Circulatory System — Set 1

Correct Answer: D. Pulmonary artery

• **Pulmonary artery** = it is the only artery in the body that carries oxygen-poor blood, pumped from the right ventricle to the lungs for re-oxygenation. • **Naming exception** — arteries are defined by direction of flow (away from the heart), not by oxygen content, which is why the pulmonary artery carries deoxygenated blood despite being an artery. • This vessel completes the pulmonary (right-side) circuit before oxygenated blood returns via the pulmonary veins. • 💡 Option A (Pulmonary vein) is wrong because it carries oxygenated blood back from the lungs to the left atrium; Option B (Aorta) is wrong because it carries oxygenated blood from the left ventricle to the entire body; Option C (Vena cava) is wrong because it carries deoxygenated blood from the body back to the right atrium, not to the lungs.

Correct Answer: A. 120/80 mmHg

• **120/80 mmHg** = the WHO and most medical guidelines define this as the standard normal blood pressure, where 120 is the systolic (heart contracting) and 80 is the diastolic (heart relaxing) pressure. • **Hypertension threshold** — readings consistently at or above 140/90 mmHg are classified as hypertension (Stage 1), making that option a disease state, not a healthy value. • Blood pressure is measured in millimetres of mercury (mmHg) using a sphygmomanometer. • 💡 Option B (100/60 mmHg) is wrong because it falls in the low blood pressure (hypotension) range; Option C (150/100 mmHg) is wrong because it indicates Stage 2 hypertension; Option D (140/90 mmHg) is wrong because it marks the threshold for Stage 1 hypertension.

Correct Answer: A. SA node

• **SA node** = the Sinoatrial node, located in the upper wall of the right atrium, spontaneously generates electrical impulses at 60–100 times per minute, setting the heart's natural rhythm. • **Automaticity** — unlike most heart cells, SA node cells can self-depolarize, which is why it is called the natural or inherent pacemaker. • When the SA node fails, the AV node takes over as a backup pacemaker at a slower rate of 40–60 beats per minute. • 💡 Option B (Purkinje fibers) is wrong because they only conduct impulses to the ventricular walls, they do not initiate them; Option C (Bundle of His) is wrong because it merely transmits impulses from the AV node to the ventricles; Option D (AV node) is wrong because it acts as a relay station and secondary pacemaker, not the primary one.

Correct Answer: A. Plasma

• **Plasma** = the pale-yellow liquid component of blood (about 55% of total volume) that carries dissolved nutrients, hormones, waste products like urea, and proteins throughout the body. • **Composition** — plasma is about 92% water, with the remaining 8% being proteins (albumin, globulins, fibrinogen), electrolytes, glucose, and metabolic waste. • It transports carbon dioxide from tissues to the lungs largely as bicarbonate ions dissolved in plasma. • 💡 Option B (Platelets) is wrong because platelets are cell fragments involved in blood clotting, not nutrient transport; Option C (Red blood cells) is wrong because their role is oxygen transport via haemoglobin; Option D (White blood cells) is wrong because their role is immune defence against pathogens.

Correct Answer: B. Left ventricle

• **Left ventricle** = it must pump oxygenated blood through the aorta to the entire body against high systemic resistance, requiring the greatest muscular force and therefore the thickest walls (about 1–1.5 cm). • **Pressure difference** — the left ventricle generates pressures around 120 mmHg, compared to only ~25 mmHg generated by the right ventricle, which only pumps to the nearby lungs. • The thick myocardium of the left ventricle is why heart attacks affecting this chamber are particularly life-threatening. • 💡 Option A (Right ventricle) is wrong because it only pumps blood to the lungs over a short distance at lower pressure; Option C (Left atrium) is wrong because atria receive blood at low pressure and require thinner walls; Option D (Right atrium) is wrong because it receives venous blood and does very little work.

Correct Answer: D. Pericardium

• **Pericardium** = the two-layered fibrous sac enclosing the heart that anchors it to surrounding structures, prevents over-distension, and provides a frictionless environment via pericardial fluid between its layers. • **Pericardial fluid** — about 15–50 mL of fluid between the visceral and parietal layers acts as a lubricant, reducing friction during every heartbeat. • Inflammation of the pericardium is called pericarditis, which causes chest pain aggravated by breathing. • 💡 Option A (Pleura) is wrong because it is the protective sac surrounding the lungs, not the heart; Option B (Endocardium) is wrong because it is the innermost lining of the heart chambers; Option C (Peritoneum) is wrong because it lines the abdominal cavity and its organs.

Correct Answer: B. O negative

• **O negative** = it lacks A antigens, B antigens, and the Rh factor on the surface of red blood cells, so its red cells do not trigger an immune reaction in any recipient regardless of their blood group. • **Emergency use** — O negative blood is kept in emergency units for use before a patient's blood type is determined, but it is rare (about 7% of the population). • The concept of universal donor applies only to red blood cell transfusions, not whole blood or plasma. • 💡 Option A (AB positive) is wrong because it has A, B, and Rh antigens and is actually the universal recipient; Option C (B positive) is wrong because it has B and Rh antigens that can cause reactions in A, O, and Rh-negative recipients; Option D (A negative) is wrong because the A antigen it carries would cause transfusion reactions in B and O group recipients.

Correct Answer: B. Stethoscope

• **Stethoscope** = it amplifies and transmits body sounds to the examiner's ears, allowing doctors to hear the characteristic 'lub-dub' sounds produced when heart valves close during the cardiac cycle. • **'Lub-dub' sounds** — 'lub' (S1) is produced by the closure of the mitral and tricuspid valves; 'dub' (S2) is produced by the closure of the aortic and pulmonary valves. • Abnormal sounds called murmurs detected by a stethoscope can indicate defective heart valves or septal defects. • 💡 Option A (Sphygmomanometer) is wrong because it measures blood pressure, not heart sounds; Option C (Thermometer) is wrong because it measures body temperature; Option D (ECG) is wrong because it records the electrical activity of the heart, not its sounds.

Correct Answer: C. Mitral valve

• **Mitral valve** = also called the bicuspid valve because it has two cusps (flaps), it guards the opening between the left atrium and left ventricle, preventing backflow of oxygenated blood into the atrium during ventricular contraction. • **Bicuspid vs. tricuspid** — the mitral valve on the left side has 2 leaflets, while the tricuspid valve on the right side (between the right atrium and right ventricle) has 3 leaflets. • Mitral valve prolapse and mitral stenosis are common cardiac conditions affecting this valve. • 💡 Option A (Tricuspid valve) is wrong because it sits between the right atrium and right ventricle; Option B (Pulmonary valve) is wrong because it is located between the right ventricle and the pulmonary artery; Option D (Aortic valve) is wrong because it is located between the left ventricle and the aorta.

Correct Answer: A. Immune defense

• **Immune defense** = leukocytes are the soldiers of the immune system; they identify, engulf, and destroy pathogens (bacteria, viruses, fungi), and also produce antibodies to neutralize foreign substances. • **Types** — there are five main types: neutrophils (most common, engulf bacteria), lymphocytes (produce antibodies and T-cells), monocytes, eosinophils, and basophils. • Normal WBC count is 4,000–11,000 per microlitre; elevated counts may indicate infection or leukaemia. • 💡 Option B (Nutrient absorption) is wrong because nutrients are absorbed by the intestinal villi and transported in plasma; Option C (Blood clotting) is wrong because that is the function of platelets (thrombocytes) and clotting factors; Option D (Oxygen transport) is wrong because red blood cells carry oxygen via haemoglobin.