Circulatory System — Set 2

Correct Answer: C. Calcium

• **Calcium (Ca²⁺)** = calcium ions act as an indispensable cofactor at multiple steps of the coagulation cascade, including the conversion of prothrombin to thrombin and fibrinogen to fibrin. • **Factor IV** — calcium is officially listed as Factor IV in the blood clotting cascade, one of only 13 numbered coagulation factors. • This is why blood banks store donated blood in citrate solution — citrate chelates calcium ions to prevent the blood from clotting during storage. • 💡 Option A (Sodium) is wrong because it regulates fluid balance and nerve impulses, not coagulation; Option B (Iron) is wrong because it is part of haemoglobin and involved in oxygen transport, not clotting; Option D (Magnesium) is wrong because while it is a cofactor for many enzymes, it is not part of the clotting cascade.

Correct Answer: D. Pulse

• **Pulse** = each time the left ventricle contracts, it sends a wave of increased pressure through the arteries; this pressure wave causes the arterial walls to expand and recoil rhythmically, which is felt as the pulse. • **Radial artery** — the pulse is most commonly measured at the radial artery of the wrist; normal resting pulse in adults is 60–100 beats per minute. • Pulse rate equals heart rate in a healthy person, but in arrhythmias a 'pulse deficit' can occur where some heartbeats are too weak to produce a palpable pulse. • 💡 Option A (Heartbeat) is wrong because it refers to the actual contraction of the heart muscle itself, not the peripheral wave felt in arteries; Option B (Stroke volume) is wrong because it is the volume of blood ejected per beat, not a physical sensation; Option C (Blood pressure) is wrong because it is the force exerted by blood on vessel walls, measured by a sphygmomanometer.

Correct Answer: D. Pulmonary vein

• **Pulmonary vein** = the four pulmonary veins (two from each lung) are the only veins in the human body that carry oxygen-rich blood, delivering it from the alveolar capillaries to the left atrium. • **Naming exception** — veins are defined by direction of flow (toward the heart), not by oxygen content, making the pulmonary vein unique for carrying oxygenated blood. • After entering the left atrium, the oxygenated blood moves to the left ventricle, which then pumps it out through the aorta to the body. • 💡 Option A (Hepatic artery) is wrong because it supplies oxygenated blood to the liver from the abdominal aorta; Option B (Superior vena cava) is wrong because it drains deoxygenated blood from the upper body to the right atrium; Option C (Pulmonary artery) is wrong because it carries deoxygenated blood from the right ventricle to the lungs.

Correct Answer: D. Hemoglobin

• **Hemoglobin** = a metalloprotein inside red blood cells composed of four globin chains, each carrying a haem group with an iron (Fe²⁺) atom that reversibly binds one oxygen molecule, allowing each haemoglobin molecule to carry up to four O₂. • **Color significance** — when haemoglobin is bound to oxygen (oxyhaemoglobin) it is bright red; when it releases oxygen (deoxyhaemoglobin) it becomes dark red/purple, explaining why venous blood looks darker. • A healthy adult has about 14–18 g/dL (males) or 12–16 g/dL (females) of haemoglobin; low levels cause anaemia. • 💡 Option A (Globulin) is wrong because it is a plasma protein involved in immune defence (antibodies), not oxygen transport; Option B (Fibrinogen) is wrong because it is a clotting protein that converts to fibrin during coagulation; Option C (Albumin) is wrong because it maintains osmotic pressure and transports non-water-soluble substances, not oxygen.

Correct Answer: B. Diastole

• **Diastole** = the phase of the cardiac cycle when the heart muscle relaxes, allowing the chambers (especially the ventricles) to passively fill with blood; the diastolic pressure (80 mmHg in normal BP) is recorded during this phase. • **Cardiac cycle duration** — at a heart rate of 75 beats/min, each cardiac cycle lasts about 0.8 seconds; diastole lasts roughly 0.5 seconds and systole about 0.3 seconds. • During diastole, the SA node generates the next impulse, preparing the heart for the next contraction. • 💡 Option A (Systole) is wrong because it is the contraction phase when the heart actively pumps blood out; Option C (Hypertension) is wrong because it is a chronic condition of persistently high blood pressure, not a phase of the cardiac cycle; Option D (Fibrillation) is wrong because it describes a pathological state of rapid, disorganised electrical activity in the heart.

