Blood, Lymph, and the Heart — Understanding Circulation from the Ground Up
Every cell in your body needs nutrients and oxygen delivered to it, and metabolic waste removed from it. That’s the entire job of the circulatory system — a remarkably efficient delivery network that the heart drives non-stop, from the moment you’re born.
This chapter carries significant weightage in NEET: expect 3–5 questions directly from here, covering blood composition, cardiac cycle, ECG, and disorders. In CBSE board exams, the cardiac cycle and double circulation are the most frequently tested diagrams.
Let’s work through this systematically — composition of body fluids first, then the heart’s structure and function, and finally the regulatory mechanisms.
Key Terms & Definitions
Blood is a connective tissue consisting of a fluid matrix called plasma (~55% of blood volume) and formed elements (~45%).
Plasma is mostly water (90–92%), with dissolved proteins (albumin, globulin, fibrinogen), glucose, amino acids, lipids, and inorganic ions. Fibrinogen is critical — it participates in clotting.
Formed elements include:
- Erythrocytes (RBCs) — 5–5.5 million/mm³ in males, 4.5–5 million/mm³ in females
- Leucocytes (WBCs) — 6,000–8,000/mm³
- Thrombocytes (Platelets) — 1.5–3.5 lakh/mm³
Lymph is the fluid that leaks from blood capillaries into tissue spaces. It’s similar to plasma but lacks large proteins and RBCs. Lymph carries fats absorbed from the intestine via lacteals.
Serum = Plasma − Fibrinogen. This distinction trips up many students — serum is what remains after blood clots.
NEET frequently asks the difference between plasma and serum. Remember: plasma has fibrinogen (so it can clot); serum doesn’t. Serum is essentially plasma without clotting factors.
Blood Composition in Detail
Erythrocytes (RBCs)
Human RBCs are biconcave, enucleate (no nucleus) discs. The biconcave shape increases surface area for gas exchange. They’re produced in red bone marrow (haemopoiesis) and live approximately 120 days.
Haemoglobin (Hb) is the oxygen-carrying protein inside RBCs. Each Hb molecule has 4 globin chains, each carrying one haem group with an iron atom at the centre. One Hb binds 4 O₂ molecules.
Normal Hb values:
- Males: 14–16 g/dL
- Females: 12–14 g/dL
Leucocytes (WBCs)
WBCs are nucleated and colourless. They’re classified as:
Granulocytes (have granules in cytoplasm):
- Neutrophils — phagocytic, 60–65% of total WBCs
- Eosinophils — involved in allergic reactions, 2–3%
- Basophils — release histamine, rarest at 0.5–1%
Agranulocytes (no granules):
- Lymphocytes — immune response (B and T cells), 20–25%
- Monocytes — largest WBCs, become macrophages, 6–8%
A classic NEET question: “Which WBC is most abundant?” Answer: Neutrophils (60–65%). And “Which is least abundant?” Answer: Basophils (0.5–1%). Memorise these percentages — they appear almost every year.
Blood Groups (ABO and Rh)
The ABO system is based on antigens (agglutinogens) on RBC surface and antibodies (agglutinins) in plasma:
| Blood Group | Antigen on RBC | Antibody in Plasma | Can Donate To | Can Receive From |
|---|---|---|---|---|
| A | A | Anti-B | A, AB | A, O |
| B | B | Anti-A | B, AB | B, O |
| AB | A and B | None | AB only | All (Universal Recipient) |
| O | None | Anti-A, Anti-B | All (Universal Donor) | O only |
Rh factor: If the Rh antigen is present, the person is Rh+ (positive). About 80% of humans are Rh+. The Rh system is critical in erythroblastosis foetalis — a condition where an Rh− mother carrying an Rh+ foetus develops antibodies that attack foetal RBCs in subsequent pregnancies.
Coagulation of Blood (Clotting)
When a blood vessel is injured, a cascade of reactions leads to clot formation:
- Injured tissue releases thromboplastin
- Thromboplastin activates prothrombin → thrombin (in presence of Ca²⁺)
- Thrombin converts soluble fibrinogen → insoluble fibrin
- Fibrin forms a mesh that traps RBCs → clot
Thromboplastin + Ca²⁺ → Prothrombin → Thrombin
Thrombin + Fibrinogen → Fibrin (clot mesh)
Students confuse fibrinogen and fibrin. Fibrinogen is soluble (found in plasma). Fibrin is insoluble (the actual clot). Thrombin is the enzyme that converts one to the other.
