Human Heart and Cardiac Cycle

Heart Structure and Chambers

The human heart is located in the chest cavity and is enclosed in a double membranous sac called the Pericardium. This sac contains Pericardial Fluid that lubricates the heart, protecting it from friction and preventing over-extension during vigorous contractions.
Double Membranous Sac: The pericardium has an outer fibrous layer and an inner serous layer that secretes pericardial fluid
Protective Function: The pericardial fluid reduces friction as the heart beats continuously, allowing smooth movement within the chest cavity
Prevents Over-Extension: The pericardium limits excessive expansion of the heart during filling, maintaining structural integrity
The wall of the heart is composed of three distinct layers, each with a specific role in protecting, powering, and lining the heart.
Myocardium Dominance: The myocardium is the thickest layer and constitutes the bulk of the heart wall, especially in the ventricles

Heart Wall Layers

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Epicardium: The outermost thin layer, also called the visceral pericardium
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Myocardium: The thick middle layer made of cardiac muscle tissue responsible for contraction
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Endocardium: The smooth inner lining that provides a frictionless surface for blood flow
The Myocardium is made up of Cardiac Muscle, a specialised type of muscle tissue that contains myofibrils and myofilaments of myosin and actin. Their arrangement is similar to skeletal muscle fibres and their mechanism of contraction is essentially the same, but they are branched cells separated by junctions called Intercalated Discs.
Branched Cells: Unlike skeletal muscle fibres, cardiac muscle cells branch and connect to neighbouring cells, allowing rapid signal spread
Intercalated Discs: These junctions between successive cardiac cells contain gap junctions that allow electrical impulses to pass directly from cell to cell
Automatic Rhythmicity: The heart contracts automatically with its own rhythm under the control of the autonomic nervous system — it does not require external nerve stimulation to beat
The heart has four chambers: two upper thin-walled Atria (singular: atrium) and two lower thick-walled Ventricles. The heart functions as a double pump maintaining complete separation of deoxygenated blood on the right side and oxygenated blood on the left side.
Atria: The right and left atria are thin-walled receiving chambers that collect blood returning to the heart
Ventricles: The right and left ventricles are thick-walled pumping chambers that eject blood into the arterial system
Left Ventricle Thickness: The wall of the left ventricle is approximately three times thicker than the right ventricle because it must generate enough pressure to push blood throughout the entire body via the aorta
Complete Separation: Deoxygenated blood is confined to the right side and oxygenated blood to the left side — there is no mixing in a normal heart

Valves and Blood Flow Through the Heart

Deoxygenated blood from the body enters the right atrium through the Venae Cavae — the superior vena cava drains the upper body and the inferior vena cava drains the lower body. From the right atrium, blood passes through the Tricuspid Valve into the right ventricle. When the right ventricle contracts, blood is pumped into the pulmonary trunk, which divides into the right and left Pulmonary Arteries carrying blood to the lungs for gas exchange.
Superior Vena Cava: Collects deoxygenated blood from the head, neck, arms, and upper chest and delivers it to the right atrium
Inferior Vena Cava: Collects deoxygenated blood from the lower limbs, kidneys, liver, and abdominal organs before entering the right atrium
Tricuspid Valve: So named because it has three flaps (cusps); it allows one-way flow from the right atrium to the right ventricle and prevents backflow during ventricular contraction
Pulmonary Trunk: A single vessel that exits the right ventricle and splits into left and right pulmonary arteries heading to each lung

Right Side Blood Flow Pathway

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Venae cavae (SVC + IVC) → Right atrium
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Right atrium → Tricuspid valve → Right ventricle
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Right ventricle → Pulmonary trunk → Pulmonary arteries → Lungs
After oxygenation in the lungs, blood returns to the left atrium through the Pulmonary Veins. It then passes through the Bicuspid Valve (also called the mitral valve) into the left ventricle. When the left ventricle contracts, it forcefully ejects blood into the Aorta, which distributes oxygenated blood to all body tissues except the lungs.
Pulmonary Veins: Four pulmonary veins (two from each lung) carry oxygenated blood from the lungs to the left atrium — these are the only veins that carry oxygenated blood
Bicuspid Valve: Has two flaps and controls flow between the left atrium and left ventricle; also called the mitral valve
Aorta: The largest artery in the body; it arches from the left ventricle and branches to supply every organ except the lungs

Left Side Blood Flow Pathway

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Lungs → Pulmonary veins → Left atrium
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Left atrium → Bicuspid valve → Left ventricle
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Left ventricle → Aorta → Body tissues
The heart contains four sets of valves that ensure one-way blood flow. The atrioventricular valves (Tricuspid Valve and Bicuspid Valve) are supported by Chordae Tendinae — fibrous cords that anchor the valve flaps to Papillary Muscles on the ventricle walls. The Semilunar Valves are located at the base of the pulmonary trunk and the aorta.
Chordae Tendinae: Thin, strong fibrous cords that prevent the AV valve flaps from inverting (blowing backwards) into the atria during ventricular contraction
Papillary Muscles: Extensions of the ventricle wall that contract during systole, pulling the chordae tendinae taut to stabilise the valve flaps
Semilunar Valves at Pulmonary Trunk: Three crescent-shaped cusps that prevent backflow from the pulmonary arteries into the right ventricle
Semilunar Valves at Aorta: Three crescent-shaped cusps that prevent backflow from the aorta into the left ventricle
One-Way Flow: All heart valves open and close passively in response to pressure differences — they never actively push blood

