Body Fluids and Circulation — NEET notes

Blood — The River of Life Running Through You

Blood is a special connective tissue (~5-5.5 L in adults) with two components: plasma (55%, straw-colored fluid with water, proteins, and ions) and formed elements (45%, RBCs/WBCs/platelets). Plasma proteins include albumin (most abundant, maintains osmotic pressure), globulins (immune defense), and fibrinogen (clotting). Serum = plasma minus clotting factors.

Blood Overview

Blood is a fluid connective tissue with a liquid matrix (plasma) carrying formed elements. It constitutes about 6-8% of body weight. pH is slightly alkaline at 7.35-7.45.

Blood is the only fluid connective tissue in the body. Unlike other connective tissues (bone, cartilage, tendons), blood has a liquid matrix called plasma. An adult human has roughly 5 to 5.5 litres of blood, which constitutes about 6-8% of body weight.

• Blood = Plasma (55%) + Formed Elements (45%)
• Volume: ~5-5.5 litres in a healthy adult
• pH: 7.35-7.45 (slightly alkaline — narrow range, tightly regulated)
• Blood is a connective tissue because it develops from mesoderm and connects/transports between all body parts
• Color: bright red (oxygenated, arteries) vs dark red/purplish (deoxygenated, veins)

Plasma Composition

Plasma is the straw-colored liquid portion of blood. It is ~90% water, 6-8% proteins, and the rest includes ions, nutrients, gases, hormones, and waste products.

Plasma makes up 55% of blood volume. It's a straw-colored, slightly alkaline fluid. About 90-92% is water — the remaining 6-8% consists of proteins, and the rest is minerals, glucose, amino acids, lipids, hormones, dissolved gases, and waste products like urea.

• Water — ~90-92% of plasma
• Proteins — ~6-8% of plasma (most critical component after water)
• Mineral ions — Na+, K+, Ca2+, Cl-, HCO3- (maintain osmotic balance and pH)
• Nutrients — glucose, amino acids, fatty acids, vitamins
• Gases — O2, CO2 (dissolved in small amounts)
• Hormones — transported from endocrine glands to target organs
• Waste products — urea, uric acid, creatinine (transported to kidneys for excretion)

Plasma Proteins

Three major plasma proteins: albumin (60%, osmotic pressure), globulins (alpha, beta, gamma — transport & immunity), fibrinogen (clotting). All are produced by the liver except gamma globulins (by B-lymphocytes).

Plasma contains three major protein groups, all primarily made by the liver (except gamma globulins). Together they constitute 6-8% of plasma and perform vital functions from maintaining blood volume to immune defense to clotting.

• Albumin (~60% of plasma proteins) — Most abundant. Maintains osmotic pressure (keeps water in blood vessels). Also transports fatty acids, hormones, drugs
• Globulins (~35%) — Three subtypes: alpha & beta (transport proteins, made by liver) and gamma globulins = antibodies/immunoglobulins (made by B-lymphocytes, NOT liver)
• Fibrinogen (~4%) — Essential for blood clotting. Converted to fibrin during coagulation. Made by liver
• Serum = Plasma - Clotting factors (fibrinogen removed). When blood clots in a test tube, the liquid that oozes out is serum

Formed Elements — RBCs, WBCs & Platelets, the cellular workforce

Formed elements make up 45% of blood. RBCs (erythrocytes) are biconcave, enucleated, packed with hemoglobin, and live ~120 days. WBCs (leukocytes) come in 5 types: neutrophils (most abundant, 60-65%), eosinophils, basophils, lymphocytes, monocytes. Platelets (thrombocytes) are cell fragments essential for clotting.

RBC count: 5 - 5.5 million/mm^3 (males), 4.5 - 5 million/mm^3 (females)

Red Blood Cells (Erythrocytes)

RBCs are biconcave disc-shaped cells that lack a nucleus and most organelles in their mature form. They are packed with hemoglobin for O2 transport. Formed in bone marrow, live ~120 days, and destroyed in spleen.

