Circulation in the Blood Vessels

Mammalian Transport Systems

What are the three main types of blood vessel?

Arteries, veins and capillaries.

What is the main function of arteries?

To carry blood away from the heart.

What is the main function of veins?

To carry blood towards the heart.

What is the main function of capillaries?

To allow exchange of substances between the blood and body tissues.

What is a lumen?

The central space inside a blood vessel through which blood flows.

What is the arterial system?

The system of arteries in the body.

What type of blood do most arteries carry?

Oxygenated blood.

Which arteries carry deoxygenated blood?

The pulmonary artery and the umbilical artery.

What does the pulmonary artery do?

It carries deoxygenated blood from the heart to the lungs.

What does the umbilical artery do?

During pregnancy, it carries deoxygenated blood from the fetus to the placenta.

Why do arteries have thick walls?

They must withstand the high pressure produced when the heart pumps blood.

What are the main tissues found in artery walls?

Elastic fibres, smooth muscle and collagen fibres.

What is the function of elastic fibres in artery walls?

They allow the artery to stretch when blood enters at high pressure and recoil between heartbeats.

Why is elastic recoil important in arteries?

It helps maintain blood pressure and keeps blood moving continuously between heartbeats.

Do elastic fibres generate blood pressure?

No. The heart generates the pressure; elastic recoil helps maintain it.

What is the function of collagen fibres in arteries?

They provide strength and prevent the artery wall from overstretching or bursting under high pressure.

What is the function of smooth muscle in artery walls?

It can contract or relax to change the diameter of the lumen and therefore control blood flow.

What is the endothelium?

The smooth inner lining of a blood vessel.

Why is the endothelium of an artery smooth?

It reduces friction and allows blood to flow easily.

Why do arteries have relatively small lumens?

A smaller lumen helps maintain the high pressure of blood travelling away from the heart.

What happens to an artery when the heart contracts?

High-pressure blood enters the artery and stretches its elastic fibres.

What happens to an artery between heartbeats?

The elastic fibres recoil, helping maintain pressure and continuous blood flow.

What causes the pulse felt in an artery?

The surge of high-pressure blood entering the artery with each heartbeat.

What happens to blood pressure as blood travels further away from the heart through arteries?

It gradually decreases.

What are peripheral arteries?

Arteries further away from the heart but before the arterioles.

What are arterioles?

The smallest branches of the arterial system, furthest from the heart.

How does the structure of arteries change as they get further from the heart?

The proportion of elastic tissue decreases and the proportion of smooth muscle generally becomes greater.

Why does the aorta contain a large proportion of elastic fibres?

It receives blood directly from the heart at very high pressure and must stretch and recoil with each heartbeat.

Why do arterioles contain a relatively large proportion of smooth muscle?

The smooth muscle allows their lumen diameter to be changed to control blood flow into different tissues.

What happens when smooth muscle in an arteriole contracts?

The lumen becomes narrower, increasing resistance and reducing blood flow through the vessel.

What happens when smooth muscle in an arteriole relaxes?

The lumen becomes wider, decreasing resistance and allowing more blood to flow through the vessel.

How can arterioles regulate the activity of an organ?

They alter their lumen diameter to change the amount of blood supplied to the organ.

Why might blood flow to the skin decrease when a person is cold?

Arterioles supplying the skin constrict, reducing blood flow and therefore reducing heat loss.

What is vasoconstriction?

Contraction of smooth muscle in blood vessel walls, causing the lumen to narrow and blood flow to decrease.

What is vasodilation?

Relaxation of smooth muscle in blood vessel walls, causing the lumen to widen and blood flow to increase.

What do arterioles lead into?

Networks of capillaries.

What is a capillary network?

A branching network of very small blood vessels that spreads throughout body tissues.

What vessels connect arterioles to venules?

Capillaries.

Why are capillaries found close to almost every cell?

This creates a short diffusion distance between the blood and cells.

How wide is a capillary lumen?

Approximately the width of a single red blood cell.

Why is the narrow lumen of a capillary useful?

It forces red blood cells close to the capillary wall, reducing the diffusion distance for exchange.

How thick are capillary walls?

Only one cell thick.

What are capillary walls mainly made from?

A single layer of endothelial cells.

Why are capillary walls only one cell thick?

This creates a very short diffusion distance, allowing rapid exchange of substances between blood and tissues.

Do capillary walls contain large amounts of smooth muscle, elastic tissue or collagen?

No. Their walls are extremely thin and specialised for exchange.

How does the extensive branching of capillary networks aid exchange?

It creates a very large total surface area for diffusion.

Why does blood flow relatively slowly through capillaries?

The capillary network has a very large total cross-sectional area, reducing the velocity of blood.

