Circulatory System

The heart is a muscular organ with FOUR chambers:

• Right atrium: receives deoxygenated blood from the body (via vena cava).
• Right ventricle: pumps deoxygenated blood to the lungs (via pulmonary artery).
• Left atrium: receives oxygenated blood from the lungs (via pulmonary vein).
• Left ventricle: pumps oxygenated blood to the rest of the body (via aorta).

Why TWO sides? Mammals (and birds) have DOUBLE circulation — blood goes through the heart twice per complete loop:

• Right side handles the SHORT pulmonary loop (heart → lungs → heart).
• Left side handles the LONG systemic loop (heart → body → heart).

Keeping the two sides separate means oxygenated and deoxygenated blood don't mix → tissues get full-strength O₂.

The LEFT VENTRICLE has the thickest muscle wall because it has to pump blood to the whole body — much further than the right ventricle's pulmonary loop.

Valves prevent backflow:

• Between atria and ventricles (tricuspid on right, bicuspid/mitral on left).
• At exits from ventricles (pulmonary and aortic semilunar valves).
• The familiar 'lub-dub' heart sound is these valves snapping shut.

Vessel	Function	Wall	Pressure	Valves?
Artery	Away from heart	Thick, muscular, elastic	HIGH	No (only at heart exits)
Vein	Towards heart	Thin walls, larger lumen	Low	YES (prevent backflow)
Capillary	Exchange with cells	One cell thick	Falling	No

Arteries take HIGH-PRESSURE blood (pumped straight from the ventricle). Thick muscular walls stretch and recoil to smooth out the pulses — this is the pulse you feel at your wrist.

By the time blood reaches the capillaries, the pressure has dropped. Capillaries are tiny (5-10 μm — only one red blood cell can pass at a time) and one-cell thick → nutrients, gases and waste can diffuse easily between blood and cells.

Veins carry blood back to the heart at LOW pressure. They have:

• Wider LUMEN (less resistance).
• Thinner walls.
• VALVES to prevent backflow — important because there's not enough pressure to keep blood flowing forwards on its own. Leg muscles squeezing veins as you walk pushes blood up against gravity.

Common confusion: arteries are NOT 'red' and veins 'blue'. Arteries carry oxygenated blood (bright red) EXCEPT the pulmonary artery (deoxygenated, dark red). Veins carry deoxygenated blood EXCEPT the pulmonary vein (oxygenated). The blue you see in your wrist veins is an optical effect of light through skin — blood is never blue.

Blood is a tissue with four main components:

• Plasma (~55% of blood). Yellow liquid. Carries dissolved CO₂, nutrients, hormones, waste, heat.
• Red blood cells (erythrocytes) (~45%). Contain HAEMOGLOBIN → binds O₂. Adapted: no nucleus (more room for haemoglobin), biconcave disc shape (big surface area + flexible to squeeze through capillaries).
• White blood cells (leucocytes). Fight infection. Two main types: PHAGOCYTES (engulf bacteria) and LYMPHOCYTES (make antibodies).
• Platelets. Cell fragments. Help blood CLOT at wounds. Without platelets, even a small cut would bleed indefinitely (this happens in haemophilia).

Numbers at a glance: ~5 million red cells per microlitre, ~7,000 white cells, ~250,000 platelets.

When you donate blood, ~470 mL is taken. Your body replaces the plasma in a day or two, the red cells in 4-6 weeks. One donation can help up to 3 patients (if separated into red cells, plasma, platelets).

Trace one drop of blood through the body:

1. Deoxygenated blood arrives at the right atrium from the body (via vena cava).
2. Through the tricuspid valve into the right ventricle.
3. Pumped via the pulmonary artery to the LUNGS.
4. Picks up O₂ in alveoli; releases CO₂.
5. Returns to the left atrium via the pulmonary vein.
6. Through the bicuspid valve into the left ventricle.
7. Pumped via the AORTA to the body.
8. Travels through arteries → arterioles → capillaries; delivers O₂ + nutrients; collects CO₂ + waste.
9. Returns via veins → vena cava → right atrium. Loop complete.

This DOUBLE circulation is more efficient than the single circulation of fish (which would deliver O₂ at the falling lung-pressure, slow). Mammals get a fresh pressure boost from the left ventricle after the lungs.

A famous medical case: 'hole in the heart' (septal defect) — a hole between the right and left ventricles lets blood mix. Less efficient O₂ delivery → fatigue + bluish skin. Surgically repairable.