Heart Valves Summary

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Tricuspid valve: Right atrium → Right ventricle (3 flaps)
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Bicuspid (mitral) valve: Left atrium → Left ventricle (2 flaps)
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Pulmonary semilunar valves: Right ventricle → Pulmonary trunk
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Aortic semilunar valves: Left ventricle → Aorta

Pulmonary and Systemic Circulation

The human heart functions as a double pump simultaneously driving two circulatory pathways: Pulmonary Circulation, in which the right ventricle pumps deoxygenated blood to the lungs via the pulmonary arteries for oxygenation, and Systemic Circulation, in which the left ventricle pumps oxygenated blood to all body tissues via the aorta.
Pulmonary Circulation: Right ventricle → pulmonary trunk → pulmonary arteries → lungs (gas exchange) → pulmonary veins → left atrium
Systemic Circulation: Left ventricle → aorta → body tissues (gas and nutrient exchange) → venae cavae → right atrium
Simultaneous Operation: Both circulations occur at the same time with each heartbeat — the right and left ventricles contract together
The heart muscle itself requires a continuous supply of oxygen and nutrients. The Coronary Arteries are the first pair of arteries branching from the base of the Aorta, and they supply blood directly to the heart tissue.
First Branch of Aorta: The coronary arteries arise right at the aortic root, ensuring the heart receives the freshest, most oxygen-rich blood available
Critical Supply: Blockage of a coronary artery leads to a heart attack (myocardial infarction) because the heart muscle downstream is deprived of oxygen
Cardiac Veins: After supplying the heart tissue, blood is collected by cardiac veins and drained into the right atrium via the coronary sinus
The Aorta forms an arch after leaving the left ventricle, giving off branches to the head, arms, and shoulders before descending through the chest and abdominal cavities. The venous system returns blood from the body through a network of veins that ultimately drain into the Venae Cavae. The Hepatic Portal Vein is a special vessel that carries nutrient-rich blood from the digestive tract to the liver for processing.
Aortic Arch Branches: Three branches supply the head, neck, arms, and shoulders with oxygenated blood
Descending Aorta: Gives branches to the chest wall, then passes into the abdominal region supplying the alimentary canal, kidneys, and lower abdomen
Iliac Arteries: The aorta bifurcates into the left and right iliac arteries, each supplying a leg
Superior Vena Cava: Formed by the union of veins collecting blood from the upper body — drains into the right atrium
Inferior Vena Cava: Formed by the union of two iliac veins from the legs; receives the renal vein from each kidney and the hepatic vein from the liver before entering the right atrium
Hepatic Portal Vein: Formed by veins collecting deoxygenated, nutrient-rich blood from the alimentary canal — delivers it to the liver for metabolism and detoxification

The Cardiac Cycle

The Cardiac Cycle is the complete sequence of events in one heartbeat. It consists of three distinct phases: Diastole (relaxation), Atrial Systole (atrial contraction), and Ventricular Systole (ventricular contraction). One complete cardiac cycle lasts approximately 0.8 seconds at a normal resting rate.
One complete heartbeat (systole + diastole) takes 0.8 seconds at a resting heart rate of approximately 75 beats per minute
=Duration of one complete cardiac cycle(seconds (s))
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Heart rate is approximately 75 beats per minute
Three Phases: Each cycle follows the same sequence — diastole, atrial systole, ventricular systole
Continuous Process: The heart contracts about 2.5 billion times in an average lifetime without stopping
Fixed Sequence: The phases always occur in the same order; atria must finish contracting before ventricles begin
During Diastole, all four chambers of the heart are relaxed. Deoxygenated blood enters the right atrium through the Venae Cavae and oxygenated blood enters the left atrium through the Pulmonary Veins. As the Atria fill with blood, they become distended and their internal pressure exceeds that of the relaxed Ventricles, setting the stage for the next phase.
General Relaxation: Both atria and both ventricles are completely relaxed during this phase
Passive Filling: Blood flows into the atria by venous return — no muscular contraction is needed at this stage
Pressure Buildup: As the atria fill and distend, pressure inside them rises above ventricular pressure, which will soon push the AV valves open
When the Atria are filled and distended with blood, the atrial muscles contract simultaneously — this is Atrial Systole. The increased pressure forces the remaining blood through the Tricuspid Valve and Bicuspid Valve into the two relaxed Ventricles, topping up the blood already received by passive filling during diastole.
Simultaneous Contraction: Both atria contract at the same time, ensuring coordinated filling of both ventricles
AV Valves Open: The tricuspid and bicuspid valves are pushed open by the pressure difference between atria and ventricles
Completes Ventricular Filling: Atrial systole delivers the final portion of blood to the ventricles, ensuring they are fully loaded before contraction
After receiving blood from the atria, both Ventricles contract simultaneously in Ventricular Systole. Blood is forcefully ejected into the Pulmonary Arteries from the right ventricle and into the Aorta from the left ventricle. The Tricuspid Valve and Bicuspid Valve snap shut to prevent backflow into the atria, producing the first heart sound.
Simultaneous Ventricular Contraction: Both ventricles contract together, pushing blood out through their respective arteries
AV Valves Close (Lubb): The tricuspid and bicuspid valves close forcefully as ventricular pressure exceeds atrial pressure — this produces the first heart sound, "lubb"
Semilunar Valves Open: The pressure in the ventricles exceeds the pressure in the aorta and pulmonary trunk, forcing the semilunar valves open
Semilunar Valves Close (Dubb): As ventricular systole ends and ventricles begin to relax, blood in the aorta and pulmonary trunk briefly flows backwards, snapping the semilunar valves shut — this produces the second heart sound, "dubb"

