All Free Biology MCQs with Answers

111. A normal resting blood pressure for a young adult is recorded as

• A. 80/120 mm Hg
• B. 120/80 mm Hg
• C. 200/100 mm Hg
• D. 60/40 mm Hg

Explanation: The larger figure is the systolic pressure during ventricular contraction and the smaller is the diastolic pressure while the ventricles are relaxed and filling, and the systolic value is always written first. A sustained reading at or above about 140/90 is hypertension, which strains the heart and damages arteries, kidneys and the retina. Reversing the two numbers is the classic careless error.

112. The hepatic portal vein is unusual because it

• A. carries oxygenated blood to the liver
• B. carries blood directly from the heart to the liver
• C. has thicker walls than any artery
• D. begins in the capillaries of the gut and ends in the capillaries of the liver

Explanation: A portal vessel runs between two capillary beds instead of returning blood to the heart, so absorbed glucose and amino acids reach the liver before entering the general circulation. This lets the liver regulate blood glucose and remove toxins straight after a meal. The blood it carries is deoxygenated, since it has already passed through the intestinal capillaries.

113. Lymph is formed from

• A. tissue fluid that drains into blind ending lymph capillaries
• B. blood plasma filtered by the kidney
• C. the secretion of the lymph nodes
• D. bile leaving the liver

Explanation: Plasma is forced out of the capillaries at their arterial end to become tissue fluid, most of which returns to the blood at the venous end, and the surplus enters lymph capillaries as lymph. It travels in one direction only, through nodes and along the thoracic duct, and is finally returned to the blood at the subclavian veins. Lymph therefore resembles plasma but has far less protein and no red cells.

114. The main function of lymph nodes is to

• A. pump lymph around the body
• B. produce red blood cells
• C. filter lymph and provide a site where lymphocytes multiply
• D. absorb glucose from the small intestine

Explanation: Nodes are packed with macrophages that trap bacteria and debris carried in the lymph, and with lymphocytes that divide there when an antigen is met, which is why nodes in the neck swell during a throat infection. The lymphatic system has no pump of its own and relies on skeletal muscle movement and valves. Red cell formation belongs to red bone marrow.

115. The lacteals of the intestinal villi are part of the lymphatic system and are responsible for absorbing

• A. glucose
• B. amino acids
• C. water and mineral ions
• D. the products of fat digestion

Explanation: Fatty acids and glycerol are reassembled into fats inside the villus cells and packaged into droplets too large for the blood capillaries, so they enter the lacteal instead and reach the blood later at the thoracic duct. Glucose and amino acids are water soluble and pass directly into the blood capillaries of the villus. This is why a lymph sample after a fatty meal looks milky.

116. Blockage of the lymph vessels, as occurs in filariasis, causes swelling of the limb because

• A. blood cannot leave the limb through the veins
• B. excess tissue fluid cannot be drained away and accumulates
• C. the arteries dilate and overfill the tissue
• D. plasma proteins are digested in the tissue spaces

Explanation: The lymphatic system is the only route by which the surplus tissue fluid returns to the blood, so blocking it lets fluid build up in the spaces between cells, and the result is the oedema seen in elephantiasis. Venous return is unaffected in this condition, which is why the swelling is soft and painless at first. The same reasoning explains the oedema of severe protein deficiency, where too little fluid is drawn back osmotically.

117. A mature human red blood cell

• A. has no nucleus and is biconcave in shape
• B. has a large nucleus and many mitochondria
• C. is amoeboid and can leave the capillaries
• D. lives for about ten days

Explanation: Losing the nucleus makes room for more haemoglobin, and the biconcave disc shape gives a large surface area for gas exchange and enough flexibility to squeeze through capillaries. Having no mitochondria means the cell cannot use the oxygen it carries. Its life span is about 120 days, after which it is broken down in the liver and spleen; white cells, not red, can squeeze out of capillaries.

118. The human circulation is described as a double circulation because

• A. the heart has two sides
• B. arteries and veins run side by side
• C. blood passes through the heart twice in one complete circuit of the body
• D. there are two types of blood cell

Explanation: Blood goes heart to lungs to heart in the pulmonary circuit and then heart to body to heart in the systemic circuit, so it passes through the pump twice for each full journey. The advantage is that pressure lost in the lung capillaries is restored before the blood is sent to the body, so the systemic circuit can be supplied at high pressure. A fish has a single circulation and cannot do this.

119. A myocardial infarction, or heart attack, is most often caused by

• A. failure of the sinoatrial node to fire
• B. leakage of the bicuspid valve
• C. an increase in the number of red blood cells
• D. blockage of a coronary artery by a thrombus on an atherosclerotic plaque

Explanation: Fatty plaques narrow a coronary artery over years, and when one ruptures a clot forms and cuts off the blood supply, so the muscle beyond it dies within minutes for lack of oxygen. Damaged cardiac muscle is not replaced, which is why the loss is permanent. Failure of the sinoatrial node disturbs rhythm rather than starving the muscle, and a leaking valve reduces efficiency without killing tissue.

120. The receptors of the retina that allow vision in dim light are the

• A. cones
• B. rods
• C. ganglion cells
• D. cells of the blind spot

Explanation: Rods contain the pigment rhodopsin, which is broken down by very low light intensities, and several rods share one sensory neurone so their effects add up, making them extremely sensitive. The price of that pooling is poor detail and no colour discrimination, which is why night vision is grey and blurred. Cones need bright light and give sharp colour vision instead.