Erythrocytes (RBCs) are the most abundant cells in blood — numbering 5-5.5 million per mm3 in males and 4.5-5 million per mm3 in females. Their job is simple but critical: carry oxygen from lungs to tissues and carry CO2 back. To maximize hemoglobin capacity, mature RBCs kick out their nucleus and most organelles.

• Shape: Biconcave disc — increases surface area for gas exchange (~30% more than a sphere of same volume)
• No nucleus in mature mammalian RBCs (camel and llama are exceptions — their RBCs are nucleated and oval)
• No mitochondria — RBCs rely entirely on anaerobic glycolysis for energy (so they don't consume the O2 they carry!)
• Hemoglobin content: ~33% of cell weight. Each Hb molecule has 4 heme groups, each binding 1 O2 → max 4 O2 per Hb
• Formed in red bone marrow (process = erythropoiesis), regulated by erythropoietin (hormone from kidneys)
• Lifespan: ~120 days. Old RBCs destroyed in spleen ('graveyard of RBCs')
• Hematocrit = % of blood volume occupied by RBCs. Normal: ~40-45% in males, ~36-40% in females

White Blood Cells (Leukocytes)

WBCs are nucleated, colorless blood cells that defend against infections. Two categories: granulocytes (neutrophils, eosinophils, basophils — have granules) and agranulocytes (lymphocytes, monocytes — no visible granules). Total WBC count: 6,000-8,000 per mm3.

Leukocytes (WBCs) are the defense force of the blood. Unlike RBCs, they are nucleated and colorless (leuko = white). Total count: 6,000-8,000 per mm3 — much less than RBCs. They are classified into granulocytes (have cytoplasmic granules) and agranulocytes (no prominent granules).

• Neutrophils (60-65%) — Most abundant WBC. First responders to bacterial infection. Phagocytic. Multi-lobed nucleus (3-5 lobes). Short-lived (~12 hours in blood)
• Eosinophils (2-3%) — Fight parasitic infections (worms). Also involved in allergic reactions. Bilobed nucleus. Granules stain red with eosin
• Basophils (0.5-1%) — Rarest WBC. Release histamine (vasodilator) and heparin (anticoagulant). Involved in inflammatory and allergic responses. Similar to mast cells in tissues
• Lymphocytes (20-25%) — B-cells (antibody production) and T-cells (cell-mediated immunity). Key players in adaptive/specific immunity. Large round nucleus with thin cytoplasm
• Monocytes (6-8%) — Largest WBC. Kidney/bean-shaped nucleus. Migrate into tissues and become macrophages — the big eaters that engulf pathogens and dead cells

Quick recall

Which WBC is the first responder to bacterial infection?

Neutrophils — they are the most abundant WBCs (60-65%) and the first to arrive at infection sites via phagocytosis. An increase in neutrophil count in a blood report often indicates bacterial infection.

Platelets (Thrombocytes)

Platelets are tiny, enucleated cell fragments derived from megakaryocytes in bone marrow. Count: 1.5-3.5 lakh per mm3. They initiate clotting by forming a platelet plug at injury sites and releasing clotting factors.

Thrombocytes (platelets) are not true cells — they are cell fragments pinched off from giant bone marrow cells called megakaryocytes. They have no nucleus but contain granules packed with clotting factors and chemicals. Normal count: 1.5-3.5 lakh (150,000-350,000) per mm3.

• Not true cells — they are cytoplasmic fragments of megakaryocytes
• No nucleus (like mature RBCs)
• Lifespan: ~5-10 days
• Primary function: Initiate blood clotting by forming a temporary platelet plug
• At wound sites, platelets adhere, aggregate, and release clotting factors (platelet-derived growth factor, serotonin, thromboxane A2)
• Thrombocytopenia (low platelet count < 50,000/mm3) → excessive bleeding
• Thrombocytosis (high platelet count) → risk of unwanted blood clots

Blood Groups — ABO & Rh, the life-saving labels on your RBCs

ABO blood grouping is based on antigens (A, B) on RBC surface and antibodies (anti-A, anti-B) in plasma. O is universal donor, AB is universal recipient. Rh factor: Rh+ (antigen present) or Rh- (absent). Rh incompatibility between Rh- mother and Rh+ fetus causes erythroblastosis fetalis.

