NEET Body Fluids and Circulation Previous Year Questions with Solutions

A specialized cardiac musculature called nodal tissue is distributed in the heart. A patch of this tissue called the Sino-atrial node (SAN) is located in the upper right corner of the right atrium.

• • Nodal cells possess inherent autorhythmicity (spontaneous diastolic depolarization) without external neural innervation.

Different parts of the conducting system can generate action potentials at different inherent frequencies: SAN generates 70–75 min⁻¹, AVN generates 40–60 min⁻¹, and Purkinje fibers generate 20–40 min⁻¹. Because SAN possesses the highest intrinsic rate, it commands heart contraction.

• • Dictates the average resting human heart rate of ~72 beats per minute.

Option (1) gives 28-30 (characteristic of severe heart block / slow Purkinje rhythm). Option (3) gives 100-110 (tachycardia). Option (4) gives 120-140 (strenuous exercise). The basal rate is 70-75.

• • Matches official NTA final answer key Option B / (2).

When a blood vessel suffers trauma or injury, platelets and injured tissues release thromboplastin, triggering an enzymatic cascade leading to the formation of thrombokinase (prothrombinase).

• • Thrombokinase converts inactive plasma prothrombin into active thrombin.

Thrombin then acts as an enzyme to cleave soluble inactive fibrinogen into insoluble fibrin monomers, which polymerize into long threads trapping blood cells to form the clot (coagulum).

• • Therefore, Fibrin is derived from Fibrinogen, never Fibrinogen from Fibrin!

(1) Blood coagulates in response to injury: Correct physiological defense mechanism. (2) Blood clot consists of fibrins: Correct, fibrin meshwork forms the structural framework. (3) Fibrin is produced from fibrinogen: Correct. (4) Fibrinogen is produced from fibrin: Incorrect (inverted).

• • Matches official NTA final answer key Option D / (4).

The human heart possesses four chambers: two atria and two ventricles. To ensure unidirectional blood flow and prevent backflow during ventricular systole, the atrioventricular apertures are guarded by specialized fibrous valves.

• • NCERT verbatim: "The opening between the right atrium and the right ventricle is guarded by a valve formed of three muscular flaps or cusps, the tricuspid valve."

Bicuspid (mitral) valve guards the left atrioventricular aperture. Semilunar valves guard the openings of the right ventricle into the pulmonary artery and left ventricle into the systemic aorta. The sino-atrial node (SAN) is the neuromuscular pacemaker patch, not a mechanical valve.

• • Option (2) is the solely accurate response.

The tricuspid valve guards the right atrioventricular aperture.

• • Matches official NTA final answer key Option B / (2).

Normal activities of the human heart are regulated intrinsically, i.e., auto-regulated by specialized cardiac muscle tissues called nodal tissue (Sinoatrial node, Atrioventricular node, bundle of His, Purkinje fibers). Hence, human heart is termed myogenic. Thus, Statement A is correct.

A special neural center in the medulla oblongata moderates cardiac function through the autonomic nervous system (ANS):

• • Sympathetic neural signals increase the rate of heartbeat, the strength of ventricular contraction, and thereby the cardiac output.
• • Parasympathetic neural signals (via vagus nerve) decrease the rate of heartbeat, speed of action potential conduction, and cardiac output.
• • Thus, Statement B is correct.

Hormonal regulation of heart rate:

• • Adrenal medullary hormones (epinephrine and norepinephrine / catecholamines) act during emergency / fight-or-flight states to significantly increase the rate of heartbeat, stroke volume, and cardiac output. Thus, Statement C is correct.
• • Adrenal cortical hormones (glucocorticoids like cortisol and mineralocorticoids like aldosterone) are involved in carbohydrate metabolism, anti-inflammatory responses, and electrolyte balance (Na⁺/water retention in renal tubules), not direct cardiac pacing. Thus, Statement D is incorrect.

Only A, B, and C are true regulators of cardiac activities according to the canonical NCERT text.

The sino-atrial node (SAN) can generate the maximum number of action potentials, i.e., 70-75 min⁻¹, and is responsible for initiating and maintaining the rhythmic contractile activity of the heart. Therefore, it is called the pacemaker (Stage E).

The action potential spreads from the SAN to the atrio-ventricular node (AVN) situated in the lower left corner of the right atrium close to the atrio-ventricular septum (Stage C). From the AVN, an atrioventricular bundle (AV bundle / bundle of His) continues that divides into a right and left bundle (Stages A and D).

These branches give rise to minute fibres throughout the ventricular musculature of the respective sides called Purkinje fibres (Stage B), causing simultaneous ventricular contraction.

Each peak in the ECG is identified with a letter from P to T that corresponds to a specific electrical activity of the heart: - The P-wave represents the electrical excitation (or depolarisation) of the atria, which leads to the contraction of both the atria (A -> III). - The QRS complex represents the depolarisation of the ventricles, which initiates the ventricular contraction (B -> II). - The T-wave represents the return of the ventricles from excited to normal state (repolarisation) (C -> IV).

