the plasma in an inactive form.

Definition: Serum is plasma without the clotting factors.

What are the formed elements of blood?

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

A row of labelled drawings, left to right: RBC, a small plain red disc with no nucleus; platelets, a scatter of small purple fragments; eosinophil, a large cell packed with red-orange granules; basophil, a cell densely dotted with dark purple granules.

Then a neutrophil, a pink cell with a lobed purple nucleus; a monocyte, the largest cell, with a big curved nucleus; and a T lymphocyte and a B lymphocyte, small round cells whose nucleus fills most of the cell.

See Fig. 15.1 in your NCERT textbook

Erythrocytes (red blood cells)

Erythrocytes or red blood cells (RBC) 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 mm⁻³ of blood.

In adults, RBCs are formed in the red bone marrow. RBCs are devoid of nucleus in most of the mammals and are biconcave in shape.

They contain a red coloured, iron containing complex protein called haemoglobin, which gives these cells their colour and their name. A healthy individual has 12 to 16 g of haemoglobin in every 100 mL of blood. These molecules play 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. For this reason the spleen is called the graveyard of RBCs.

Leucocytes (white blood cells)

Leucocytes are also known as white blood cells (WBC) because they are colourless, owing to the lack of haemoglobin. They are nucleated and relatively fewer in number, averaging 6000 to 8000 mm⁻³ of blood. Leucocytes are generally short lived.

There are two main categories of WBCs. Granulocytes are the neutrophils, eosinophils and basophils, while agranulocytes are the lymphocytes and monocytes.

Platelets (thrombocytes)

Platelets, also called thrombocytes, are cell fragments produced from megakaryocytes, which are special cells in the bone marrow. Blood normally contains 1,50,000 to 3,50,000 platelets mm⁻³.

Platelets can release a variety of substances, most of which are involved in the coagulation or clotting of blood. A reduction in their number can lead to clotting disorders, which lead to excessive loss of blood from the body.

How are human blood groups classified?

Human blood differs in certain aspects, though it appears similar. Two groupings, the ABO and Rh groupings, are widely used all over the world.

ABO grouping

ABO grouping is based on the presence or absence of two surface antigens, A and B, on the RBCs. Antigens are chemicals that can induce an immune response.

Similarly, the plasma of different individuals contains two natural antibodies, which are proteins produced in response to antigens. The table shows the four blood groups, A, B, AB and O.

During blood transfusion, any blood cannot be used. The blood of a donor has to be carefully matched with the blood of the recipient before transfusion, to avoid severe problems of clumping, that is, destruction of RBCs.

The table shows that group O blood can be donated to persons of any other blood group, so group O individuals are called universal donors. Persons with group AB can accept blood from AB as well as from the other groups, so they are called universal recipients.

Rh grouping

Another antigen, the Rh antigen, is similar to one present in Rhesus monkeys, hence the name Rh. It is observed on the surface of the RBCs of the majority of humans, nearly 80 per cent.

Individuals with this antigen are called Rh positive (Rh+ve), and those in whom it is absent are called Rh negative (Rh-ve). An Rh-ve person exposed to Rh+ve blood forms specific antibodies against the Rh antigens. The Rh group should therefore also be matched before transfusions.

Erythroblastosis foetalis

A special case of Rh incompatibility (mismatching) has been observed between the Rh-ve blood of a pregnant mother and the Rh+ve blood of her foetus. It develops in this order.

1. In the first pregnancy, the Rh antigens of the foetus do not get exposed to the Rh-ve blood of the mother, because the two bloods are well separated by the placenta.
2. During the delivery of the first child, the maternal blood may be exposed to small amounts of Rh+ve blood from the foetus.
3. In such cases the mother starts preparing antibodies against the Rh antigen in her blood.
4. In her subsequent pregnancies, the Rh antibodies from the Rh-ve mother can leak into the blood of an Rh+ve foetus and destroy the foetal RBCs.
5. This could be fatal to the foetus, or could cause severe anaemia and jaundice in the baby. The condition is called erythroblastosis foetalis.

This condition can be avoided by administering anti-Rh antibodies to the mother immediately after the delivery of the first child.

How does blood clot?

When you cut your finger, the wound does not continue to bleed for a long time; usually the blood stops flowing after some time. Blood shows coagulation or clotting in response to an injury or trauma. This mechanism prevents excessive loss of blood from the body.