ABO Blood Group System

Karl Landsteiner discovered the ABO system (Nobel Prize 1930). Blood groups A, B, AB, O are based on the presence/absence of antigen A and antigen B on RBC surfaces. Plasma contains the opposite antibodies — this determines compatibility.

The ABO system is based on two types of sugar antigens (A and B) on the surface of RBCs, and corresponding antibodies in the plasma. Your blood group depends on which antigens your RBCs carry. The critical rule: your plasma NEVER contains antibodies against your own antigens — that would destroy your own RBCs.

• 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 antigens A & B on RBCs, NO antibodies in plasma → Universal recipient
• Group O — No antigens on RBCs, Both anti-A & anti-B antibodies in plasma → Universal donor
• Discovered by Karl Landsteiner (1900) — won Nobel Prize in 1930
• ABO grouping is determined by genes on chromosome 9 — gene I has three alleles: IA, IB, and i

Rh Factor

Rh factor is an antigen (D antigen) on RBC surface. ~80% of humans are Rh+ (have the antigen). Rh incompatibility is dangerous when an Rh- mother carries an Rh+ fetus — her body makes anti-Rh antibodies that can attack the fetus in subsequent pregnancies.

The Rh factor (named after the Rhesus monkey in which it was first discovered) is another antigen on RBC surfaces — the D antigen. If you have it, you're Rh positive (Rh+). If you lack it, you're Rh negative (Rh-). About 80% of the human population is Rh+.

• Rh+ = D antigen present on RBCs (~80% of population)
• Rh- = D antigen absent from RBCs (~20% of population)
• Unlike ABO, Rh- individuals do NOT naturally have anti-Rh antibodies — they only develop them after exposure to Rh+ blood
• Full blood type = ABO + Rh → e.g., A+, B-, O+, AB-
• An Rh- person receiving Rh+ blood for the first time → no immediate reaction (body starts making anti-Rh antibodies). Second exposure → severe hemolytic reaction

Erythroblastosis Fetalis

Occurs when an Rh- mother carries an Rh+ fetus (father is Rh+). During first pregnancy, small amounts of fetal Rh+ blood enter mother's circulation at delivery, sensitizing her. In subsequent Rh+ pregnancies, maternal anti-Rh antibodies (IgG) cross the placenta and destroy fetal RBCs.

Erythroblastosis fetalis (Hemolytic Disease of the Newborn, HDN) is a dangerous condition that occurs when an Rh- mother carries an Rh+ fetus. The mother's immune system treats the fetal Rh antigen as foreign and produces antibodies that destroy the baby's RBCs.

How Erythroblastosis Fetalis Develops

1. First pregnancy — Rh- mother carries Rh+ baby. During delivery, some fetal Rh+ blood enters mother's circulation (mixing at placenta).
2. Sensitization — Mother's immune system recognizes Rh antigen as foreign → produces anti-Rh antibodies (IgG). This takes time, so the first baby is usually safe.
3. Second Rh+ pregnancy — Mother already has anti-Rh antibodies (memory B cells activated quickly). These IgG antibodies cross the placenta → attack fetal Rh+ RBCs → hemolysis (RBC destruction).
4. Consequences for fetus — Severe anemia, jaundice (excess bilirubin from RBC breakdown), edema (hydrops fetalis). Can be fatal if untreated.
5. Prevention — Inject mother with anti-Rh antibodies (RhoGAM) within 72 hours of first delivery. These destroy any fetal Rh+ RBCs in mother's blood BEFORE her immune system gets sensitized.

