Blood Vessels

Three types of vessel, three completely different jobs. An artery has to survive a punch of pressure every second. A capillary has to be thin enough for a gas to cross it. A vein has to get blood home uphill with almost no push behind it. Every difference in their walls comes straight from those jobs.

The five vessels, in order

Blood leaves the heart in an artery, which branches into narrower arterioles, which feed a network of capillaries in the tissues. The capillaries join up into venules, and the venules join into veins that return the blood to the heart.

Arteries: built for pressure

Every time the ventricles contract, a slug of blood is forced into the arteries at high pressure. The artery wall has three layers that deal with it.

• Inner layer (endothelium) – one cell thick, very smooth, so friction is low and blood flows freely. Every vessel has this lining.
• Middle layer – thick smooth muscle plus a thick layer of elastic tissue. The muscle strengthens the wall so it withstands high pressure, and can contract or relax to change the lumen diameter. The elastic tissue stretches when blood surges in and recoils afterwards.
• Outer layer – mostly collagen with more elastic fibres. Collagen is a strong protein that stops the vessel over-stretching or rupturing.

Arteries also have a narrow lumen compared with the thickness of their wall, and that helps keep the pressure high.

Systolic and diastolic pressure

• Systolic pressure is the peak. Blood is forced out of the ventricles, the artery walls are pushed outwards and the elastic fibres stretch.
• Diastolic pressure is the low point, as the heart relaxes. The stretched fibres recoil, squeezing the blood onwards and keeping the pressure up between beats.

Without that recoil, blood flow would come in stop-start pulses with nothing in between. The elastic wall turns a series of pushes into a continuous stream.

Arteries can also change their own diameter:

• Vasoconstriction – the circular muscle contracts, the lumen gets narrower and blood pressure rises.
• Vasodilation – the muscle relaxes, the lumen gets wider and blood pressure falls.

Capillaries: built for exchange

Capillaries are where the whole circulatory system finally does its job. They form dense networks called capillary beds that run in between the cells of a tissue.

• The wall is a single layer of endothelial cells. One cell thick means a very short diffusion distance for oxygen, carbon dioxide and glucose.
• There are gaps between those cells called fenestrations. Plasma leaks out through them to form tissue fluid, which bathes the cells. Large molecules such as plasma proteins normally cannot fit through.
• Branching into a bed gives a huge surface area and puts a capillary close to almost every cell, cutting the diffusion distance again.
• The lumen is so narrow that red blood cells pass through in single file. This slows the blood, giving more time for diffusion, and presses the red cells right up against the wall.

Veins: built to get blood home

By the time blood reaches a vein it has crossed a capillary bed and lost almost all of its pressure. Everything about a vein is a response to that.

• The middle layer is thin – there is no high pressure to withstand, so a thick muscular layer would be wasted tissue.
• The walls are flexible, so surrounding skeletal muscles can squeeze them as you move. That squeeze is what actually pushes venous blood along.
• Valves stop the blood falling backwards. Any blood pushed the wrong way is caught in a valve and sent forwards by the next muscle movement.
• A wide lumen maximises the volume of blood that can flow at once and reduces friction with the wall.

Worked examples