Over a period of time a dark reddish brown scum forms at the site of a cut or injury. This is a clot or coagulam. It is formed mainly of a network of threads called fibrins, in which dead and damaged formed elements of blood are trapped.

Steps in coagulation

1. An injury or trauma stimulates the platelets in the blood to release certain factors that activate the mechanism of coagulation. Certain factors released by the tissues at the site of injury can also initiate coagulation.
2. A series of linked enzymic reactions, a cascade process, involving a number of factors present in the plasma in an inactive state, forms an enzyme complex called thrombokinase.
3. With thrombokinase, the enzyme thrombin is formed from another inactive substance present in the plasma, prothrombin.
4. Thrombin converts the inactive fibrinogen of the plasma into fibrin.
5. The fibrin threads form a network in which dead and damaged formed elements are trapped, and the clot is formed.

Calcium ions play a very important role in clotting.

Write "Injury: platelets and tissues release factors", then an arrow to "Cascade of inactive plasma factors", then to "Thrombokinase". Below, draw prothrombin → thrombin (thrombokinase on the arrow) and fibrinogen → fibrin (thrombin on the arrow). End with "Fibrin network traps formed elements: clot", and note that calcium ions play a very important role.

What are tissue fluid and lymph?

A second body fluid forms from blood in the tissues and is returned to it through a separate set of vessels, in these stages.

1. As the blood passes through the capillaries in tissues, some water along with many small water soluble substances moves out into the spaces between the cells of the tissues.
2. The larger proteins and most of the formed elements are left behind in the blood vessels.
3. The fluid released out is called the interstitial fluid or tissue fluid. It has the same mineral distribution as plasma. Exchange of nutrients, gases, etc., between the blood and the cells always occurs through this fluid.
4. An elaborate network of vessels called the lymphatic system collects this fluid and drains it back to the major veins.
5. 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, etc.

Fats are absorbed through lymph in the lacteals present in the intestinal villi. Lymph is almost similar to blood except for the protein content and the formed elements.

How do circulatory systems differ among animals?

The circulatory patterns of animals are of two types, open and closed.

All vertebrates and a few invertebrates have a closed circulatory system.

Hearts of vertebrates

All vertebrates possess a muscular, chambered heart.

Single, incomplete double and double circulation

In fishes the heart pumps out deoxygenated blood, which is oxygenated by the gills and supplied to the body parts. From there deoxygenated blood is returned to the heart. This is single circulation.

In amphibians and reptiles (except crocodiles), the left atrium receives oxygenated blood from the gills, lungs or skin, and the right atrium gets deoxygenated blood from the other body parts. The two get mixed up in the single ventricle, which pumps out mixed blood. This is incomplete double circulation.

In birds and mammals, oxygenated and deoxygenated blood received by the left and right atria respectively passes on to the ventricles of the same sides. The ventricles pump it out without any mixing, so two separate circulatory pathways are present. These animals have double circulation.

What is the structure of the human heart?

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 fluid which is circulated.

Position and covering

The heart is a mesodermally derived organ situated in the thoracic cavity, in between the two lungs, slightly tilted to the left. It has the size of a clenched fist.

It is protected by a double walled membranous bag, the pericardium, which encloses the pericardial fluid.

Chambers and septa

The heart has four chambers: two relatively small upper chambers called atria and two larger lower chambers called ventricles. The entire heart is made of cardiac muscles, and the walls of the ventricles are much thicker than those of the atria.

A thin, muscular wall called 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 called the atrio-ventricular septum.

The atrio-ventricular septum is provided with an opening on each side through which the atrium and ventricle of the same side are connected.

Valves of the heart

The valves allow blood to flow in only one direction, from the atria to the ventricles and from the ventricles to the pulmonary artery or aorta. They prevent any backward flow.

The heart is drawn cut open to show its chambers, with the apex at the bottom. Labels on the left read vena cava (a blue vessel), sino-atrial node, right atrium, atrio-ventricular node, Chordae tendinae (drawn as thin threads) and right ventricle.

Labels on the right read aorta (the large red arch at the top), pulmonary artery (blue), pulmonary veins (red), left atrium, bundle of His, left ventricle, interventricular septum and apex. The nodal tissue is drawn in yellow: small patches for the sino-atrial and atrio-ventricular nodes, and a yellow band running down the interventricular septum and round both ventricles.

