Transport in humans

Biology · Transport. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Blood is plasma (about 55%) plus cells (about 45%). Red cells carry oxygen, white cells defend the body and platelets start clotting. The heart is a double pump. The right side sends deoxygenated blood to the lungs; the left side sends oxygenated blood to the body at much higher pressure.

Each heartbeat starts at the sino-atrial node, the pacemaker. The atria contract first, then the ventricles, and valves keep the flow one-way. Arteries carry blood away from the heart, veins return it, and exchange with tissues happens only in capillaries.

Fluid that leaks from capillaries returns to the blood as lymph. Lymphocytes give immunity: B cells make antibodies and T cells attack infected cells directly.

Rules to remember

White cells: granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (monocytes, lymphocytes). Neutrophils are the most numerous and are phagocytic.
Clotting: prothrombin → thrombin; thrombin changes soluble fibrinogen into insoluble fibrin.
Leukaemia: cancer with uncontrolled production of white cells. Thalassaemia: inherited defect in making haemoglobin chains.
Cardiac cycle about 0.8 s: atrial systole 0.1 s, ventricular systole 0.3 s, diastole 0.4 s. "Lubb" = atrioventricular valves close; "dubb" = semilunar valves close.
Impulse path: SA node → AV node → bundle of His → Purkinje fibres. cardiac output = stroke volume × heart rate.
Arteries: thick muscular elastic walls, narrow lumen, high pressure. Veins: thin walls, wide lumen, valves. Capillaries: wall one cell thick. Normal blood pressure is about 120/80 mm Hg.
Active immunity: the body makes its own antibodies (infection or vaccine). Passive immunity: ready-made antibodies are received (placenta, milk or antiserum).

Common mistake

Students define an artery as a vessel carrying oxygenated blood. The pulmonary artery carries deoxygenated blood. Define arteries as vessels that carry blood away from the heart.

Worked example

An athlete at rest has a heart rate of 60 beats per minute and a stroke volume of 80 mL. Calculate the cardiac output.

cardiac output = stroke volume × heart rate.
= 80 mL × 60 per minute = 4800 mL per minute.
4800 ÷ 1000 = 4.8 L per minute.

Answer: 4.8 L per minute.

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Chapter 14: Transport

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