30 free MCQs on Body Fluids and Circulation with worked answers and explanations. Blood, lymph, the heart, the cardiac cycle, blood pressure, and ECG. High NEET weightage.
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A single ECG cycle: the P wave marks atrial depolarisation (contraction signal), the large QRS complex marks ventricular depolarisation, and the T wave marks ventricular repolarisation (recovery) before the next cycle begins.
Q1.
The pumping chamber of the heart that sends blood to the body is:
Answer: D. Left ventricle
Why: The left ventricle pumps oxygenated blood to the body via the aorta (systemic circulation). It has the thickest walls.
Q2.
The main gas carried by red blood cells is:
Answer: C. Oxygen (via haemoglobin)
Why: RBCs contain haemoglobin which binds and transports oxygen from lungs to tissues. CO2 is also transported but mostly in plasma.
Q3.
Normal blood pressure is approximately:
Answer: B. 120/80 mmHg
Why: Normal blood pressure is 120/80 mmHg (systolic/diastolic). Above 140/90 is hypertension.
Q4.
Haemoglobin contains which metal?
Answer: C. Iron
Why: Haemoglobin contains iron (Fe) in its heme group. Iron binds to oxygen. Iron deficiency causes anaemia.
Q5.
Which blood cells fight infection?
Answer: C. White blood cells
Why: White blood cells (WBCs/leukocytes) are part of the immune system. They fight infections and foreign particles.
Q6.
Double circulation means blood passes through the heart:
Answer: B. Twice per circuit (to lungs AND to body)
Why: Humans have double circulation: pulmonary (heart to lungs and back) and systemic (heart to body and back). Blood passes through the heart twice per circuit.
Q7.
Platelets are important for:
Answer: C. Blood clotting
Why: Platelets (thrombocytes) are involved in blood clotting. They aggregate at wound sites and release clotting factors.
Q8.
Which blood vessels carry blood away from the heart to the rest of the body?
Answer: C. Arteries
Why: Arteries are blood vessels that carry blood away from the heart, typically under high pressure, to tissues throughout the body.
Q9.
The fluid part of blood, in which blood cells are suspended, is called:
Answer: D. Plasma
Why: Plasma is the straw-coloured fluid matrix of blood that carries blood cells, nutrients, hormones, and waste products throughout the body.
Q10.
Red blood cells (erythrocytes) mainly function to:
Answer: A. Transport oxygen to body tissues
Why: Red blood cells contain haemoglobin, which binds oxygen in the lungs and transports it to tissues throughout the body.
Q11.
The sinoatrial (SA) node is called the pacemaker because:
Answer: B. It initiates electrical impulses that set heart rate (~72 bpm)
Why: SA node (in right atrium wall) generates spontaneous electrical impulses at ~72/min, setting the heart rate. It is the natural pacemaker.
Q12.
Erythropoietin (EPO) stimulates:
Answer: B. Red blood cell production in bone marrow
Why: Erythropoietin (EPO): produced by kidneys in response to low O2. Stimulates red blood cell (erythrocyte) production in bone marrow.
Q13.
The refractory period after a heartbeat prevents:
Answer: B. Tetanic (continuous) contraction of the heart (ensures rhythmic pumping)
Why: The refractory period of cardiac muscle cells prevents tetanus. The heart must relax between beats to fill with blood for the next contraction.
Q14.
The Frank-Starling law of the heart states:
Answer: B. Greater cardiac filling → greater stretch of ventricle → greater force of contraction
Why: Frank-Starling law: the more ventricular muscle is stretched by filling (preload), the greater the subsequent contraction force and stroke volume.
Q15.
The lub sound of the heartbeat (S1) is mainly produced by the closure of the:
Answer: C. Tricuspid and bicuspid (atrioventricular) valves
Why: The first heart sound (lub) results from the closure of the atrioventricular valves (tricuspid and bicuspid) at the start of ventricular systole.
Q16.
A blood pressure reading of 160/100 mm Hg, when persistently elevated, is referred to as:
Answer: D. Hypertension, a rise above the normal range
Why: Hypertension (high blood pressure) is diagnosed when blood pressure is persistently above the normal range of about 120/80 mm Hg, raising risk of heart disease.
Q17.
