Circulatory System — Set 5

Correct Answer: D. Serum

• **Serum** = the clear, yellowish liquid that remains after blood has clotted and the clot (containing fibrin, platelets, and blood cells) has been removed; it is essentially plasma minus the clotting factors (fibrinogen, prothrombin, etc.). • **Uses of serum** — blood serum is used in diagnostic tests (serological tests, antibody detection), and vaccines/antitoxins are often prepared as serum from immunized animals. • The key distinction: Plasma = blood liquid WITH clotting factors; Serum = blood liquid WITHOUT clotting factors. • 💡 Option A (Lymph) is wrong because it is the fluid that drains from tissues into the lymphatic vessels, not derived from clotted blood; Option B (Cytosol) is wrong because it is the liquid component inside cells, not blood; Option C (Plasma) is wrong because plasma still contains clotting factors like fibrinogen, making it distinct from serum.

Correct Answer: A. Capillary

• **Capillary** = the smallest and thinnest-walled blood vessels in the body, with walls only one cell thick (a single layer of endothelial cells), allowing efficient exchange of oxygen, carbon dioxide, nutrients, and waste products between blood and surrounding tissues. • **Diameter ~5–10 µm** — capillaries form dense networks (capillary beds) in all tissues; red blood cells must squeeze through them in single file, maximizing surface contact for gas exchange. • Capillaries connect arterioles (small arteries) to venules (small veins), completing the circuit between supply and return. • 💡 Option B (Arteriole) is wrong because arterioles have smooth muscle in their walls to regulate blood flow and are thicker than capillaries; Option C (Aorta) is wrong because it has the thickest walls of any vessel, with three layers including thick elastic and muscular tissue; Option D (Vena cava) is wrong because as the largest vein it also has multilayered walls, much thicker than capillaries.

Correct Answer: C. Fibrin

• **Fibrin** = an insoluble, thread-like protein formed when the enzyme thrombin cleaves fibrinogen (a soluble plasma protein), creating a mesh-like network that traps red blood cells and platelets to form a stable blood clot (thrombus). • **Clotting cascade** — the sequence is: damaged tissue → prothrombin activated to thrombin → thrombin converts fibrinogen to fibrin → fibrin mesh seals the wound. • Vitamin K is essential for producing several clotting factors; its deficiency causes excessive bleeding. • 💡 Option A (Prothrombin) is wrong because prothrombin is the inactive precursor that gets converted to thrombin, not the final clot material; Option B (Thrombin) is wrong because thrombin is the enzyme that acts on fibrinogen — it is not the product of fibrinogen conversion; Option D (Platelet) is wrong because platelets are cell fragments already present in blood and are not formed from fibrinogen.

Correct Answer: C. Right atrium and Right ventricle

• **Right atrium and Right ventricle** = the tricuspid valve (having three cusps/flaps) is positioned on the right side of the heart, separating the right atrium from the right ventricle, and prevents backflow of blood when the ventricle contracts. • **Function** — when the right atrium fills with deoxygenated blood from the body, the tricuspid valve opens to let it flow into the right ventricle; it then closes when the ventricle pumps blood toward the lungs via the pulmonary artery. • Tricuspid valve disease (stenosis or regurgitation) is often caused by rheumatic fever and disrupts normal blood flow on the right side. • 💡 Option A (Left ventricle and Aorta) is wrong because they are separated by the aortic semilunar valve; Option B (Left atrium and Left ventricle) is wrong because they are separated by the bicuspid (mitral) valve, which has only two cusps; Option D (Right ventricle and Pulmonary artery) is wrong because they are separated by the pulmonary semilunar valve.

Correct Answer: B. Arteries

• **Arteries** = blood vessels that carry blood away from the heart (the word 'artery' comes from Greek meaning 'air-carrier'); all arteries except the pulmonary artery carry oxygenated blood to organs and tissues throughout the body. • **Thick, elastic walls** — arteries have three layers (tunica intima, media, adventitia) with abundant smooth muscle and elastic tissue to withstand and dampen the high pressure generated by each heartbeat. • The exception is the pulmonary artery, which carries deoxygenated blood from the right ventricle to the lungs — arteries are defined by direction (away from heart), not oxygen content. • 💡 Option A (Veins) is wrong because veins carry blood toward the heart, typically deoxygenated blood returning from the body; Option C (Capillaries) is wrong because they are microscopic vessels where exchange occurs between blood and tissues, not the main transport route; Option D (Lymphatics) is wrong because lymphatic vessels carry lymph fluid, not blood.

