Blood Vessel Structure and Rate of Flow

A transport network built for different jobs

Blood must travel away from the heart, pass close to body cells and then return to the heart. One type of vessel could not perform all three jobs efficiently, so the circulatory system contains arteries, capillaries and veins. Their structures differ because the pressure and purpose of the blood flow differ in each part of the system.

The lumen is the space through the centre of a vessel. Blood flows through this space, while the vessel wall surrounds it.

Arteries carry blood away from the heart

When a ventricle contracts, it forces blood into an artery at high pressure. Arteries therefore have thick walls containing muscle and elastic tissue. The strong wall withstands the pressure, while the elastic tissue allows the wall to stretch as blood is forced into it and then recoil towards its original shape. This recoil helps maintain the pressure and keeps blood moving between heartbeats.

An artery has a relatively narrow lumen and no valves along most of its length. The high pressure produced by the heart is sufficient to keep blood moving forwards.

An artery is defined by the direction of flow, not by how much oxygen its blood contains: it always carries blood away from the heart. Most arteries carry oxygenated blood, but the pulmonary artery carries deoxygenated blood from the heart to the lungs.

Veins return blood to the heart

After blood has passed through networks of tiny capillaries, its pressure is much lower. Veins carry this blood towards the heart. They have thinner walls with less muscle and elastic tissue because they do not have to withstand the high pressure found in arteries. Their wide lumen provides plenty of space for blood to flow at low pressure.

Low-pressure blood could move backwards, especially in the limbs, so veins contain valves. These open to allow movement towards the heart and close if blood begins to flow the other way. Contractions of nearby skeletal muscles squeeze veins and help push blood forwards; the valves prevent each squeeze from simply forcing it backwards again.

Most veins carry deoxygenated blood, but the pulmonary vein carries oxygenated blood from the lungs towards the heart. Again, direction—not oxygen content—determines whether a vessel is an artery or a vein.

Capillaries are exchange surfaces

Arteries branch repeatedly into increasingly small vessels and ultimately into networks of capillaries within tissues. Capillaries then connect to vessels that lead into veins. Their function is not simply to transport blood quickly: it is to bring blood close enough to cells for substances to be exchanged.

A capillary wall is only one cell thick, giving substances a very short diffusion distance. Capillaries form extensive networks, producing a large surface area for exchange and placing blood close to many cells. Their lumen is extremely narrow, so red blood cells pass close to the wall.

Oxygen and glucose can diffuse from the blood to respiring cells, while carbon dioxide can diffuse from cells into the blood. Other dissolved substances are also exchanged according to their concentration gradients. The thin capillary wall makes this exchange rapid enough to supply cells and remove waste products.

Comparing structure with function

A strong explanation links each feature to the conditions and job of the vessel:

• Artery: high pressure $\rightarrow$ thick muscular and elastic wall to withstand pressure, stretch and recoil.
• Vein: low pressure $\rightarrow$ thinner wall and wide lumen; valves prevent backflow, while skeletal muscles help move blood.

Calculating the rate of blood flow

Rate of blood flow is a compound measure because it combines volume and time:

$$\text{rate of blood flow}=\frac{\text{volume of blood}}{\text{time}}$$

The units depend on the units supplied. A volume in cubic centimetres and a time in seconds give a rate in $\text{cm}^3/\text{s}$. A volume in litres and a time in minutes give $\text{L}/\text{min}$.

For example, if $24\text{ cm}^3$ of blood passes in one minute, first convert one minute to $60\text{ s}$. The rate is therefore $24\div60=0.4\text{ cm}^3/\text{s}$. This result means that, on average, of blood passes the stated point every second.

Rearranging the relationship gives:

$$\text{volume}=\text{rate}\times\text{time}$$ $$\text{time}=\frac{\text{volume}}{\text{rate}}$$

Always make the time unit in the calculation match the time unit required in the answer. To change minutes to seconds, multiply by 60; to change seconds to minutes, divide by 60.