See Fig. 15.2 in your NCERT textbook

Nodal tissue and the pacemaker

A specialised cardiac musculature called the nodal tissue is distributed in the heart, arranged as follows.

1. The sino-atrial node (SAN) is a patch of nodal tissue in the right upper corner of the right atrium.
2. The atrio-ventricular node (AVN) is another mass of this tissue, in the lower left corner of the right atrium, close to the atrio-ventricular septum.
3. The atrio-ventricular bundle (AV bundle), a bundle of nodal fibres, continues from the AVN, passes through the atrio-ventricular septa and emerges on the top of the inter-ventricular septum.
4. There it immediately divides into a right and a left bundle.
5. These branches give rise to minute fibres throughout the ventricular musculature of their own sides, called Purkinje fibres.

The nodal musculature can generate action potentials without any external stimuli; it is autoexcitable. The number of action potentials generated in a minute varies in different parts of the nodal system.

The SAN generates the maximum number, 70 to 75 min⁻¹, and is responsible for initiating and maintaining the rhythmic contractile activity of the heart. It is therefore called the pacemaker. Our heart normally beats 70 to 75 times in a minute, an average of 72 beats min⁻¹.

What happens during the cardiac cycle?

The cardiac cycle is the sequence of events in the heart that is cyclically repeated. It consists of the systole (contraction) and diastole (relaxation) of both the atria and the ventricles. The events follow this order.

1. To begin with, all four chambers are relaxed, in joint diastole. The tricuspid and bicuspid valves are open, so blood from the pulmonary veins and the vena cava flows through the left and right atria into the left and right ventricles respectively. The semilunar valves are closed.
2. The SAN generates an action potential that makes both atria contract simultaneously, the atrial systole. This increases the flow of blood into the ventricles by about 30 per cent.
3. The action potential is conducted to the ventricular side by the AVN and the AV bundle, from where the bundle of His transmits it through the entire ventricular musculature.
4. The ventricular muscles contract, the ventricular systole. The atria relax (diastole) at the same time.
5. Ventricular pressure rises and closes the tricuspid and bicuspid valves, because blood attempts to flow back into the atria.
6. As the pressure rises further, the semilunar valves guarding the pulmonary artery (right side) and the aorta (left side) are forced open, and blood flows from the ventricles into the circulatory pathways.
7. The ventricles now relax, the ventricular diastole. Ventricular pressure falls and the semilunar valves close, preventing backflow into the ventricles.
8. As ventricular pressure declines further, the tricuspid and bicuspid valves are pushed open by the pressure of the blood that the veins have been emptying into the atria. Blood again moves freely into the ventricles.
9. The heart is again in joint diastole. Soon the SAN generates a new action potential and the events repeat in the same sequence.

Duration, stroke volume and cardiac output

The heart beats 72 times per minute, so 72 cardiac cycles are performed per minute. The duration of one cardiac cycle is therefore 60 seconds ÷ 72, which is about 0.8 seconds.

During a cardiac cycle each ventricle pumps out approximately 70 mL of blood. This is the stroke volume. The stroke volume multiplied by the heart rate (number of beats per minute) gives the cardiac output.

Definition: Cardiac output is the volume of blood pumped out by each ventricle per minute. It averages 5000 mL, or 5 litres, in a healthy individual (70 mL × 72 beats min⁻¹ = 5040 mL, about 5 litres).

The body can alter the stroke volume as well as the heart rate, and thereby the cardiac output. For example, the cardiac output of an athlete is much higher than that of an ordinary man.

Heart sounds

Two prominent sounds are produced during each cardiac cycle, and they can be easily heard through a stethoscope. These sounds are of clinical diagnostic significance.

What does an electrocardiogram (ECG) show?

A machine called the electro-cardiograph is used to obtain an electrocardiogram (ECG). The ECG is a graphical representation of the electrical activity of the heart during a cardiac cycle.

To obtain a standard ECG, a patient is connected to the machine with three electrical leads, one to each wrist and one to the left ankle, which continuously monitor the heart activity. For a detailed evaluation of the heart's function, multiple leads are attached to the chest region.

A single green trace runs across a pale panel along a flat baseline. First comes a small rounded upward wave labelled P. After a flat stretch there is a small downward dip labelled Q, then a tall, sharp upward spike labelled R, then a downward dip labelled S.

After another flat stretch comes a broad, rounded upward wave labelled T, and a further small bump appears at the right-hand end.