• Only happens when mother is Rh- and fetus is Rh+ (father must be Rh+)
• First child is usually safe — sensitization happens during/after first delivery
• Subsequent Rh+ pregnancies are at risk — anti-Rh IgG antibodies cross the placenta
• Symptoms: severe anemia, jaundice, edema in the newborn
• Prevention: Anti-D injection (RhoGAM) to Rh- mother after each Rh+ delivery

Quick recall

If an Rh- woman marries an Rh- man, will erythroblastosis fetalis occur?

No. Both parents are Rh-, so all children will be Rh-. The fetal RBCs won't have Rh antigen, so there's nothing for the mother's immune system to attack. The condition ONLY occurs when the fetus is Rh+ (inherited from an Rh+ father).

Coagulation — How your body plugs a leak in under 5 minutes

Blood clotting (coagulation) is a cascade of enzymatic reactions that converts soluble fibrinogen into insoluble fibrin threads. Key players: platelets (form initial plug), thromboplastin (tissue factor), prothrombin → thrombin, fibrinogen → fibrin. Calcium ions (Ca2+) are essential at multiple steps. Anticoagulants like heparin prevent unwanted clotting.

Prothrombin Thromboplastin + Ca^ 2+ Thrombin Fibrinogen arrow Fibrin (clot)

Clotting Cascade

When a blood vessel is injured, clotting involves three main steps: formation of thromboplastin, conversion of prothrombin to thrombin, and conversion of fibrinogen to fibrin. This cascade requires calcium ions and involves both intrinsic and extrinsic pathways.

Blood clotting (coagulation) is the body's emergency response to stop bleeding. It's a cascade — a series of enzyme activations where each step activates the next, amplifying the signal exponentially. The end product is a mesh of fibrin threads that trap blood cells, forming a solid clot.

Simplified Clotting Cascade (NCERT Level)

1. Injury occurs — damaged tissue and platelets release thromboplastin (tissue factor/factor III). Platelets also aggregate at the wound site to form a temporary platelet plug.
2. Prothrombin → Thrombin — Thromboplastin, along with Ca2+ ions and other clotting factors, converts inactive prothrombin (made by liver, needs Vitamin K) into active thrombin (an enzyme).
3. Fibrinogen → Fibrin — Thrombin acts on soluble fibrinogen (plasma protein) and converts it into insoluble fibrin threads.
4. Clot formation — Fibrin threads form a mesh/network that traps RBCs, platelets, and other cells → solid clot (thrombus) that seals the wound.
5. Clot retraction — Platelets in the clot contract, pulling wound edges together and squeezing out serum (plasma without clotting factors).

• Calcium ions (Ca2+) are required at multiple steps — removing Ca2+ prevents clotting (basis of lab anticoagulants like EDTA, citrate)
• Vitamin K is essential for liver to synthesize prothrombin and several other clotting factors (II, VII, IX, X)
• Clotting factors are numbered I through XIII — a cascade of sequential activations
• Fibrinogen (Factor I) is the final substrate → converted to fibrin by thrombin
• The entire process takes about 3-6 minutes in a healthy individual

Anticoagulants

The body has natural anticoagulants to prevent clots from forming inside healthy blood vessels. Heparin (from basophils/mast cells) inactivates thrombin. Clinical anticoagulants include EDTA and sodium citrate (chelate Ca2+).

If clotting cascades ran unchecked, your entire blood would solidify inside your vessels — that would be instant death. The body uses anticoagulants to keep clotting confined to injury sites only.

