Body Fluids and Circulation for NEET: Blood, Lymph and the Cardiac Cycle

Body fluids and circulation isn't just theory—it's worth 8-12 marks on NEET, and the questions are notoriously specific. You could lose 4-5 marks easily by confusing plasma composition, misremembering valve names, or mixing up systolic and diastolic pressures. This chapter (Chapter 18 in NCERT Biology Class 11) sits at the intersection of three major topics: what's IN your blood, what it does, and HOW it moves. Understand the mechanics deeply, and you'll crack almost every variation the exam throws at you.

Understanding Blood Composition and Plasma Components

NEET loves asking about blood composition in two formats: percentage-based MCQs and reasoning questions about WHY certain components matter. Let's break this down the way examiners test it.

Blood is roughly 55% plasma and 45% cells by volume—but that 55% plasma is where most students falter. Plasma isn't just water. It contains 7-8% solutes, distributed as:

• Proteins (6-8%): Mainly albumin (produced in liver, maintains osmotic pressure), globulins (antibodies and transport), and fibrinogen (clotting)
• Glucose (0.1%): Normal range 70-100 mg/dL—NEET often asks about hyperglycemia vs hypoglycemia effects
• Ions (0.9%): Na⁺, K⁺, Ca²⁺, Cl⁻—crucial for nerve and muscle function
• Urea, creatinine, uric acid: Nitrogenous wastes being transported to kidneys
• Hormones, enzymes, vitamins: In trace amounts but tested in reasoning MCQs

The cellular component (45%) breaks into 99% RBCs, 0.5% WBCs, and 0.5% platelets. Here's what you MUST memorize for NEET: RBC count is 4.5-5.5 million/µL in males and 4-5 million/µL in females. WBC count is 4,500-11,000/µL. Platelet count is 1.5-4 lakh/µL. Examiners test these numbers because they differentiate between anaemia, polycythaemia, and infection.

The Cardiac Cycle: Systole, Diastole, and Pressure Changes

The cardiac cycle is mechanics—and NEET tests your understanding by asking WHERE in the cycle certain pressure changes occur. Most students memorize 120/80 mm Hg and move on. That's a mistake.

The complete cardiac cycle takes 0.8 seconds at a resting heart rate of 75 bpm. It's divided into two main phases:

Atrial Systole (0.1 sec)

Both atria contract simultaneously, pushing blood into the ventricles. This is sometimes called "atrial kick" because it contributes only 20-30% of ventricular filling—the other 70% happens during ventricular diastole. Atrial pressure rises to about 5-10 mm Hg. Atrioventricular (AV) valves (mitral on left, tricuspid on right) are OPEN. Semilunar valves are CLOSED.

Ventricular Systole (0.3 sec)

Ventricles contract, raising ventricular pressure rapidly. This is split into two sub-phases:

• Isovolumetric contraction: Pressure rises from 0 to 80 mm Hg in the left ventricle (LV), but no blood moves because AV valves snap shut and aortic valve hasn't opened yet. This closure of AV valves creates the first heart sound (S1, "lub")
• Ejection phase: When LV pressure exceeds aortic pressure (120 mm Hg), the aortic valve opens and blood is ejected. Systolic pressure is defined as the maximum pressure during this phase—normally 120 mm Hg in the aorta

Ventricular Diastole (0.5 sec)

Ventricles relax. Ventricular pressure drops rapidly below aortic pressure, the aortic valve snaps shut (creating the second heart sound, S2, "dub"), and the AV valves open. Blood fills the ventricles again from the atria. This is the longest phase of the cardiac cycle.

Diastolic pressure (80 mm Hg) is measured when the aortic valve closes and the ventricle is refilling. NEET questions often ask: "At which point is the aortic valve open?" Answer: during ventricular systole ejection phase only. "When are both sets of valves closed?" Answer: during isovolumetric contraction and isovolumetric relaxation.

Blood Pressure Regulation and the Cardiac Output Formula

Cardiac output (CO) = Heart Rate (HR) × Stroke Volume (SV). At rest, this is roughly 70 bpm × 70 mL = 4.9 L/min (often rounded to 5 L/min in NCERT). NEET tests this by asking: if heart rate increases but stroke volume decreases due to blood loss, does cardiac output increase or decrease? You need to calculate.

Blood pressure is maintained by three factors: cardiac output, peripheral resistance, and blood volume. The autonomic nervous system regulates all three. Sympathetic stimulation increases HR and force of contraction (positive inotropic effect), raising both cardiac output and blood pressure. Parasympathetic (vagal) stimulation does the opposite.

The baroreceptor reflex is crucial: when blood pressure rises, baroreceptors in the carotid sinus and aortic arch sense this, and the parasympathetic nervous system is activated