The interval between the end of the T wave and the beginning of the next P wave (T-P segment) represents the period of complete cardiac relaxation (joint diastole) during which ventricles and atria are relaxing and the heart is electrically silent (D -> I).

A-III, B-II, C-IV, D-I corresponds to Option (4).

A standard electrocardiogram represents the sequential propagation of action potentials across cardiac chambers:

NCERT states under Electrocardiograph (ECG): - "The P-wave represents the electrical excitation (or depolarisation) of the atria..." - "The QRS complex represents the depolarisation of the ventricles..." - "The contraction starts shortly after Q and marks the beginning of the systole." - "The T-wave represents the return of the ventricles from excited to normal state (repolarisation)."

Matching A-III immediately isolates Option (2), making this solvable within seconds.

Basophils are circulating granulocytes containing coarse basophilic granules (staining with basic dyes like methylene blue). Their granules release potent inflammatory vasoactive mediators:

Analyzing the errors in the false statements: - Statement A: Neutrophils are the most abundant ($60-65\%$). Basophils are the least abundant ($0.5-1\%$). - Statement D: Monocytes have a characteristic large kidney-shaped/bean-shaped nucleus. Basophils have an S-shaped or bilobed nucleus obscured by dark granules. - Statement E: Granulocytes include neutrophils, eosinophils, and basophils. Agranulocytes include lymphocytes and monocytes.

NCERT states verbatim: "Basophils are least (0.5-1 per cent) among them. Basophils secrete histamine, serotonin, heparin, etc., and are involved in inflammatory reactions."

The ABO blood group system is governed by surface oligosaccharide antigens on red blood cell membranes and naturally occurring isoantibodies in blood plasma: - Group A: Antigen A on RBCs; anti-B antibody in plasma. - Group B: Antigen B on RBCs; anti-A antibody in plasma. - Group AB: Both Antigen A and Antigen B on RBCs; NEITHER anti-A nor anti-B antibodies in plasma. - Group O: No A or B antigens on RBCs; both anti-A and anti-B antibodies in plasma.

When donor blood is transfused, the critical clinical danger is agglutination and haemolysis of the donor RBCs by antibodies present in the recipient's plasma. Because an individual with blood group AB has zero circulating anti-A and anti-B antibodies, transfused erythrocytes of type A, B, AB, or O encounter no opposing agglutinins, allowing safe transfusion without immune destruction.

Coagulation is an amplified biological cascade where inactive proenzymes are sequentially converted to active serine proteases: 1. Injured tissues and activated platelets release thromboplastin, forming the enzyme complex Thrombokinase (prothrombinase). 2. In the presence of ionic calcium ($Ca^{2+}$), thrombokinase catalyzes the proteolytic conversion of the plasma zymogen Prothrombin into active Thrombin. 3. Thrombin acts directly on soluble globular Fibrinogen (Factor I), cleaving fibrinopeptides A and B to yield insoluble fibrin monomers that polymerize and cross-link into a meshwork trapping blood cells to form the definitive clot.

- Renin: An aspartic protease secreted by juxtaglomerular cells of the kidney that converts angiotensinogen into angiotensin I (RAAS pathway). - Epinephrine (Adrenaline): An amino acid derivative hormone secreted by the adrenal medulla during fight-or-flight emergencies. - Thrombokinase: Activates prothrombin to thrombin; it does not directly convert fibrinogen to fibrin.

A standard ECG tracing records the electrical changes propagating through the myocardium during a cardiac cycle, consisting of a P wave, a QRS complex, and a T wave.

The QRS complex represents the electrical depolarisation of the ventricles, which initiates the ventricular contraction (ventricular systole shortly after Q).

The P-wave represents the electrical excitation (or depolarisation) of the atria, which leads to atrial contraction. The T-wave represents the return of the ventricles from excited to normal state (repolarisation of ventricles).

By counting the number of QRS complexes that occur in a given time period, one can determine the heart beat rate of an individual.

Eosinophils (2-3% of total WBCs) resist infections and are associated with allergic reactions; they contain granules releasing histaminase and destructive enzymes to combat parasitic infections. Thus, (a) matches (iii).

Basophils (0.5-1% of total WBCs) secrete histamine, serotonin, and heparin, and are involved in inflammatory reactions. Thus, (b) matches (iv).

Neutrophils (60-65% of total WBCs) are the most abundant phagocytic cells that engulf and destroy foreign microbes entering the body. Thus, (c) matches (ii).

Lymphocytes (20-25% of total WBCs) occur in two major types—B and T cells—both of which are responsible for the immune responses of the body (humoral and cell-mediated immunity). Thus, (d) matches (i).