Lymphatic System
Lymph capillaries are present in all tissues except the CNS. They collect tissue fluid (lymph) and return it to blood via the left subclavian vein.
Key functions of lymph:
- Returns proteins and excess fluid from tissues to blood
- Transports dietary fats absorbed by lacteals in the small intestine
- Involved in immune surveillance via lymph nodes
Lymph nodes filter lymph and are sites where lymphocytes proliferate to fight infection.
Structure of the Human Heart
The heart is a muscular, four-chambered organ located in the mediastinum, tilted slightly to the left. It’s enclosed in a double-layered pericardium.
Chambers and Valves
The heart has 4 chambers:
- Right Atrium (RA) — receives deoxygenated blood from body via superior and inferior vena cava
- Right Ventricle (RV) — pumps blood to lungs via pulmonary artery
- Left Atrium (LA) — receives oxygenated blood from lungs via pulmonary veins
- Left Ventricle (LV) — pumps blood to the entire body via aorta
Valves prevent backflow:
- Tricuspid valve — between RA and RV (3 cusps)
- Bicuspid (Mitral) valve — between LA and LV (2 cusps)
- Semilunar valves — at the exit of RV (pulmonary) and LV (aortic)
Memory trick: TRIcuspid is on the Right side (both have ‘R’ sound). BIcuspid is on the Left. Or remember: Right = Tricuspid, Left = Bicuspid (alphabetical: B comes before T, Left before Right… actually just memorise it directly).
Nodal Tissue and Conduction System
The heart generates its own electrical impulses — it’s myogenic (the impulse originates in muscle, not nerves).
The conduction pathway:
- SA Node (Sinoatrial node) — in the wall of RA, called the pacemaker. Sets heart rate (~72 beats/min).
- AV Node (Atrioventricular node) — at the atrioventricular junction. Delays impulse slightly so atria contract before ventricles.
- Bundle of His — runs through the interventricular septum
- Purkinje fibres — spread through ventricular walls, cause ventricular contraction
Cardiac Cycle
One complete cardiac cycle takes 0.8 seconds at 72 beats/min.
| Phase | Duration | What Happens |
|---|---|---|
| Atrial systole | 0.1 s | Atria contract, push blood into ventricles |
| Ventricular systole | 0.3 s | Ventricles contract, push blood to lungs and body |
| Joint diastole | 0.4 s | All chambers relax, heart fills with blood |
Stroke volume = volume of blood pumped per beat ≈ 70 mL
The “lub-dub” heart sounds:
- “Lub” (S1) — closure of tricuspid and bicuspid valves (start of ventricular systole)
- “Dub” (S2) — closure of semilunar valves (end of ventricular systole)
ECG (Electrocardiogram)
An ECG records the electrical activity of the heart. The normal ECG has:
- P wave — atrial depolarisation (atria contract)
- QRS complex — ventricular depolarisation (ventricles contract)
- T wave — ventricular repolarisation (ventricles relax)
NEET 2023 asked directly about what QRS complex represents. Answer: ventricular depolarisation. Also frequently asked: what does the T wave represent? Answer: ventricular repolarisation. Atrial repolarisation is hidden within the QRS complex.
Double Circulation
Humans have double circulation — blood passes through the heart twice for every complete circuit:
Pulmonary circulation: RV → Pulmonary artery → Lungs → Pulmonary veins → LA (deoxygenated blood gets oxygenated)
Systemic circulation: LV → Aorta → Body tissues → Vena cava → RA (oxygenated blood delivers O₂ to tissues)
This ensures oxygenated and deoxygenated blood never mix, giving higher efficiency — crucial for warm-blooded animals with high metabolic rates.
Solved Examples
Example 1 — Easy (CBSE Level)
Q: A person has blood group AB. Which antigens are present on their RBCs, and which antibodies in their plasma?
Both A and B antigens are present on RBCs. Since the immune system doesn’t make antibodies against its own antigens, AB individuals have neither anti-A nor anti-B antibodies in plasma. This is why AB is the universal recipient — no antibodies to react with donor blood.
Example 2 — Moderate (NEET Level)
Q: The cardiac output of a person is 3.5 L/min with a heart rate of 50 beats/min. Calculate their stroke volume.
Convert litres to mL before dividing, otherwise you get 0.07 — which is the right answer in litres but looks wrong and causes confusion in MCQs.