See Fig. 15.3 in your NCERT textbook

Each peak in the ECG is identified by a letter from P to T that corresponds to a specific electrical activity of the heart.

Uses of the ECG

By counting the number of QRS complexes that occur in a given time period, one can determine the heart beat rate of an individual.

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, so the ECG is of great clinical significance.

How does double circulation work in humans?

Blood flows strictly by a fixed route through the blood vessels, the arteries and the veins. Each artery and vein basically consists of three layers.

Layers of the wall of a blood vessel

Pulmonary and systemic circulation

Humans have a complete double circulation: two circulatory pathways, the pulmonary and the systemic. The blood travels round both in this order.

1. The right ventricle pumps deoxygenated blood into the pulmonary artery, which passes it on to the lungs.
2. From the lungs, oxygenated blood is carried by the pulmonary veins into the left atrium. This pathway is the pulmonary circulation.
3. The left ventricle pumps oxygenated blood into the aorta.
4. A network of arteries, arterioles and capillaries carries it to the tissues.
5. From the tissues, deoxygenated blood is collected by a system of venules, veins and the vena cava and emptied into the right atrium. This is the systemic circulation.

The systemic circulation provides nutrients, O₂ and other essential substances to the tissues, and takes CO₂ and other harmful substances away for elimination.

The heart sits in the middle with its chambers marked RA, LA, RV and LV. An upper loop in purple, the pulmonary artery, runs from the right side of the heart to the lungs at the top, and a red pulmonary vein returns from the lungs to the left side.

A larger lower loop runs from the heart through the red dorsal aorta down to the body parts, and back up through the purple vena cava (great veins). Small arrows along both loops show the direction of flow.

Side drawings show cross-sections of a vein and an artery, each labelled with its smooth muscle and lumen. The artery has a thicker ring of smooth muscle around a small round lumen, while the vein has a thinner ring around a wider lumen. A capillary is drawn below as a thin tube whose wall is made of flattened cells.

See Fig. 15.4 in your NCERT textbook

Hepatic portal and coronary systems

A unique vascular connection exists between the digestive tract and the liver, called the hepatic portal system. The hepatic portal vein carries blood from the intestine to the liver before it is delivered to the systemic circulation.

A special coronary system of blood vessels is present in our body exclusively for the circulation of blood to and from the cardiac musculature.

How is the activity of the heart regulated?

The normal activities of the heart are regulated intrinsically, that is, auto regulated by specialised muscles, the nodal tissue. The heart is therefore called myogenic.

Though the heart is autoexcitable, its functions can be moderated by neural and hormonal mechanisms. A special neural centre in the medulla oblongata can moderate cardiac function through the autonomic nervous system (ANS).

Note: The rhythmic activity of the heart is initiated and maintained by the SAN, so the heart is myogenic. The neural centre in the medulla oblongata and the adrenal medullary hormones moderate this activity; sympathetic and parasympathetic signals act in opposite directions on the heart rate.

What are the disorders of the circulatory system?

High blood pressure (hypertension)

Hypertension is the term for blood pressure that is higher than normal (120/80). In this measurement, 120 mm Hg (millimetres of mercury pressure) is the systolic, or pumping, pressure, and 80 mm Hg is the diastolic, or resting, pressure.

If repeated checks of the blood pressure of an individual are 140/90 (140 over 90) 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)

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

Angina

Angina, also called angina pectoris, is a symptom of acute chest pain that appears when not enough oxygen is reaching the heart muscle. It occurs due to conditions that affect the blood flow. Angina can occur in men and women of any age, but it is more common among the middle-aged and elderly.

Heart failure

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 the main symptoms.

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.

Glossary

• Plasma — The straw coloured, viscous fluid matrix of blood, nearly 55 per cent of it; 90 to 92 per cent water and 6 to 8 per cent proteins.
• Serum — Plasma without the clotting factors, the factors for coagulation that are present in plasma in an inactive form.
• Haemoglobin — The red coloured, iron containing complex protein of RBCs that plays a significant role in the transport of respiratory gases.
• Antigen — A chemical that can induce an immune response; the A, B and Rh antigens may be present on the surface of RBCs.
• Antibody — A protein produced in response to an antigen; natural anti-A and anti-B antibodies may be present in plasma, depending on the blood group.
• Erythroblastosis foetalis — Destruction of the RBCs of an Rh+ve foetus by Rh antibodies from its Rh-ve mother, which could be fatal or cause severe anaemia and jaundice.
• Lymph — The colourless fluid present in the lymphatic system, containing specialised lymphocytes and carrying nutrients, hormones and absorbed fats.
• Pacemaker — The sino-atrial node, which generates the maximum number of action potentials, 70 to 75 per minute, and sets the pace of the activities of the heart.
• Cardiac output — The volume of blood pumped out by each ventricle per minute; stroke volume (about 70 mL) multiplied by heart rate, about 5 litres.
• Myogenic — Describes a heart whose normal activities are regulated intrinsically, that is auto regulated, by specialised muscles, the nodal tissue.