• Heparin — Natural anticoagulant secreted by basophils (in blood) and mast cells (in tissues). Inhibits thrombin and other clotting factors. Used clinically as a blood thinner
• Antithrombin III — Plasma protein that slowly inactivates thrombin; heparin speeds up this process ~1000-fold
• EDTA and Sodium Citrate — Lab anticoagulants that chelate (bind and remove) Ca2+ ions → prevent clotting in blood collection tubes
• Warfarin — Drug that blocks Vitamin K → prevents liver from making prothrombin → used to prevent clots in patients with heart disease
• Aspirin — Inhibits platelet aggregation (prevents platelets from clumping) → used as a preventive measure against heart attacks and strokes

Quick recall

Why doesn't blood clot inside normal, healthy blood vessels?

Three reasons: (1) Smooth endothelial lining of vessels prevents platelet activation, (2) Heparin and other natural anticoagulants in blood inactivate any stray thrombin, (3) Blood flow dilutes and washes away any activated clotting factors before they can accumulate.

Heart — The four-chambered pump that never takes a break

The human heart is a muscular organ with 4 chambers: 2 atria (receiving chambers) and 2 ventricles (pumping chambers). Valves prevent backflow — tricuspid (right AV), bicuspid/mitral (left AV), pulmonary and aortic semilunar valves. The cardiac cycle lasts ~0.8s with three phases: atrial systole, ventricular systole, and joint diastole. Cardiac output = stroke volume x heart rate (~5 L/min).

Cardiac Output = Stroke Volume Heart Rate 70 mL 72 beats/min 5 L/min

Heart Structure

The heart is a hollow, muscular organ roughly the size of a fist, located slightly left of center in the thoracic cavity. It has 4 chambers — right and left atria (thin-walled, receive blood) and right and left ventricles (thick-walled, pump blood). A septum completely separates the right and left sides.

The human heart is enclosed in a double-walled pericardium (fibrous outer + serous inner with pericardial fluid for lubrication). The heart wall has three layers: epicardium (outer), myocardium (thick muscular middle — the actual pump muscle), and endocardium (inner lining).

• Right Atrium (RA) — Receives deoxygenated blood from body via Superior Vena Cava (upper body) and Inferior Vena Cava (lower body). Also receives from coronary sinus (heart's own deoxygenated blood)
• Right Ventricle (RV) — Pumps deoxygenated blood to lungs via pulmonary artery (only artery carrying deoxygenated blood)
• Left Atrium (LA) — Receives oxygenated blood from lungs via 4 pulmonary veins (only veins carrying oxygenated blood)
• Left Ventricle (LV) — Pumps oxygenated blood to entire body via aorta. Has the thickest wall (3x thicker than RV) because it pumps against the highest pressure
• Inter-atrial septum separates RA and LA; inter-ventricular septum separates RV and LV. A small fossa ovalis on inter-atrial septum marks where the foramen ovale existed in fetal heart

Heart Valves

Four valves prevent backflow of blood. Two AV valves (tricuspid on right, bicuspid/mitral on left) between atria and ventricles. Two semilunar valves (pulmonary and aortic) at the exit of ventricles. The 'lub-dub' sounds are valve closures.

Heart valves are like one-way doors — they open to let blood through and snap shut to prevent backflow. There are four valves arranged in two pairs: atrioventricular (AV) valves between atria and ventricles, and semilunar valves at the exits of ventricles.

• Tricuspid valve — Right AV valve (between RA and RV). Has 3 flaps/cusps. Held by chordae tendineae attached to papillary muscles on ventricular wall
• Bicuspid (Mitral) valve — Left AV valve (between LA and LV). Has 2 flaps/cusps. Also anchored by chordae tendineae
• Pulmonary semilunar valve — At the exit of RV into pulmonary artery. Has 3 half-moon shaped cusps
• Aortic semilunar valve — At the exit of LV into aorta. Has 3 half-moon shaped cusps
• Chordae tendineae = tendon-like cords connecting valve flaps to papillary muscles — prevent valves from flipping inside out during ventricular contraction

Cardiac Cycle

One complete cardiac cycle lasts ~0.8 seconds at 72 beats/min. It has three phases: atrial systole (0.1s), ventricular systole (0.3s), and joint/complete diastole (0.4s). During each cycle, the heart pumps ~70 mL of blood (stroke volume).