Heart Conduction System and ECG

The cardiac cycle begins when the SA Node (sino-atrial node), located at the upper end of the right atrium, sends out electrical impulses to the atrial muscles causing both atria to contract. The SA node is the heart's natural pacemaker — it consists of a small number of diffusely oriented cardiac fibres with few myofibrils and few nerve endings from the autonomic nervous system.
Natural Pacemaker: The SA node initiates every normal heartbeat and sets the heart rate
Location: Positioned at the upper end of the right atrium near the opening of the superior vena cava
Specialised Fibres: SA node cells have few myofibrils, meaning they contract weakly — their primary role is generating electrical impulses, not pumping
Autonomic Influence: While the SA node generates impulses automatically, the autonomic nervous system can speed up (sympathetic) or slow down (parasympathetic) the heart rate
Electrical impulses from the SA Node travel to the AV Node (atrioventricular node) and then through the AV Bundle (bundle of His), which propagates the signal via excitable fibres in the interventricular septum to the myocardium of both ventricles. A critical delay of approximately 0.15 seconds at the AV node ensures atrial systole completes before ventricular systole begins.
AV Node Function: Receives impulses from the SA node and relays them to the ventricles with a built-in delay
0.15 Second Delay: This delay permits the atria to finish contracting and empty completely into the ventricles before the ventricles begin to contract
AV Bundle (Bundle of His): A tract of specialised muscle fibres that carries impulses from the AV node down through the interventricular septum
Purkinje Fibres: The AV bundle branches into smaller fibres (Purkinje fibres) that spread the impulse throughout the ventricular myocardium, causing simultaneous ventricular contraction
As the cardiac impulse spreads through the heart, small electrical currents reach the body surface. An Electrocardiogram (ECG) is a recording of these electrical potentials made by placing electrodes on the skin on opposite sides of the heart. The ECG helps diagnose abnormalities in the heart's rhythmicity and conduction system.
Recording Method: Electrodes placed on the skin detect the tiny electrical currents generated by cardiac muscle activity
P Wave: Occurs just prior to atrial contraction — represents the spread of the impulse through both atria
QRS Complex: Occurs just prior to ventricular contraction — represents the rapid spread of the impulse through the ventricular myocardium via the AV bundle and Purkinje fibres
T Wave: Occurs when the ventricles are recovering from contraction (repolarisation) — indicates the ventricles are resetting electrically

ECG Wave Interpretation

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P wave: Atrial depolarisation (impulse spreading through atria)
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QRS complex: Ventricular depolarisation (impulse spreading through ventricles)
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T wave: Ventricular repolarisation (ventricles recovering electrically)
When the heart's natural conduction system fails, medical interventions can restore normal rhythm. An Artificial Pacemaker is a battery-operated device that generates electrical stimuli to replace or regulate a defective SA node or blocked conduction pathway. A congenital condition known as Blue Babies occurs when the interatrial foramen or ductus arteriosus fails to close after birth, causing mixing of oxygenated and deoxygenated blood and resulting in cyanosis.
Artificial Pacemaker: If the SA node produces weak impulses or the AV pathway is blocked, electrodes of an artificial pacemaker are surgically attached to the ventricle to provide continued rhythmic impulses that take over control of the ventricles
Artificial Pacemaker Mechanism: The device detects the heart's natural rhythm and delivers electrical shocks only when the heart skips a beat or beats too slowly
Blue Babies — Interatrial Foramen: Failure of the opening in the inter-atrial septum to close after birth allows mixing of blood between the two atria
Blue Babies — Ductus Arteriosus: Failure of the ductus arteriosus (a fetal bypass vessel) to fully constrict after birth allows mixing of blood between the pulmonary artery and aorta
Cyanosis: The mixed blood supplied to the body of affected newborns has reduced oxygen, causing a bluish discolouration of the skin — hence the name blue babies