Common errors and misconceptions

• Misconception: People with group O blood are universal recipients. Correct: Group O blood can be given to any group, so group O people are universal donors; group AB people are universal recipients.
• Misconception: Erythroblastosis foetalis usually harms the first Rh+ve baby of an Rh-ve mother. Correct: In the first pregnancy the placenta keeps the two bloods separate; the mother makes Rh antibodies after exposure during the first delivery, and these can harm a later Rh+ve foetus.
• Misconception: The heart beat is started by impulses from the brain. Correct: The heart is myogenic; the SAN starts each beat, and the neural centre in the medulla oblongata moderates the heart through the ANS.
• Misconception: The first heart sound, lub, comes from the semilunar valves. Correct: Lub is associated with the closure of the tricuspid and bicuspid valves; dub is associated with the closure of the semilunar valves.
• Misconception: All reptiles have a three-chambered heart. Correct: Crocodiles, like birds and mammals, have a four-chambered heart with two atria and two ventricles.
• Misconception: Heart failure, cardiac arrest and heart attack are the same thing. Correct: In heart failure the heart does not pump effectively enough for the body's needs; in cardiac arrest it stops beating; in a heart attack the muscle is suddenly damaged by an inadequate blood supply.

Exam-style questions with model answers

Q1. Distinguish between plasma and serum. [2 marks]

1. Plasma is the straw coloured, viscous fluid matrix of blood, nearly 55 per cent of it, and it contains the factors for coagulation in an inactive form.
2. Serum is plasma without the clotting factors.

Q2. Name the formed elements of blood and mention one major function of each. [3 marks]

1. Erythrocytes (RBCs) contain haemoglobin and play a significant role in the transport of respiratory gases.
2. Leucocytes (WBCs): neutrophils and monocytes are phagocytic and destroy foreign organisms, and lymphocytes are responsible for immune responses; eosinophils resist infections and basophils are involved in inflammatory reactions.
3. Platelets (thrombocytes) are cell fragments that release substances, most of which are involved in the coagulation or clotting of blood.

Q3. What is erythroblastosis foetalis? How can it be avoided? [3 marks]

1. It is a special case of Rh incompatibility between an Rh-ve pregnant mother and her Rh+ve foetus.
2. The placenta keeps the two bloods separate in the first pregnancy, but during the first delivery the mother's blood may be exposed to some Rh+ve foetal blood, and she starts preparing antibodies against the Rh antigen.
3. In subsequent pregnancies these antibodies can leak into the blood of an Rh+ve foetus and destroy its RBCs, which could be fatal or cause severe anaemia and jaundice in the baby.
4. It can be avoided by administering anti-Rh antibodies to the mother immediately after the delivery of the first child.

Q4. Describe the evolutionary change in the pattern of the heart among the vertebrates. [4 marks]

1. All vertebrates possess a muscular, chambered heart.
2. Fishes have a 2-chambered heart (an atrium and a ventricle) that pumps out deoxygenated blood; it is oxygenated by the gills, supplied to the body and returned to the heart. This is single circulation.
3. Amphibians and reptiles (except crocodiles) have a 3-chambered heart with two atria and a single ventricle; blood from the two atria mixes in the ventricle: incomplete double circulation.
4. Crocodiles, birds and mammals have a 4-chambered heart with two atria and two ventricles. In birds and mammals the ventricles pump out the two kinds of blood without mixing: double circulation.