A cardiac cycle is one complete heartbeat — from the start of one contraction to the start of the next. At a normal heart rate of 72 beats/minute, each cycle takes 0.8 seconds. The cycle involves coordinated systole (contraction) and diastole (relaxation) of atria and ventricles.

The Three Phases of Cardiac Cycle

1. Atrial Systole (0.1 s) — Both atria contract simultaneously. AV valves (tricuspid & bicuspid) are OPEN. Blood is pushed from atria into ventricles. Semilunar valves are CLOSED. Atrial contraction tops up the ventricles (which are already ~70% full from passive filling during diastole).
2. Ventricular Systole (0.3 s) — Both ventricles contract. AV valves CLOSE ('lub' sound). Pressure in ventricles rises sharply → semilunar valves OPEN → blood ejected into pulmonary artery (from RV) and aorta (from LV). ~70 mL blood ejected per ventricle = stroke volume.
3. Joint/Complete Diastole (0.4 s) — Both atria AND ventricles are relaxed. Semilunar valves CLOSE ('dub' sound) as blood tries to flow back. AV valves open as ventricular pressure drops below atrial pressure → blood flows passively from atria (filling from veins) into ventricles. Heart is refilling for the next cycle.

• Atrial systole: 0.1 s | Atrial diastole: 0.7 s (rests most of the cycle)
• Ventricular systole: 0.3 s | Ventricular diastole: 0.5 s
• Joint diastole: 0.4 s — both chambers relaxed, heart refilling
• Stroke Volume (SV) = blood ejected per ventricle per beat ≈ 70 mL
• Cardiac Output (CO) = SV × Heart Rate = 70 mL × 72 beats/min ≈ 5040 mL/min ≈ 5 L/min
• During exercise: heart rate increases, SV increases → CO can reach 20-25 L/min

Quick recall

If heart rate increases to 120 beats/min, what happens to the cardiac cycle duration?

Duration = 60/120 = 0.5 seconds (shorter cycle). The diastole phase gets shorter (less filling time), which is why extremely high heart rates can actually REDUCE cardiac output — the heart doesn't get enough time to fill.

Double Circulation — Blood visits the heart twice in one complete loop

In humans, blood passes through the heart twice in one complete circuit — once through pulmonary circulation (heart → lungs → heart for oxygenation) and once through systemic circulation (heart → body → heart for delivery). The hepatic portal system carries nutrient-rich blood from gut to liver. Coronary circulation supplies the heart itself. Humans have a closed circulatory system.

Pulmonary vs Systemic Circulation

Pulmonary circulation carries deoxygenated blood from the right ventricle to lungs for oxygenation and back to the left atrium. Systemic circulation carries oxygenated blood from left ventricle to all body tissues and returns deoxygenated blood to the right atrium.

In double circulation, blood completes two circuits for every one trip around the body. This system ensures that oxygenated and deoxygenated blood never mix (complete separation), allowing highly efficient oxygen delivery — essential for warm-blooded animals with high metabolic rates.

The Two Circuits of Double Circulation

1. Pulmonary Circuit: RV → Pulmonary artery → Lungs (CO2 released, O2 picked up) → Pulmonary veins → LA. Purpose: Gas exchange — deoxygenated blood becomes oxygenated.
2. Systemic Circuit: LV → Aorta → Body tissues (O2 delivered, CO2 picked up) → Superior/Inferior Vena Cava → RA. Purpose: Nutrient/O2 delivery and waste collection from all body organs.

Portal & Coronary Circulation

The hepatic portal system is a unique circuit where blood from the intestines goes to the liver first (via hepatic portal vein) before entering general circulation. Coronary circulation supplies the heart muscle itself via coronary arteries branching from the aorta.

Two special circulatory pathways deserve attention: the hepatic portal system (serves the liver) and the coronary circulation (serves the heart muscle itself).

