All Free Biology MCQs with Answers

521. Blood pressure is lowest in the

• A. aorta
• B. arteries
• C. arterioles
• D. venae cavae

Explanation: Pressure falls steadily through arteries, arterioles and capillaries as energy is lost to friction, reaching near zero in the great veins. This is why venous return depends on valves, muscle contraction and breathing rather than on pressure.

522. The build up of fatty deposits called plaques within the walls of arteries is known as

• A. atherosclerosis
• B. arthritis
• C. anaemia
• D. haemophilia

Explanation: Plaques of cholesterol and debris narrow and stiffen arteries, raising blood pressure and predisposing to clots that cause heart attacks and strokes. Controlling diet, smoking and blood pressure slows the process.

523. The largest lymphoid organ of the body, which filters blood and removes worn out red blood cells, is the

• A. tonsils
• B. spleen
• C. thymus
• D. lymph node

Explanation: The spleen sits in the abdomen and acts on blood the way lymph nodes act on lymph, trapping pathogens and recycling old erythrocytes. Its size and blood filtering role distinguish it from the other lymphoid organs.

524. The tonsils are lymphoid tissues located

• A. along the spinal cord
• B. in the thorax around the heart
• C. in the throat, guarding the entrances of the respiratory and digestive tracts
• D. inside the bone marrow

Explanation: Their position at the back of the mouth and nose lets them sample inhaled and swallowed material and mount early immune responses. This sentinel role explains why they swell during throat infections.

525. The lymphatic system finally returns lymph to the blood at the

• A. capillaries of the tissues
• B. aorta
• C. pulmonary vein
• D. subclavian veins near the neck

Explanation: The thoracic duct and right lymphatic duct empty lymph into the subclavian veins, restoring fluid and proteins that leaked from the capillaries. Without this return, tissues would swell with trapped fluid.

526. The thymus gland is essential for immunity because it is the site where

• A. T lymphocytes mature
• B. red blood cells are produced
• C. antibodies are stored
• D. platelets are formed

Explanation: Immature lymphocytes from the bone marrow migrate to the thymus, where they learn to distinguish self from non self. This education is why a defective thymus causes severe immune deficiency in childhood.

527. Lymph differs from blood in that lymph

• A. contains more red blood cells than blood
• B. has no red blood cells and less protein
• C. flows away from the heart through arteries
• D. contains no white blood cells

Explanation: Lymph is essentially filtered tissue fluid, so it lacks erythrocytes and is poorer in plasma proteins, though it carries lymphocytes. It moves towards the heart in one direction, unlike blood in arteries.

528. Lymph nodes often swell during an infection because

• A. they fill with excess tissue fluid
• B. bacteria dissolve inside them
• C. lymphocytes multiply rapidly inside them to fight the pathogen
• D. their valves close and trap lymph

Explanation: Trapped antigens trigger lymphocyte proliferation within the node, enlarging it and making it tender. Swollen glands in the neck during a sore throat are immune factories at work, not fluid accumulation.

529. Like veins, the larger lymphatic vessels contain

• A. thick muscular walls like arteries
• B. red blood cells
• C. pacemakers
• D. valves that keep the lymph moving in one direction

Explanation: Lymph flows under very low pressure, so valves prevent backflow while surrounding muscle contractions squeeze it onwards. This one way design is why lymphoedema develops when the vessels are blocked.

530. Receptors that respond to changes in temperature are called

• A. photoreceptors
• B. thermoreceptors
• C. chemoreceptors
• D. mechanoreceptors

Explanation: Thermoreceptors in the skin and hypothalamus detect heat and cold and feed the thermoregulatory centre. Photoreceptors respond to light, chemoreceptors to chemicals and mechanoreceptors to pressure and touch, so each stimulus has its own receptor family.