Circulatory System — Set 3

Correct Answer: D. 120 days

• **120 days** = red blood cells survive for approximately 120 days (about 4 months) in circulation; they lack a nucleus and cannot repair themselves, so they gradually degrade as their membranes become fragile. • **Destruction site** — old, worn-out erythrocytes are removed and broken down mainly by macrophages in the spleen and liver; the haem group is recycled into bilirubin (which gives bile its colour), and iron is reclaimed for new haemoglobin synthesis. • About 2–3 million red blood cells are destroyed and replaced every second in the human body. • 💡 Option A (5 days) is wrong because that is approximately the lifespan of platelets; Option B (365 days) is wrong because no blood cell type survives a full year in circulation; Option C (10 days) is wrong because that value does not correspond to any major blood cell's normal lifespan.

Correct Answer: B. Right atrium

• **Right atrium** = it receives oxygen-depleted blood from the entire body through two large veins — the superior vena cava (from the upper body) and the inferior vena cava (from the lower body) — as well as from the coronary sinus (from the heart muscle itself). • **Next step** — from the right atrium, blood passes through the tricuspid valve into the right ventricle, which then pumps it to the lungs via the pulmonary artery. • The right atrium also houses the SA node in its upper wall, making it the site where each heartbeat electrically originates. • 💡 Option A (Left ventricle) is wrong because it receives oxygenated blood from the left atrium and pumps it to the body; Option C (Right ventricle) is wrong because it receives deoxygenated blood from the right atrium (not directly from the body) and pumps it to the lungs; Option D (Left atrium) is wrong because it receives oxygenated blood from the pulmonary veins.

Correct Answer: B. Capillaries

• **Capillaries** = they are the smallest blood vessels (diameter 5–10 micrometres), with walls only one cell (endothelium) thick and no smooth muscle, allowing rapid diffusion of oxygen, carbon dioxide, glucose, and waste products between blood and surrounding tissues. • **Total length** — if all the capillaries in the human body were laid end to end, they would stretch approximately 100,000 km — enough to circle the Earth more than twice. • Exchange occurs by simple diffusion (O₂, CO₂), osmosis (water), and active transport (nutrients), making capillaries the functional units of the circulatory system. • 💡 Option A (Veins) is wrong because they carry blood back to the heart at low pressure and their thick walls prevent direct exchange with tissues; Option C (Arteries) is wrong because their thick, muscular walls and high pressure prevent diffusion; Option D (Arterioles) is wrong because they are small arteries that regulate blood flow into capillary beds and are still too thick-walled for exchange.

Correct Answer: D. AB positive

• **AB positive** = individuals with AB positive blood have both A and B antigens on their red cells and the Rh factor, meaning their immune system does not produce antibodies against A, B, or Rh antigens and will not reject red blood cells of any type. • **No agglutination** — since AB positive plasma contains no anti-A or anti-B antibodies, there is no risk of agglutination (clumping of donated red cells) regardless of the donor's blood type. • AB positive individuals can also receive platelets from any donor and are universal plasma donors for red cell transfusions. • 💡 Option A (O negative) is wrong because it is the universal donor, not recipient; Option B (B negative) is wrong because it carries anti-A antibodies, so it cannot receive A or AB type blood; Option C (A positive) is wrong because it carries anti-B antibodies, so it cannot receive B or AB type blood.

Correct Answer: B. Erythropoietin

• **Erythropoietin (EPO)** = produced mainly by peritubular cells in the kidney cortex in response to low blood oxygen (hypoxia), EPO travels to the red bone marrow and stimulates the differentiation and proliferation of red blood cell precursors (erythroblasts). • **Athletic doping** — synthetic EPO is banned in sports because it artificially raises haematocrit, increasing oxygen delivery to muscles and improving endurance performance. • In chronic kidney disease, EPO production falls, causing renal anaemia; patients are treated with recombinant EPO injections. • 💡 Option A (Insulin) is wrong because it is produced by the pancreatic beta cells to regulate blood glucose, not red blood cell production; Option C (Thyroxine) is wrong because it is a thyroid hormone that regulates metabolic rate; Option D (Adrenaline) is wrong because it is produced by the adrenal medulla to prepare the body for 'fight or flight' responses.