• Hepatic Portal System — Blood from stomach, intestines, spleen, and pancreas drains into the hepatic portal vein → liver. The liver processes absorbed nutrients, detoxifies harmful substances, and stores/releases glucose. Blood then exits the liver via hepatic veins → inferior vena cava
• Portal vein = a vein that connects two capillary beds (intestinal capillaries → liver sinusoids) WITHOUT passing through the heart in between. This is unusual — normally veins go directly to the heart
• Coronary Circulation — The heart feeds itself via coronary arteries (branch from the base of the aorta, just above the aortic valve). Deoxygenated blood from heart muscle drains via coronary sinus into the right atrium
• Coronary artery blockage → reduced blood supply to heart muscle → Coronary Artery Disease (CAD) → can lead to myocardial infarction (heart attack)

Open vs Closed Circulation

In open circulatory system (arthropods, molluscs), blood flows through open spaces (sinuses/lacunae) and directly bathes organs. In closed circulatory system (annelids, vertebrates), blood flows entirely within vessels. Humans have a closed system with complete double circulation.

Not all animals pump blood through closed tubes. In many invertebrates, blood (called hemolymph) flows freely through body cavities. Understanding the difference between open and closed systems is essential for NEET comparative questions.

• Arthropods (insects, crabs, spiders) — Open system. Heart pumps hemolymph into sinuses
• Annelids (earthworm, leech) — Closed system. Blood in dorsal and ventral vessels
• Molluscs — Mostly open (snails, clams). Exception: Cephalopods (octopus, squid) have a closed system — they're active predators needing high O2 delivery
• All vertebrates — Closed system. Fish have single circulation (2-chambered heart). Amphibians have incomplete double (3-chambered heart, some mixing). Mammals and birds have complete double (4-chambered heart, no mixing)

Quick recall

Does an earthworm have an open or closed circulatory system?

Closed circulatory system. Earthworms have dorsal, ventral, and lateral vessels with blood confined within them. Don't confuse annelids with arthropods — arthropods (insects) have OPEN systems, annelids (earthworms) have CLOSED systems.

ECG & Disorders — Reading the heart's electrical signature

The heart's electrical activity is recorded as an ECG (electrocardiogram) with characteristic waves: P wave (atrial depolarization), QRS complex (ventricular depolarization), T wave (ventricular repolarization). The conduction system runs: SA node → AV node → Bundle of His → Bundle branches → Purkinje fibers. Major disorders include hypertension, CAD, angina pectoris, and heart failure.

Cardiac Conduction System

The heart generates its own electrical impulse (myogenic). The SA node (pacemaker) in the right atrium initiates each heartbeat. The impulse travels: SA node → AV node → Bundle of His → Left and Right bundle branches → Purkinje fibers, ensuring coordinated contraction.

The heart is myogenic — it generates its own electrical impulse without needing external nerve signals. This is why a heart can continue beating even when removed from the body (as long as it has nutrients and oxygen). The electrical system ensures all parts of the heart contract in the correct sequence.

Pathway of Cardiac Impulse

1. SA Node (Sinoatrial Node) — Located in the upper wall of the right atrium near the opening of the superior vena cava. Generates impulses at ~72 beats/min. Called the pacemaker because it sets the heart rate.
2. Atrial conduction — Impulse spreads through both atrial walls → both atria contract simultaneously (atrial systole).
3. AV Node (Atrioventricular Node) — Located in the lower right corner of the right atrium, near the AV septum. Introduces a slight delay (~0.1 s) — this is critical! It ensures atria finish contracting before ventricles begin.
4. Bundle of His (AV Bundle) — Impulse passes from AV node through the AV bundle, which penetrates the inter-ventricular septum and divides into right and left bundle branches.
5. Purkinje Fibers — Bundle branches break into a network of fine Purkinje fibers that spread through the ventricular walls. Impulse reaches all ventricular muscle cells → both ventricles contract simultaneously (ventricular systole) from apex upward.

