60 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. CO<sub>2</sub> 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 fluid connective tissue that transports materials around the body is:
Answer: A. blood
Why: Blood is a fluid connective tissue carrying gases, nutrients and wastes.
Q12.
The red colour of blood is due to the pigment:
Answer: A. haemoglobin
Why: Haemoglobin in red blood cells gives blood its red colour.
Q13.
The blood cells that carry oxygen are the:
Answer: A. red blood cells
Why: Red blood cells (erythrocytes) transport oxygen via haemoglobin.
Q14.
The organ that pumps blood through the body is the:
Answer: A. heart
Why: The heart is the muscular pump of the circulatory system.
Q15.
White blood cells mainly help in:
Answer: A. defence against disease
Why: White blood cells (leucocytes) defend the body against infection.
Q16.
Blood platelets play a key role in:
Answer: A. clotting of blood
Why: Platelets help blood to clot at the site of injury.
Q17.
The number of chambers in the human heart is:
Answer: A. four
Why: The human heart has four chambers: two atria and two ventricles.
Q18.
Blood vessels that carry blood away from the heart are the:
Answer: A. arteries
Why: Arteries carry blood away from the heart.
Q19.
Blood vessels that carry blood back toward the heart are the:
Answer: A. veins
Why: Veins return blood to the heart.
Q20.
The liquid (fluid) part of blood is called:
Answer: A. plasma
Why: Plasma is the pale-yellow liquid part of blood.
Q21.
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.
Q22.
Erythropoietin (EPO) stimulates:
Answer: B. Red blood cell production in bone marrow
Why: Erythropoietin (EPO): produced by kidneys in response to low O<sub>2</sub>. Stimulates red blood cell (erythrocyte) production in bone marrow.
Q23.
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.
Q24.
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.
Q25.
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.
Q26.
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.
Q27.
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.
Q28.
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.
Q29.
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.
Q30.
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.
Q31.
The two upper chambers of the heart are the:
Answer: A. atria
Why: The atria are the upper receiving chambers of the heart.
Q32.
The two lower chambers of the heart are the:
Answer: A. ventricles
Why: The ventricles are the lower pumping chambers.
Q33.
The largest artery in the human body is the:
Answer: A. aorta
Why: The aorta carries oxygenated blood from the heart to the body.
Q34.
Oxygenated blood is carried from the lungs to the heart by the:
Answer: A. pulmonary veins
Why: The pulmonary veins bring oxygen-rich blood from the lungs to the left atrium.
Q35.
The blood group known as the universal donor is:
Answer: A. O negative
Why: O-negative blood lacks A, B and Rh antigens, so it can be given to anyone.
Q36.
The blood group known as the universal recipient is:
Answer: A. AB positive
Why: AB-positive blood has no antibodies against A, B or Rh, so it can receive any type.
Q37.
The rhythmic sequence of contraction and relaxation of the heart is the:
Answer: A. cardiac cycle
Why: One heartbeat is a cardiac cycle of contraction (systole) and relaxation (diastole).
Q38.
The valves in the heart function to prevent the ___ of blood:
Answer: A. backflow
Why: Heart valves ensure blood flows in one direction only, preventing backflow.
Q39.
The metal present in the haemoglobin molecule is:
Answer: A. iron
Why: Each haemoglobin molecule contains iron, which binds oxygen.
Q40.
In double circulation, blood passes through the heart ___ during one complete circuit of the body:
Answer: A. twice
Why: In double circulation the blood goes through the heart twice per full circuit.
Q41.
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 Ca<sup>2+</sup> (exposing more troponin C sites) and reducing steric hindrance. More crossbridges form, increasing force.
Q42.
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.
Q43.
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 Ca<sup>2+</sup> channels near threshold
Why: SA node pacemaker potential: If (funny current through HCN channels) causes slow diastolic depolarization. T-type Ca<sup>2+</sup> channels then activate near threshold, followed by L-type Ca<sup>2+</sup> channels for action potential upstroke.
Q44.
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.
Q45.
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.
Q46.
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.
Q47.
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.
Q48.
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).
Q49.
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.
Q50.
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.
Q51.
The natural pacemaker of the heart is the:
Answer: A. SA node
Why: The sino-atrial (SA) node initiates each heartbeat.
Q52.
Blood pressure is measured with an instrument called the:
Answer: A. sphygmomanometer
Why: A sphygmomanometer measures arterial blood pressure.
Q53.
Normal human blood pressure is about:
Answer: A. 120/80 mm Hg
Why: A typical healthy reading is 120/80 mm Hg (systolic/diastolic).
Q54.
Deoxygenated blood returning from the body first enters the ___ of the heart:
Answer: A. right atrium
Why: The vena cava empties deoxygenated blood into the right atrium.
Q55.
The lymphatic system returns ___ from the tissues back to the blood:
Answer: A. tissue fluid
Why: Lymph (excess tissue fluid) is drained and returned to the bloodstream.
Q56.
The blood plasma protein essential for clotting is:
Answer: A. fibrinogen
Why: Fibrinogen is converted into fibrin threads that form the clot.
Q57.
The first heart sound "lubb" is produced by the closure of the:
Answer: A. atrioventricular valves
Why: "Lubb" is caused by the atrioventricular (bicuspid and tricuspid) valves closing.
Q58.
The Rh factor is a type of ___ found on the surface of red blood cells:
Answer: A. antigen
Why: The Rh factor is an antigen; people with it are Rh-positive.
Q59.
The proportion of blood volume made up of plasma is about:
Answer: A. 55%
Why: Plasma forms roughly 55% of blood volume, cells the remaining 45%.
Q60.
An electrocardiogram (ECG) records the ___ activity of the heart:
Answer: A. electrical
Why: An ECG traces the electrical impulses that drive each heartbeat.