Correct Answer: B. James Blundell

• **James Blundell** = a British obstetrician who performed the first successful human-to-human blood transfusion in 1818, transfusing blood from a husband to his wife who was dying from post-partum haemorrhage, saving her life. • **Significance** — Blundell conducted ten human transfusions between 1818 and 1829; five were successful, establishing that only human blood should be used for human transfusions (earlier attempts had used animal blood). • Blood groups were only discovered by Karl Landsteiner in 1901, explaining why many early transfusions failed due to incompatibility. • 💡 Option A (William Harvey) is wrong because he described the circulatory system and blood circulation in 1628 but did not perform transfusions; Option C (Karl Landsteiner) is wrong because he discovered the ABO blood group system in 1901, making transfusions safe, but was not the first to perform one; Option D (Robert Koch) is wrong because he identified the bacteria causing tuberculosis and cholera, work unrelated to transfusions.

Correct Answer: C. Thrombocytopenia

• **Thrombocytopenia** = from Greek thrombos (clot) + kytos (cell) + penia (deficiency); it is defined as a platelet count below 150,000 per microlitre, causing symptoms like petechiae (tiny red spots), easy bruising, and prolonged bleeding even from minor cuts. • **Causes** — it can result from decreased platelet production (bone marrow failure, chemotherapy), increased destruction (immune thrombocytopenia, dengue fever), or sequestration in an enlarged spleen. • Dengue fever is a common cause of thrombocytopenia in India, which is why platelet counts are closely monitored in dengue patients. • 💡 Option A (Polycythemia) is wrong because it refers to an abnormally high number of red blood cells, not low platelets; Option B (Anemia) is wrong because it is a deficiency of haemoglobin or red blood cells, not platelets; Option D (Leukemia) is wrong because it is a cancer of blood-forming tissues causing overproduction of abnormal white blood cells.

Correct Answer: D. Medulla oblongata

• **Medulla oblongata** = located in the brainstem, it contains the cardiovascular centre (cardiac centre + vasomotor centre) that automatically adjusts heart rate and blood pressure via the autonomic nervous system without conscious thought. • **Baroreceptor reflex** — stretch receptors in the aortic arch and carotid sinuses send signals to the medulla, which responds by speeding up or slowing the heart and dilating or constricting blood vessels to maintain constant blood pressure. • The medulla also controls breathing, swallowing, and vomiting, earning it the nickname 'vital centre' of the brain. • 💡 Option A (Thalamus) is wrong because it acts as a sensory relay station, routing signals to the cerebral cortex; Option B (Cerebrum) is wrong because it governs voluntary actions, thought, memory, and speech; Option C (Cerebellum) is wrong because it coordinates balance, posture, and fine motor movements.

Correct Answer: A. Coronary circulation

• **Coronary circulation** = the heart muscle (myocardium) cannot absorb oxygen directly from the blood filling its chambers, so it has its own dedicated supply — the right and left coronary arteries branch off the aorta just above the aortic valve and perfuse the myocardium. • **Diastolic filling** — coronary arteries fill mainly during diastole (when the heart relaxes), because during systole the contracting heart muscle compresses the vessels. • Blockage of a coronary artery causes myocardial infarction (heart attack), where the deprived muscle cells die from lack of oxygen. • 💡 Option B (Pulmonary circulation) is wrong because it describes the circuit between the heart and the lungs for gas exchange; Option C (Hepatic circulation) is wrong because it refers to blood supply to and from the liver; Option D (Systemic circulation) is wrong because it refers to the circuit from the left ventricle through the entire body and back to the right atrium.

Correct Answer: C. Albumin

• **Albumin** = it is the most abundant protein in blood plasma (about 60% of total plasma proteins) and its large molecular size prevents it from crossing capillary walls, creating the colloid osmotic pressure (oncotic pressure) that pulls water back from tissues into the bloodstream. • **Clinical importance** — when albumin levels fall (hypoalbuminaemia, e.g., in liver disease or malnutrition), oncotic pressure drops and water accumulates in tissues, causing oedema (swelling). • Albumin also serves as a carrier protein for fatty acids, bilirubin, hormones, and drugs that are not water-soluble. • 💡 Option A (Platelets) is wrong because they are cell fragments that function in blood clotting and wound repair, not in osmotic pressure; Option B (Glucose) is wrong because while it contributes minimally to osmolarity, it freely crosses most capillaries and is not the primary determinant of plasma oncotic pressure; Option D (Hemoglobin) is wrong because it is located inside red blood cells, not in plasma, and therefore does not directly affect plasma osmotic pressure.