Q5. Describe the events of the cardiac cycle. Define stroke volume and cardiac output and give their average values. [5 marks]

1. In joint diastole all chambers are relaxed; the tricuspid and bicuspid valves are open and the semilunar valves closed, so blood flows through the atria into the ventricles.
2. The SAN generates an action potential and both atria contract (atrial systole), increasing the flow of blood into the ventricles by about 30 per cent.
3. The AVN, AV bundle and bundle of His carry the action potential through the ventricular musculature. The ventricles contract (ventricular systole) while the atria relax.
4. Rising ventricular pressure closes the tricuspid and bicuspid valves and then forces the semilunar valves open, so blood flows into the pulmonary artery and aorta.
5. In ventricular diastole the falling pressure closes the semilunar valves; then the tricuspid and bicuspid valves are pushed open, and joint diastole returns.
6. Stroke volume is the blood pumped out by each ventricle in one cycle, about 70 mL. Cardiac output, stroke volume × heart rate, is the volume pumped by each ventricle per minute, about 5 litres.

Q6. Draw a standard ECG and explain its different parts. [5 marks]

1. Drawing: a trace along a flat baseline with a small upward P-wave, a small downward Q, a tall sharp R, a downward S, and then a broad, rounded T-wave, each labelled.
2. A standard ECG is a graphical representation of the electrical activity of the heart during a cardiac cycle, obtained with three leads, one to each wrist and one to the left ankle.
3. The P-wave represents the depolarisation of the atria, which leads to the contraction of both atria.
4. The QRS complex represents the depolarisation of the ventricles, which initiates ventricular contraction; contraction starts shortly after Q and marks the beginning of systole.
5. The T-wave represents the repolarisation of the ventricles; the end of the T-wave marks the end of systole.
6. Counting QRS complexes in a given time gives the heart rate; any deviation from the usual shape indicates a possible abnormality.

Q7. What is meant by double circulation? Describe the two pathways in humans and state its significance. [5 marks]

1. Double circulation means that two separate circulatory pathways, pulmonary and systemic, are present, and oxygenated and deoxygenated blood are pumped out of the ventricles without mixing.
2. Pulmonary circulation: the right ventricle pumps deoxygenated blood into the pulmonary artery to the lungs; oxygenated blood returns through the pulmonary veins to the left atrium.
3. Systemic circulation: the left ventricle pumps oxygenated blood into the aorta, and arteries, arterioles and capillaries carry it to the tissues; deoxygenated blood returns through venules, veins and the vena cava to the right atrium.
4. Significance: the oxygenated blood sent to the body is not mixed with deoxygenated blood, so the tissues receive fully oxygenated blood and the supply of oxygen is efficient, unlike the mixed blood pumped by the single ventricle of amphibians and reptiles (except crocodiles).

Key takeaways

• Blood is a special connective tissue: plasma forms nearly 55 per cent and the formed elements nearly 45 per cent.
• RBCs are the most abundant cells, carry haemoglobin, live 120 days and are destroyed in the spleen; neutrophils are the commonest WBCs and basophils the rarest.
• Group O people are universal donors and group AB people universal recipients; Rh incompatibility can cause erythroblastosis foetalis in a later Rh+ve foetus.
• In clotting, thrombin forms from prothrombin with the help of thrombokinase, and thrombin turns fibrinogen into a fibrin network; calcium ions play a very important role.
• Fishes have single circulation, amphibians and reptiles (except crocodiles) incomplete double circulation, and birds and mammals double circulation.
• The SAN is the pacemaker; a cardiac cycle lasts about 0.8 seconds, the stroke volume is about 70 mL and the cardiac output about 5 litres.
• In an ECG the P-wave shows atrial depolarisation, the QRS complex ventricular depolarisation and the T-wave ventricular repolarisation.

Test yourself

Which organ is called the graveyard of RBCs, and why?

The spleen, because RBCs are destroyed there at the end of their average life span of 120 days.

Match: eosinophils, RBC, AB group, platelets and systole.

Eosinophils resist infections; RBCs carry out gas transport; the AB group is the universal recipient; platelets help coagulation; systole is the contraction of the heart.

How are fats absorbed from the intestine?

Fats are absorbed through lymph in the lacteals present in the intestinal villi.

What are the two heart sounds associated with?

The first sound, lub, is associated with the closure of the tricuspid and bicuspid valves; the second, dub, with the closure of the semilunar valves.

What blood pressure reading shows hypertension?

Repeated readings of 140/90 or higher show hypertension; normal blood pressure is 120/80, with 120 mm Hg systolic and 80 mm Hg diastolic.

What causes coronary artery disease?

Deposits of calcium, fat, cholesterol and fibrous tissues make the lumen of the arteries supplying the heart muscle narrower.