Example 3 — Hard (NEET Advanced Application)
Q: An Rh− woman is pregnant with her second Rh+ child. Explain why this pregnancy is more dangerous than the first.
During the first pregnancy, small amounts of Rh+ foetal blood entered the mother’s circulation at delivery. Her immune system recognised the Rh antigen as foreign and produced anti-Rh antibodies (this is called sensitisation).
In the second pregnancy with an Rh+ foetus, these maternal anti-Rh antibodies (IgG class, which cross the placenta) attack foetal RBCs → massive haemolysis → erythroblastosis foetalis.
The first pregnancy was safe because antibody production takes time — by then, the baby was already delivered.
Exam-Specific Tips
NEET Weightage: Body Fluids and Circulation consistently gives 3–5 questions. High-yield topics: WBC percentages, blood groups (especially Rh factor and erythroblastosis foetalis), cardiac cycle timing, ECG waves, and disorders (anaemia, thrombosis, hypertension).
CBSE Board: The cardiac cycle diagram with all timings, the ECG labelled diagram, and double circulation are standard 3–5 mark questions. Practice drawing the heart diagram from memory — the examiner expects proper labelling of all 4 chambers and valves.
Chapters that connect: This chapter connects with Breathing and Gas Exchange (how O₂ and CO₂ are carried in blood) and Excretory Products (how blood delivers waste to kidneys). Strong understanding here strengthens both those chapters.
Disorders of Circulatory System
Hypertension: Persistent blood pressure above 140/90 mmHg. Normal BP is 120/80 mmHg (systolic/diastolic).
Coronary Artery Disease (CAD): Atherosclerosis (deposition of fat/calcium in coronary arteries) narrows vessel lumen, reducing blood flow to heart muscle.
Angina pectoris: Chest pain due to reduced O₂ supply to heart muscle during exertion.
Heart failure (Congestive Heart Failure): Heart cannot pump blood effectively. Oedema (fluid accumulation) occurs, especially in legs.
Anaemia: Reduced haemoglobin or RBC count, reducing O₂ carrying capacity.
Thrombosis: Formation of a clot (thrombus) inside a blood vessel, blocking flow.
Common Mistakes to Avoid
Mistake 1: Confusing pulmonary artery and pulmonary vein content. Pulmonary artery carries deoxygenated blood (from heart to lungs). Pulmonary veins carry oxygenated blood (from lungs to heart). The names refer to the vessels going to/from lungs, not to oxygen content.
Mistake 2: Saying SA node is in the left atrium. SA node is in the right atrium wall, near the opening of the superior vena cava. It’s always the right atrium — don’t mix this up.
Mistake 3: Thinking WBCs are produced only in bone marrow. RBCs, platelets, and most WBCs come from red bone marrow. But lymphocytes are also produced in lymphoid tissue (thymus, lymph nodes, spleen). T-lymphocytes mature in the thymus — a classic exam point.
Mistake 4: Getting joint diastole duration wrong. Joint diastole = 0.4 s. Atrial systole = 0.1 s. Ventricular systole = 0.3 s. Total = 0.8 s. Students often write 0.5 s for joint diastole by mistake.
Mistake 5: Saying serum contains fibrinogen. Serum = plasma − fibrinogen (and other clotting factors). If fibrinogen is present, it’s plasma, not serum. This appears as a one-mark MCQ every few years.
Practice Questions
Q1. Which blood group is the universal donor? Why can’t an O+ person donate to all recipients?
O blood group has no A or B antigens on RBCs, so no reaction with recipient’s antibodies. However, O+ has the Rh antigen. An Rh− recipient would develop anti-Rh antibodies over time. So O− (not O+) is the true universal donor. O+ can donate to all Rh+ recipients.
Q2. A patient’s ECG shows no P wave but normal QRS complexes. What does this indicate?
No P wave means atria are not depolarising normally — likely atrial fibrillation or the SA node is non-functional. Since QRS is normal, ventricular conduction is intact. The AV node may be generating impulses independently (AV nodal rhythm).
Q3. Calculate cardiac output if heart rate is 80 beats/min and stroke volume is 75 mL.
Cardiac Output = 80 × 75 = 6000 mL/min = 6 L/min
Q4. Why do human RBCs lack mitochondria? What is the advantage and disadvantage?
RBCs lack mitochondria (and nucleus) to maximise space for haemoglobin and maintain the biconcave shape. They generate ATP via anaerobic glycolysis only. Advantage: they don’t consume the O₂ they transport. Disadvantage: they can’t repair themselves, so they live only ~120 days.