Lymph differs in composition from blood plasma mainly in that lymph:
Answer: A. Carries less protein and fewer red blood cells than plasma
Why: Lymph is formed from tissue fluid and contains less protein than plasma, along with fewer red blood cells, though it does carry lymphocytes.
Q18.
During the cardiac cycle, the period when both atria and ventricles are relaxed is called:
Answer: A. Joint diastole, when both chambers rest together
Why: During joint diastole, both the atria and ventricles are relaxed simultaneously, allowing blood to passively fill the atria before the next cycle begins.
Q19.
An electrocardiogram (ECG) records the:
Answer: B. Electrical activity of the heart through a cardiac cycle
Why: An ECG is a graphical record of the electrical changes that occur during a cardiac cycle, showing characteristic P, QRS and T waves.
Q20.
Cardiac output is calculated as the product of:
Answer: D. Stroke volume and heart rate
Why: Cardiac output = stroke volume x heart rate, representing the total volume of blood pumped by one ventricle per minute.
Q21.
The Frank-Starling mechanism operates via:
Answer: B. Length-dependent activation of cardiac myofilaments: increased stretch exposes more troponin C binding sites and improves actin-myosin overlap
Why: Frank-Starling: increased ventricular filling stretches sarcomeres, increasing myofilament sensitivity to Ca2+ (exposing more troponin C sites) and reducing steric hindrance. More crossbridges form, increasing force.
Q22.
Nitric oxide (NO) as a vasodilator acts by:
Answer: B. Activating soluble guanylyl cyclase in VSM cells, increasing cGMP, which activates PKG, leading to smooth muscle relaxation
Why: NO from endothelial cells diffuses into vascular smooth muscle (VSM). Activates soluble guanylyl cyclase → cGMP rises → PKG activated → MLCK inhibited + K+ channels opened → smooth muscle relaxes → vasodilation.
Q23.
Pacemaker cells (SA node) depolarize spontaneously due to:
Answer: B. Funny current (If, HCN channels) carrying Na+/K+ inward current during diastole, plus T-type Ca2+ channels near threshold
Why: SA node pacemaker potential: If (funny current through HCN channels) causes slow diastolic depolarization. T-type Ca2+ channels then activate near threshold, followed by L-type Ca2+ channels for action potential upstroke.
Q24.
Einthoven's triangle in ECG refers to:
Answer: B. The three limb leads (I, II, III) forming an equilateral triangle around the heart for measuring cardiac electrical activity from different angles
Why: Einthoven's triangle: limb leads I (right arm-left arm), II (right arm-left leg), III (left arm-left leg) form a triangle around the heart. Different angles capture different aspects of electrical axis.
Q25.
During the cardiac cycle, the 'dub' sound of the heartbeat is produced by:
Answer: D. Closure of the semilunar (aortic and pulmonary) valves at the start of diastole
Why: The second heart sound ('dub') is produced by the closure of the semilunar valves (aortic and pulmonary) at the beginning of ventricular diastole.
Q26.
A P wave in an ECG corresponds to:
Answer: B. Depolarization of the atria, just before atrial contraction
Why: The P wave in an ECG represents the electrical depolarization of the atria, which precedes and triggers atrial contraction.
Q27.
Lymph differs from blood plasma mainly in that lymph:
Answer: C. Contains much less protein than blood plasma
Why: Lymph is formed from tissue fluid and has a lower protein content than blood plasma, since large plasma proteins do not readily filter out of blood capillaries into the surrounding tissue spaces.
Q28.
Cardiac output is the product of:
Answer: B. Heart rate and stroke volume
Why: Cardiac output (volume of blood pumped per minute) equals heart rate multiplied by stroke volume (volume ejected per beat).
Q29.
The dicrotic notch seen on an arterial pressure tracing is caused by:
Answer: A. A brief backflow of blood against the closed aortic valve during early diastole
Why: The dicrotic notch is a small dip and rebound in the arterial pressure trace, caused by elastic recoil of the aortic wall and brief backflow of blood against the recently closed aortic valve.
Q30.
Baroreceptors located in the aortic arch and carotid sinus help regulate blood pressure by:
Answer: B. Sensing arterial stretch and triggering reflex changes in heart rate and vessel diameter
Why: Baroreceptors detect stretch in arterial walls caused by changes in blood pressure and send signals to the medulla, triggering reflex changes in heart rate and vessel diameter to maintain blood pressure.