The Blood System

Double circulation. Blood passes through the heart TWICE per circuit:

1. Pulmonary circuit — right ventricle → pulmonary artery → lungs (gas exchange) → pulmonary veins → left atrium.
2. Systemic circuit — left ventricle → aorta → body tissues → vena cavae → right atrium.

Advantage: maintains high pressure for systemic circulation while allowing low pressure in the lungs (preventing damage to delicate alveolar capillaries).

Heart anatomy.

• Right atrium receives deoxygenated blood from vena cavae.
• Right ventricle pumps to pulmonary artery (to lungs).
• Left atrium receives oxygenated blood from pulmonary veins.
• Left ventricle pumps to aorta (to body) — thickest muscle (left wall ~3× thicker than right) for high pressure.

Valves prevent backflow:

• Atrioventricular valves between atrium and ventricle: tricuspid (right), bicuspid/mitral (left).
• Semilunar valves at exit of ventricles: aortic and pulmonary.

Coronary arteries branch from the aorta and supply the heart muscle itself. Blockage causes a heart attack (myocardial infarction).

Blood vessels.

Vessel	Wall	Lumen	Pressure	Function
Artery	Thick muscular + elastic	Narrow	High; pulsatile	Carries blood AWAY from heart
Arteriole	Smooth muscle	Narrow	Falling	Regulates flow into capillaries
Capillary	Single endothelial cell layer	Cell-wide	Low	Exchange of materials with tissues
Venule	Thin	Wider	Very low	Collects from capillaries
Vein	Thin walls, with valves	Wide	Low	Carries blood TO heart

Capillary exchange. Single endothelial cell layer + slow flow → time and short distance for diffusion. Hydrostatic pressure forces plasma out at arteriole end; lower pressure + osmotic pull returns most fluid at venule end. Excess returns via lymphatic system.

Cardiac cycle (~0.8 s at rest):

1. Atrial systole (~0.1 s) — atria contract, push remaining blood into ventricles. AV valves open.
2. Ventricular systole (~0.3 s) — ventricles contract. AV valves close ("lub"); pressure forces semilunar valves open; blood ejected to aorta/pulmonary artery.
3. Diastole (~0.4 s) — ventricles relax. Semilunar valves close ("dub"); atria fill from veins; some passive flow into ventricles.

Control of heart rate. The sinoatrial node (SAN) in the right atrium generates spontaneous electrical impulses (~60-100 per minute at rest) — the heart's natural pacemaker. The impulse spreads across the atria (causing atrial contraction), reaches the atrioventricular node (AVN), then travels down the bundle of His and Purkinje fibres to the ventricles (causing ventricular contraction).

Adjustment. The medulla oblongata adjusts heart rate via:

• Sympathetic nerves (release noradrenaline → faster).
• Parasympathetic nerves (release acetylcholine → slower).
• Hormones (adrenaline → faster).

This responds to exercise, stress, and blood CO₂/O₂ levels.