Double circulation separates two destinations
The right side of the heart pumps deoxygenated blood through the pulmonary circuit to the lungs. The left side pumps oxygenated blood through the systemic circuit to the body. Blood passes through the heart twice during one complete trip around both circuits, hence double circulation.
The left ventricle has a thicker wall because systemic circulation requires a larger pressure to overcome greater total resistance. The right ventricle pumps only to the nearby lungs, where excessive pressure could damage delicate pulmonary capillaries.
Venae cavae return deoxygenated blood.
Pumps blood through the pulmonary artery.
Gas exchange oxygenates the blood.
Pumps blood through the aorta to the body.
Valves respond to pressure gradients
A valve does not actively pull itself open. It opens when pressure behind it exceeds pressure ahead and closes when the gradient reverses. Atrioventricular valves lie between atria and ventricles; semilunar valves lie at the exits to the pulmonary artery and aorta.
Chordae tendineae and papillary muscles prevent atrioventricular valves from inverting during ventricular contraction. They do not close the valve; the pressure difference does that.
| Phase | AV valves | Semilunar valves | Main movement |
|---|---|---|---|
| Ventricular filling | Open | Closed | Atria to ventricles |
| Ventricular systole | Closed | Open after pressure rises | Ventricles to arteries |
| Early diastole | Closed briefly | Closed | Pressure falls before filling |
The heartbeat begins in specialised muscle
The sinoatrial node in the right atrium acts as the normal pacemaker. Its wave of depolarisation spreads across atrial muscle and causes atrial systole. Non-conducting tissue between atria and ventricles prevents immediate spread to the ventricles.
The atrioventricular node introduces a brief delay, allowing ventricular filling. The impulse then travels through the bundle of His and Purkyne fibres toward the apex, producing coordinated ventricular contraction from the lower regions upward.
Initiates atrial depolarisation.
Delays the signal briefly.
Carries excitation through the septum.
Spread excitation through ventricular walls.
Systole, diastole and heart sounds
Systole means contraction of a chamber; diastole means relaxation. During ventricular systole, rising ventricular pressure closes the atrioventricular valves, producing the first heart sound. When ventricular pressure falls below arterial pressure, semilunar valves close and contribute to the second sound.
A complete cardiac cycle includes overlapping atrial and ventricular events rather than one global contraction followed by one global relaxation. Pressure curves explain the valve sequence more reliably than memorising isolated phases.
Flow, pressure and exchange vessels
Arteries have thick elastic and muscular walls to withstand and smooth high-pressure pulsatile flow. Veins operate at lower pressure, possess a larger lumen and often contain valves. Capillaries have very thin walls and an enormous combined cross-sectional area, slowing blood and shortening diffusion distance.
Tissue fluid forms as hydrostatic pressure drives plasma components out of capillaries; most returns as hydrostatic pressure falls and osmotic forces favour re-entry. Excess tissue fluid enters lymphatic vessels and eventually returns to the blood.
The ideas to carry forward
- The right and left sides power pulmonary and systemic circuits.
- Pressure differences, not active valve movement, control one-way flow.
- The conduction system coordinates atrial then ventricular systole.
- Vessel structure reflects pressure, direction and exchange function.
Answer first. Then reveal the marking logic.
01Why do semilunar valves close at the beginning of ventricular diastole?2 marks · show the biological link
Answer: Ventricular pressure falls below pressure in the aorta and pulmonary artery, reversing the pressure gradient and filling the valve pockets.
02Why is blood velocity low in capillaries despite their narrow individual diameter?2 marks · show the biological link
Answer: The capillary network has an enormous total cross-sectional area, so flow is distributed across many parallel vessels.
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