Blood and blood vessels

MDCAT Biology · Biology · Circulation. 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 leaves the heart in arteries, passes through capillaries in the tissues and returns in veins. The structure of each vessel fits its job. Arteries have thick, muscular, elastic walls to stand high pressure. Capillary walls are one cell thick, so exchange is fast. Veins have thinner walls, a wider lumen and valves, because their pressure is low.

Blood pressure is highest in the aorta and falls all the way round to the venae cavae.

Blood is about 55% plasma and 45% cells. Plasma is mostly water and carries dissolved food, wastes, hormones, proteins and heat.

Rules to remember

Normal resting blood pressure is about 120/80 mm Hg (systolic/diastolic).
Blood flows slowest in the capillaries, because their total cross-sectional area is the greatest.
Blood returns up the leg veins by the squeezing of skeletal muscles, with valves stopping backflow.
Red blood cells: biconcave, no nucleus, carry oxygen on haemoglobin; made in red bone marrow; live about 120 days.
White blood cells (about 7 000 to 8 000 per mm³): neutrophils (most common, phagocytes), lymphocytes (immunity), monocytes (largest, become macrophages), eosinophils (allergy, worms), basophils (heparin and histamine).
Platelets are cell fragments that start clotting: prothrombin → thrombin (needs Ca²⁺); thrombin turns fibrinogen → fibrin.
Plasma proteins: albumin (osmotic pressure), globulins (antibodies), fibrinogen (clotting).

Common mistake

Blood does not flow slowest in the veins, where pressure is lowest. Speed is lowest in the capillaries; pressure is lowest in the venae cavae.

Worked example

Blood makes up about 8% of body weight. Estimate the mass of blood in a 70 kg adult, and say roughly what volume this is.

Mass of blood = 8 ÷ 100 × 70 = 5.6 kg.
Blood is only slightly denser than water (1 kg per litre), so 5.6 kg is a little over 5 litres.
Of this, about 55% is plasma: 0.55 × 5.6 ≈ 3.1 kg.

Answer: About 5.6 kg of blood, a little over 5 litres, of which roughly 3 kg is plasma.

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