Free Circulation MCQs with Answers

51. Which blood vessel has the thickest muscular wall?

• A. Vena cava
• B. Pulmonary vein
• C. Aorta
• D. Capillary

Explanation: The aorta has the thickest wall to withstand the high pressure of blood ejected from the left ventricle.

52. Blood clotting requires

• A. Vitamin A
• B. Vitamin K
• C. Vitamin C
• D. Vitamin D

Explanation: Vitamin K is required for synthesis of clotting factors II, VII, IX, and X in the liver.

53. The QRS complex in ECG represents

• A. Atrial depolarization
• B. Ventricular depolarization
• C. Atrial repolarization
• D. Ventricular repolarization

Explanation: The QRS complex represents ventricular depolarization (contraction); the P wave is atrial depolarization.

54. Which WBC is most abundant?

• A. Lymphocytes
• B. Monocytes
• C. Neutrophils
• D. Eosinophils

Explanation: Neutrophils are the most abundant WBCs (60-70%), functioning as primary phagocytes against bacteria.

55. Tissue fluid is formed by

• A. Active transport
• B. Ultrafiltration from capillaries
• C. Secretion by cells
• D. Osmosis only

Explanation: Tissue fluid forms by ultrafiltration from arterial ends of capillaries due to hydrostatic pressure.

56. Cardiac output equals

• A. Heart rate + Stroke volume
• B. Heart rate × Stroke volume
• C. Heart rate - Stroke volume
• D. Stroke volume only

Explanation: Cardiac output (mL/min) = Heart rate (beats/min) × Stroke volume (mL/beat).

57. Which is NOT a function of lymphatic system?

• A. Return tissue fluid to blood
• B. Fat absorption
• C. Immune defense
• D. Oxygen transport

Explanation: Oxygen transport is a function of blood, not lymph. Lymph returns tissue fluid, absorbs fats, and contains immune cells.

58. Fibrinogen is converted to fibrin by

• A. Prothrombin
• B. Thrombin
• C. Calcium
• D. Platelets

Explanation: Thrombin converts soluble fibrinogen to insoluble fibrin, forming the mesh of blood clots.

59. The first consequence of lymphatic blockage in tissues is:

• A. Deficiency of oxygen
• B. Loss of nerve signals
• C. Drop in blood pressure
• D. Excess fluid accumulation in tissue

Explanation: The lymphatic system is the only route by which surplus tissue fluid returns to the blood, so blocking it lets fluid collect between the cells and the limb swells, which is the oedema of filariasis. Blood flow through the arteries and capillaries continues normally, so oxygen delivery and blood pressure are not affected first. The swelling is soft and painless in the early stages for exactly this reason.

60. Which of the following blood vessels have lowest blood velocity with correct reason for low blood velocity?

• A. Arteries: Due to thick walls
• B. Capillaries: due to highest overall cross-sectional area
• C. Veins: due to blood flow against the gravity
• D. Capillaries: Due to two-way blood flow in capillaries

Explanation: Although each capillary is tiny, there are so many in parallel that their combined cross sectional area is far greater than that of the aorta, and since the same volume must pass every second, the velocity falls to about a millimetre per second. That slow flow is exactly what exchange requires, giving time for diffusion. The fourth option names the right vessel but an invented reason, since capillary flow is one way.