Q5. Differentiate between granulocytes and agranulocytes with two examples each.
Granulocytes have visible granules in cytoplasm and lobed nuclei. Examples: Neutrophils, Eosinophils (also Basophils). Agranulocytes have no visible granules and round/kidney-shaped nuclei. Examples: Lymphocytes, Monocytes.
Q6. What is the significance of the delay at the AV node during cardiac conduction?
The AV node delays the impulse by ~0.1 second before passing it to the Bundle of His. This ensures atria complete their contraction and fully empty blood into ventricles before ventricular contraction begins. Without this delay, atria and ventricles would contract simultaneously, reducing filling efficiency.
Q7. In which vessel does blood pressure first exceed atmospheric pressure? Justify.
The aorta. When the left ventricle contracts (ventricular systole), it generates pressure (~120 mmHg systolic) that forces blood into the aorta at pressures well above atmospheric. Pressure progressively drops as blood moves through arteries → arterioles → capillaries → venules → veins.
Q8. An athlete’s resting heart rate is 50 beats/min. Their cardiac output equals a normal person’s (~5 L/min). What does this tell us about their stroke volume?
Stroke Volume = Cardiac Output ÷ Heart Rate = 5000 ÷ 50 = 100 mL/beat. Normal stroke volume is ~70 mL. The athlete’s heart pumps more blood per beat due to a stronger, more muscular left ventricle — this is called athlete’s heart or cardiac hypertrophy from sustained training.
FAQs
Why is the left ventricle wall thicker than the right?
The left ventricle pumps blood to the entire body (systemic circulation), which requires generating much higher pressure than the right ventricle, which only pumps to the lungs (short distance, low resistance). The thicker muscular wall generates this higher pressure. In fact, systemic pressure (~120 mmHg) is roughly 6× pulmonary pressure (~20 mmHg).
What is the difference between arteries and veins, structurally?
Arteries carry blood away from the heart under high pressure — they have thick, elastic, muscular walls. Veins carry blood toward the heart under low pressure — thinner walls, and they have valves to prevent backflow (gravity would otherwise pull blood away from the heart in the legs). Capillaries are single-cell thick for maximum diffusion.
Why do humans have 4-chambered hearts but frogs have 3?
Frogs are ectotherms (cold-blooded) with lower metabolic demands. Their 3-chambered heart allows some mixing of oxygenated and deoxygenated blood, which is tolerable at their metabolic rate. Warm-blooded animals (mammals, birds) need constant high oxygen delivery — complete separation of oxygenated/deoxygenated blood via 4 chambers is essential for this.
What happens during joint diastole?
All four chambers relax simultaneously. Blood flows passively from the vena cava into the right atrium, and from the pulmonary veins into the left atrium. The AV valves open, allowing blood to flow into the relaxed ventricles. About 70% of ventricular filling happens passively during this phase before atrial systole pushes the remaining 30%.
Can a person with no SA node survive?
Yes, but with a much slower heart rate. If the SA node fails, the AV node takes over as pacemaker at ~40–60 beats/min (its intrinsic rate). If AV node also fails, the Purkinje fibres pace at ~20–40 beats/min. These backup pacemakers prevent complete cardiac arrest, but patients typically need an artificial pacemaker to maintain an adequate heart rate.
What causes the “lub-dub” sounds?
These are valve closure sounds, not blood hitting the heart wall. “Lub” (first sound, S1) = closure of tricuspid and bicuspid (mitral) valves at the start of ventricular systole. “Dub” (second sound, S2) = closure of semilunar valves (aortic and pulmonary) at the end of ventricular systole. A third sound (S3) in adults indicates heart failure.
How does the body regulate heart rate during exercise?
The autonomic nervous system modulates SA node firing. Sympathetic nerves release noradrenaline, which increases heart rate and force of contraction. Parasympathetic nerves (vagus) release acetylcholine, which slows the heart. During exercise, sympathetic activity dominates. This is why beta-blockers (which block sympathetic effects) slow the heart.
What is the normal ESR (Erythrocyte Sedimentation Rate) and why does it matter?
ESR measures how fast RBCs settle in a tube of blood. Normal: 0–15 mm/hr in males, 0–20 mm/hr in females. An elevated ESR indicates inflammation somewhere in the body — it’s a non-specific marker used to detect and monitor inflammatory conditions like rheumatoid arthritis, infections, or anaemia.