Correct Answer: B. It increases

• **It increases** = during vigorous exercise, working muscles demand more oxygen and produce more CO₂; the nervous system (sympathetic branch) and hormones (especially adrenaline) signal the heart to beat faster and harder to meet this demand. • **Cardiac output = Heart rate × Stroke volume** — a trained adult's resting heart rate is ~60–70 bpm, but during intense exercise it can reach 180–200 bpm, increasing cardiac output from ~5 L/min at rest to ~20–25 L/min during exercise. • Regular aerobic exercise trains the heart to pump more blood per beat (higher stroke volume), which is why athletes have lower resting heart rates (bradycardia). • 💡 Option A (remains constant) is wrong because the heart dynamically responds to the body's oxygen needs and does not maintain the same rate during exercise; Option C (stops temporarily) is wrong because this would cause fainting or death; Option D (decreases) is wrong because a decrease would reduce oxygen delivery to muscles, which is the opposite of what is needed.

Correct Answer: C. Vena cava

• **Vena cava** = the largest vein in the human body, comprising two major vessels — the superior vena cava (returning deoxygenated blood from the head, neck, arms, and upper chest) and the inferior vena cava (returning blood from the lower body, abdomen, and legs) — both emptying into the right atrium. • **Diameter ~2 cm** — the inferior vena cava is the larger of the two; it runs along the right side of the spine and is one of the largest blood vessels in the body. • Obstruction of the vena cava (superior vena cava syndrome) can cause facial swelling, headache, and breathing difficulty. • 💡 Option A (Saphenous vein) is wrong because it is the longest vein in the body (running the full length of the leg) but not the largest in diameter; Option B (Renal vein) is wrong because it drains only the kidneys and is much smaller; Option D (Jugular vein) is wrong because it drains the head and neck region and is significantly smaller than the vena cava.

Correct Answer: A. Rhesus monkey

• **Rhesus monkey** = the Rh (Rhesus) factor is named after the Rhesus macaque (Macaca mulatta), as it was first discovered in 1937 by Karl Landsteiner and Alexander Wiener when they injected Rhesus monkey red blood cells into rabbits and guinea pigs, producing an antibody that also reacted with certain human blood cells. • **Rh+ vs Rh−** — about 85% of humans are Rh-positive (have the Rh antigen on their RBCs) and 15% are Rh-negative; the Rh factor is critical in blood transfusions and pregnancy (Rh incompatibility between mother and fetus can cause hemolytic disease of the newborn). • The full blood typing system combines ABO and Rh: e.g., A+, B−, O+, AB−, giving 8 common blood types. • 💡 Option B (Rat) is wrong because rats were not involved in the discovery of the Rh factor; Option C (Rabbit) is wrong because rabbits were used in the experiment as recipients, not the source of the naming; Option D (Rhino) is wrong because despite the similar 'Rh' abbreviation, rhinos had no role in this discovery.

Correct Answer: D. Septum

• **Septum** = a thick muscular wall that divides the heart longitudinally into the left side (oxygenated blood) and the right side (deoxygenated blood), preventing any mixing between the two circuits — the interventricular septum separates the two ventricles and the interatrial septum separates the two atria. • **Congenital defects** — a hole in the septum (ventricular septal defect or atrial septal defect) is one of the most common congenital heart defects, causing oxygenated and deoxygenated blood to mix, reducing oxygen delivery to the body. • The septum is part of the four-chambered heart structure that evolved in mammals and birds for complete separation of pulmonary and systemic circulations. • 💡 Option A (Valve) is wrong because valves prevent backflow of blood between chambers or into vessels, not mixing between left and right sides; Option B (Ventricle) is wrong because ventricles are the pumping chambers themselves, not the dividing structure; Option C (Atrium) is wrong because atria are receiving chambers, not the structure that prevents mixing.

Correct Answer: A. Supply blood to the heart muscle

• **Supply blood to the heart muscle** = the coronary arteries (left and right) are the first branches of the aorta arising just above the aortic valve, and they deliver oxygenated blood directly to the myocardium (heart muscle), which cannot extract oxygen from the blood passing through its chambers. • **Two main coronary arteries** — the left coronary artery (further dividing into the left anterior descending and circumflex arteries) and the right coronary artery together supply the entire heart muscle; blockage of a coronary artery causes a heart attack (myocardial infarction). • The heart receives about 5% of total cardiac output despite being a relatively small organ, reflecting its enormous energy demands. • 💡 Option B (Supply blood to liver) is wrong because the liver receives blood from the hepatic artery (a branch of the celiac trunk) and the portal vein; Option C (Supply blood to brain) is wrong because the brain is supplied by the carotid and vertebral arteries; Option D (Supply blood to lungs) is wrong because oxygenation of the lungs' own tissue is done by bronchial arteries, while the pulmonary arteries carry deoxygenated blood for gas exchange.