The Circulatory System — ICSE Class 10 Biology Important Questions

Answer: (b) Pulmonary vein.

The pulmonary vein is the exception among veins — it carries oxygenated blood from the lungs to the left atrium of the heart.

Answer: (b) Bicuspid (mitral) valve.

The bicuspid (mitral) valve lies between the left atrium and left ventricle; the tricuspid valve lies on the right side.

Answer: (c) Pump blood at high pressure to the entire body.

The left ventricle pumps oxygenated blood through the aorta to the whole body against high resistance, so it needs a thick, muscular wall; the right ventricle only pumps to the nearby lungs.

Answer: (c) Hepatic portal vein. It begins in capillaries of the intestine and ends in capillaries of the liver. This lets the liver store extra glucose as glycogen and remove harmful substances before the blood reaches the rest of the body.

Answer: (a) Both are true and R explains A. Mammalian RBCs also lack mitochondria, so they do not use up the oxygen they carry, and their biconcave shape gives a large surface for absorbing oxygen.

(i) Group O is the universal donor: its red blood cells carry neither antigen A nor antigen B, so they are not clumped by the recipient's antibodies. Group AB is the universal recipient: its plasma has neither anti-A nor anti-B antibodies.

(ii) The Rh factor is an antigen present on the surface of red blood cells in most people. People who have it are Rh-positive ($Rh^+$Rh^+); those who do not are Rh-negative ($Rh^-$Rh^-).

1. Cells: Blood contains red blood cells, white blood cells and platelets; lymph has only white blood cells (mainly lymphocytes) and no red blood cells.

2. Colour/oxygen: Blood is red (due to haemoglobin) and rich in oxygen and nutrients; lymph is colourless/pale yellow and carries less oxygen but is rich in some products such as fats absorbed from the intestine.

Arteries (except the pulmonary artery) carry oxygenated blood at high pressure; veins (except the pulmonary vein) carry deoxygenated blood at low pressure; capillaries allow exchange of gases, nutrients and wastes with tissues.

1. At a wound, the damaged tissue and the platelets release thromboplastin (thrombokinase).

2. Thromboplastin, with calcium ions, converts the inactive plasma protein prothrombin into active thrombin.

3. Thrombin converts the soluble plasma protein fibrinogen into insoluble threads of fibrin.

4. The fibrin threads form a mesh that traps blood cells, making a clot that seals the wound and stops bleeding.

$\text{prothrombin}\xrightarrow{\text{thromboplastin},\ Ca^{2+}}\text{thrombin};\qquad\text{fibrinogen}\xrightarrow{\text{thrombin}}\text{fibrin}$prothrombinthromboplastin, Ca^2+thrombin; fibrinogen→ (thrombin)fibrin

Double circulation means that in one complete round the blood passes through the heart twice, keeping oxygenated and deoxygenated blood separate.

1. Pulmonary circulation: Deoxygenated blood goes from the right ventricle through the pulmonary artery to the lungs, is oxygenated, and returns by the pulmonary veins to the left atrium.

2. Systemic circulation: Oxygenated blood goes from the left ventricle through the aorta to all body organs, and deoxygenated blood returns through the venae cavae to the right atrium.

Blue: deoxygenated blood (right side). Red: oxygenated blood (left side).

Path of a drop of blood through the heart:

1. Deoxygenated blood from the body enters the right atrium (RA) through the superior and inferior venae cavae.

2. The right atrium contracts and blood passes through the tricuspid valve into the right ventricle (RV).

3. The right ventricle contracts and pumps the blood through the semilunar valve into the pulmonary artery, which carries it to the lungs, where it is oxygenated.

4. Oxygenated blood returns through the pulmonary veins to the left atrium (LA).

5. The left atrium contracts and blood passes through the bicuspid (mitral) valve into the left ventricle (LV).

6. The thick-walled left ventricle contracts and pumps the oxygenated blood at high pressure through the semilunar valve into the aorta, which distributes it to the whole body.

The valves ensure blood flows in only one direction, preventing backflow.

Cardiac cycle: the sequence of events in one complete heartbeat, lasting about 0.8 s in an adult at rest.

1. Atrial systole (about 0.1 s): Both atria contract and push blood through the tricuspid and bicuspid valves into the relaxed ventricles.

2. Ventricular systole (about 0.3 s): The ventricles contract. The rising pressure first closes the tricuspid and bicuspid valves (giving the first sound), then forces the semilunar valves open, pushing blood into the pulmonary artery and aorta.

3. Diastole (about 0.4 s): The whole heart relaxes. The semilunar valves close (giving the second sound) to prevent backflow, and the atria refill with blood.

Heart sounds:

• 'Lubb' (first sound): a soft, long sound caused by the closure of the bicuspid and tricuspid valves at the start of ventricular systole.
• 'Dupp' (second sound): a sharp, short sound caused by the closure of the semilunar valves at the start of diastole.

Heartbeat rate: the number of times the heart beats per minute; about 72 beats/min in a resting adult.

Blood pressure: the pressure exerted by blood on the walls of the arteries; normally about 120 mm Hg (systolic) / 80 mm Hg (diastolic) in a healthy adult.

(i) 150 is the systolic pressure (pressure during ventricular contraction) and 95 is the diastolic pressure (pressure when the ventricles relax), measured in mm Hg.

(ii) No — a normal value is about 120/80 mm Hg, so 150/95 is raised. Persistently high blood pressure is called hypertension.

(iii) The pulse is produced by an artery (the radial artery). Each ventricular systole sends a surge of blood that stretches the elastic arterial wall; where an artery lies close to the skin over a bone (the wrist), this expansion can be felt as the pulse.

(iv) Red blood cells carry oxygen using haemoglobin. A low count means less oxygen is transported to the tissues, causing tiredness and breathlessness. This condition is called anaemia.