Correct Answer: D. Lymphatic system

• **Lymphatic system** = a network of lymphatic vessels, lymph nodes, and organs that collects the excess interstitial fluid (about 3 litres per day) that leaks out of blood capillaries, filters it, and returns it to the bloodstream via the thoracic duct. • **Immune function** — lymph nodes filter lymph fluid, trapping bacteria and cancer cells; lymphocytes within nodes mount immune responses against pathogens. • The lymphatic system also absorbs dietary fats from the small intestine through specialised vessels called lacteals. • 💡 Option A (Respiratory system) is wrong because it exchanges gases (O₂ and CO₂), not tissue fluid; Option B (Digestive system) is wrong because it breaks down food and absorbs nutrients; Option C (Nervous system) is wrong because it transmits electrical signals to coordinate body functions.

Correct Answer: C. Erythrocytes

• **Erythrocytes** = from the Greek erythros (red) + kytos (cell); they are biconcave disc-shaped cells that lack a nucleus and mitochondria at maturity, making them entirely dedicated to oxygen transport via haemoglobin. • **Biconcave shape advantage** — the concave disc shape increases surface area to volume ratio compared to a sphere, maximising the rate of oxygen diffusion into and out of the cell. • There are approximately 4.5–5.5 million erythrocytes per microlitre of blood; they are produced in the red bone marrow and live for about 120 days. • 💡 Option A (Leukocytes) is wrong because leukocytes are white blood cells responsible for immune defence; Option B (Lymphocytes) is wrong because they are a subtype of white blood cells that produce antibodies; Option D (Thrombocytes) is wrong because thrombocytes are another name for platelets, the cell fragments involved in clotting.

Correct Answer: B. Veins

• **Veins** = they contain semilunar (pocket) valves at intervals along their length that open to allow blood to flow toward the heart and snap shut when blood begins to flow backward, acting like one-way gates. • **Need for valves** — unlike arteries, veins carry blood at low pressure and often against gravity (e.g., from legs to heart), so valves are essential to prevent pooling of blood. • Varicose veins develop when these venous valves become incompetent and fail to close properly, causing blood to pool and veins to bulge. • 💡 Option A (Arteries) is wrong because blood in arteries is driven by high pressure from the heart's pumping action and does not need valves; Option C (Capillaries) is wrong because they are extremely thin single-cell-layer vessels where diffusion occurs, with no valves; Option D (Arterioles) is wrong because they are small arteries that regulate blood flow into capillaries by contracting or dilating, not by valves.

Correct Answer: C. Liver

• **Liver** = the hepatic portal vein is a unique vessel that carries nutrient-rich blood absorbed from the stomach and intestines directly to the liver, so the liver can process glucose, amino acids, lipids, and detoxify any absorbed harmful substances before they reach the general circulation. • **First-pass effect** — this liver processing is called the first-pass effect and explains why orally taken drugs are metabolised by the liver before reaching the bloodstream. • The hepatic portal system also delivers venous blood from the spleen and pancreas to the liver. • 💡 Option A (Kidney) is wrong because kidneys receive blood from the renal arteries for filtration, not from the portal vein; Option B (Spleen) is wrong because the spleen drains into the portal vein rather than receiving from it; Option D (Lungs) is wrong because lungs receive deoxygenated blood via the pulmonary artery, not from the digestive tract.

Correct Answer: A. Vitamin K

• **Vitamin K** = the liver requires Vitamin K as a cofactor to synthesise clotting factors II (prothrombin), VII, IX, and X, as well as anticoagulant proteins C and S — all of which depend on a carboxylation reaction catalysed by Vitamin K. • **Sources** — Vitamin K is obtained from green leafy vegetables (K1/phylloquinone) and is also produced by gut bacteria (K2/menaquinone). • Warfarin (an anticoagulant drug) works by blocking Vitamin K recycling in the liver, thereby reducing the production of clotting factors. • 💡 Option B (Vitamin C) is wrong because it is essential for collagen synthesis and immune function, not blood clotting; Option C (Vitamin D) is wrong because it regulates calcium and phosphorus metabolism for bone health; Option D (Vitamin A) is wrong because it is needed for vision, immune function, and cell growth, not clotting factor synthesis.