• SA node = primary pacemaker (72 beats/min). If it fails, AV node takes over at ~40-60 beats/min (slower backup)
• AV node delay (0.1 s) is essential — without it, atria and ventricles would contract simultaneously, and blood wouldn't flow properly
• Ventricles contract from apex to base (bottom to top) — this pushes blood UPWARD into the arteries (aorta and pulmonary artery), which exit from the top
• Heart is myogenic (self-excitable) but is REGULATED by the autonomic nervous system — sympathetic speeds it up, parasympathetic (vagus nerve) slows it down

Electrocardiogram (ECG)

An ECG records the electrical activity of the heart using electrodes on the body surface. It shows three main waves: P wave (atrial depolarization), QRS complex (ventricular depolarization), and T wave (ventricular repolarization). ECG helps diagnose arrhythmias, heart attacks, and other cardiac conditions.

An Electrocardiogram (ECG or EKG) is a graphical recording of the electrical changes in the heart during each cardiac cycle. It's recorded by placing electrodes on the body surface (standard 12-lead ECG). Each heartbeat produces a characteristic pattern of waves that doctors use to diagnose heart problems.

• P wave — Represents atrial depolarization (electrical activation of both atria → leads to atrial contraction). Small, rounded, upward deflection
• QRS complex — Represents ventricular depolarization (electrical activation of ventricles → leads to ventricular contraction). Sharp, tall spike. Also masks atrial repolarization which happens simultaneously
• T wave — Represents ventricular repolarization (ventricles returning to resting electrical state → relaxation). Broad, rounded wave
• P-Q interval — Time from start of atrial depolarization to start of ventricular depolarization. Includes AV node delay (~0.1s). Prolonged PQ interval = AV block
• S-T segment — Period between end of ventricular depolarization and start of repolarization. Elevation/depression of ST segment indicates myocardial infarction (heart attack)

Quick recall

A patient's ECG shows an abnormally tall and wide QRS complex. What does this suggest?

An abnormal QRS complex suggests problems with ventricular depolarization — possibly ventricular hypertrophy (thickened walls taking longer to depolarize), bundle branch block (delayed conduction), or other ventricular conduction abnormalities.

Cardiac Disorders

Major cardiovascular disorders include hypertension (high BP), coronary artery disease (atherosclerosis), angina pectoris (chest pain from reduced blood flow), and heart failure (inability to pump sufficient blood). These are leading causes of death worldwide.

Cardiovascular diseases (CVDs) are the number one cause of death globally. NCERT covers four major disorders that are frequently tested in NEET. Understanding the pathology behind each helps answer application-based questions.

• Hypertension (High Blood Pressure) — Persistently high BP (>140/90 mmHg). Normal BP = 120/80 mmHg (120 = systolic, 80 = diastolic). Called the 'silent killer' because it often has no symptoms but damages heart, brain, kidneys, and eyes. Risk factors: high salt intake, obesity, stress, smoking, genetics
• Coronary Artery Disease (CAD) — Narrowing/blockage of coronary arteries due to atherosclerosis (fat/cholesterol deposits called plaques). Reduced blood flow to heart muscle → oxygen starvation. Can lead to heart attack
• Angina Pectoris — Chest pain caused by reduced (but not blocked) blood flow to heart muscle (usually during exertion or stress). It's a warning sign — the heart is getting less O2 than it needs. Relieved by rest or nitroglycerin (vasodilator)
• Myocardial Infarction (Heart Attack) — Complete blockage of a coronary artery → death of heart muscle tissue supplied by that artery. Symptoms: severe crushing chest pain, shortness of breath, sweating. Emergency!
• Heart Failure (Congestive Heart Failure) — Heart can't pump enough blood to meet body's needs. NOT the same as cardiac arrest (heart stopping). In heart failure, the heart is still beating but inefficiently. Symptoms: fatigue, breathlessness, edema (fluid buildup)