Blood vessels, arteries, veins and capillaries

Blood travels through three main types of blood vessel, arteries, veins and capillaries, and each is structurally adapted to the role it plays in circulation.

Blood leaves the heart through the aorta and pulmonary artery, passes through smaller arteries and arterioles, spreads through capillary beds in every organ, then collects into venules and veins before returning through the vena cava and pulmonary vein. Along that route the pressure falls steadily, and the wall of each vessel matches the pressure it must withstand and the job it must do.

A labelled cross-section of an artery and a vein side by side is one of the standard diagrams in this chapter.

Comparing arteries, veins and capillaries

Parts of the vessel wall

How structure suits function

An artery has a thick wall containing muscle and elastic fibres so it can withstand the high pressure of blood pumped directly from the heart, and the elastic tissue stretches and recoils to smooth out the pulsing flow into a steadier one. A vein carries blood at much lower pressure, so its wall can be thinner, but it has valves at intervals to prevent blood flowing backwards, which is especially important when blood is returning from the limbs against gravity.

A capillary wall is only one cell thick, giving the shortest possible diffusion distance for oxygen, carbon dioxide, glucose and other substances to exchange between the blood and body tissues.

• Artery: thick muscular wall, withstands high pressure without bursting.
• Artery: elastic fibres, stretch and recoil, keeping blood moving between heartbeats and producing the pulse you can feel at the wrist.
• Artery: narrow lumen, keeps the pressure high so blood reaches distant organs.
• Vein: wide lumen, offers little resistance, so low-pressure blood still returns to the heart.
• Vein: valves and position between skeletal muscles, when the muscles contract they squeeze the vein, and the valves make sure the blood can only move towards the heart.
• Capillary: wall one cell thick and lumen the width of one red blood cell, every red blood cell passes close to the wall, so the diffusion distance for oxygen is minimal.
• Capillary beds: enormous number of branches, a large total surface area and slow flow give time for exchange with every cell in the tissue.

Related processes

At the arteriole end of a capillary bed, the blood pressure forces water and dissolved substances, glucose, amino acids, oxygen and salts, out through the thin wall to form tissue fluid, which bathes the cells. Cells take what they need from the tissue fluid and release carbon dioxide and other wastes into it.

At the venule end most of the fluid returns to the capillary by osmosis; the remainder drains into lymph vessels as lymph and rejoins the blood near the heart.

Gas exchange in the lungs and in respiring tissues depends on the capillary. Oxygen diffuses from the alveolus into the pulmonary capillary and binds to haemoglobin; in a muscle, oxyhaemoglobin releases oxygen, which diffuses through the capillary wall into the cells.

The muscular wall of arterioles is also the site of vasodilation and vasoconstriction, which redirect blood to the skin during heat regulation and to the muscles during exercise.

Common exam errors

Worked question

Question (in the style of Paper 2 Section A): Diagram W shows cross-sections of two blood vessels, K and L. Vessel K has a thick wall and a small lumen; vessel L has a thin wall and a large lumen.

(a) Identify K and L. [2 marks] (b) Explain how the wall of K is adapted to its function. [3 marks] (c) Vessel L contains valves. State the function of the valves and explain why K does not need them. [2 marks]

Model answer: (a) K is an artery; L is a vein. (b) The wall of K is thick and muscular, so it can withstand the high pressure of blood pumped from the heart.

It contains elastic fibres that stretch when the heart contracts and recoil between beats, which maintains the pressure and smooths the flow of blood. (c) The valves prevent backflow of blood, keeping it moving towards the heart.

K does not need them because the blood in an artery is at high pressure and is pushed forward continuously by the heart.

Marking note: in part (b) the three marks are for thick wall linked to pressure, elastic fibres linked to recoil, and the effect on flow. Listing features without the linked consequence earns at most one mark.

Source:SRC-DSKP-EN

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