Why is slow blood flow through capillaries useful?

It provides more time for substances to diffuse between the blood and surrounding cells.

What substances diffuse from capillaries into body tissues?

Oxygen and useful molecules such as digested food molecules and hormones.

What substances diffuse from body tissues into capillaries?

Carbon dioxide and other metabolic waste products.

What happens to the oxygen content of blood as it passes through systemic capillaries?

It decreases as oxygen diffuses into body cells.

What happens to the carbon dioxide content of blood as it passes through systemic capillaries?

It increases as carbon dioxide diffuses from respiring cells into the blood.

Why are capillaries particularly efficient exchange surfaces?

They have walls one cell thick, narrow lumens, a large total surface area and slow blood flow, producing rapid diffusion between blood and tissues.

What are venules?

The smallest branches of the venous system that collect blood from capillary networks.

What do venules eventually join to form?

Larger veins.

What is the venous system?

The system of veins in the body.

What type of blood do most veins carry?

Deoxygenated blood.

Which veins carry oxygenated blood?

The pulmonary vein and the umbilical vein.

What does the pulmonary vein do?

It carries oxygenated blood from the lungs back to the heart.

What does the umbilical vein do?

During pregnancy, it carries oxygenated blood from the placenta to the fetus.

How do vein walls compare with artery walls?

Vein walls are thinner and contain less smooth muscle and elastic tissue.

Why do veins have thinner walls than arteries?

Blood in veins is at much lower pressure, so thick walls are unnecessary.

Why do veins contain less elastic tissue than arteries?

Blood pressure in veins is low and there are no large pressure surges that require extensive stretching and recoil.

Why do veins have relatively large lumens?

The large lumen reduces resistance to blood flow and allows a large volume of blood to be carried at low pressure.

What is the function of collagen in vein walls?

It provides strength and support to the vessel.

How much of the body's blood can be present in the veins at one time?

More than half of the body's blood volume.

Why are veins described as a blood reservoir?

They can contain a large proportion of the body's total blood volume.

Why is blood pressure low by the time blood reaches veins?

Pressure generated by the heart is progressively lost as blood passes through arteries, arterioles and the extensive capillary network.

How is blood returned to the heart through veins despite its low pressure?

Skeletal muscle contractions and one-way valves help move blood towards the heart.

How do skeletal muscles help venous return?

Contracting muscles squeeze nearby veins, forcing blood through them towards the heart.

Where are many large veins located?

Between large skeletal muscles, particularly in the arms and legs.

What type of valves are found in veins?

Semilunar one-way valves.

What is the function of valves in veins?

They prevent the backflow of blood and ensure it travels towards the heart.

Why are valves particularly important in veins?

Blood pressure is low, so valves prevent blood from flowing backwards between muscle contractions.

How do semilunar valves work when blood moves towards the heart?

The valves open and allow blood to pass through.

What happens if blood begins to flow backwards in a vein?

The semilunar valves close, preventing backflow.

Why are valves generally unnecessary in arteries?

The high pressure generated by the heart keeps blood moving away from the heart.

What is the inferior vena cava?

The large vein that carries deoxygenated blood from the lower parts of the body to the heart.

What is the superior vena cava?

The large vein that carries deoxygenated blood from the upper parts of the body to the heart.

Where do the inferior and superior vena cava carry blood?

Back to the heart.

How does blood pressure change through the circulatory system?

It is highest in the arteries and decreases through the arterioles and capillaries until it is very low in the veins.

Where is blood pressure highest?

In the large arteries closest to the heart.

Why is blood pressure highest in arteries?

They receive blood directly from the pumping action of the heart.

Where is blood pressure lowest?

In the veins.

How does blood velocity change through the circulatory system?

It is relatively high in arteries, falls to its lowest level in capillaries, then increases again in veins.

Where is blood velocity lowest?

In the capillaries.

Why is blood velocity lowest in the capillaries?

The capillary network has the greatest total cross-sectional area.

Why is low blood velocity in capillaries advantageous?

It allows sufficient time for diffusion and exchange between the blood and tissues.

Which blood vessels have the greatest total cross-sectional area?

The capillaries.

Why do capillaries have such a large total cross-sectional area?

There are enormous numbers of tiny capillaries arranged in highly branched networks.

What happens to total cross-sectional area from arteries to capillaries?

It increases greatly and reaches its maximum in the capillary network.

What happens to total cross-sectional area from capillaries to veins?

It decreases as vessels merge into larger venules and veins.

What is the relationship between total cross-sectional area and blood velocity?

As total cross-sectional area increases, blood velocity decreases; as total cross-sectional area decreases, velocity increases.

Compare artery and vein lumen size.

Arteries have relatively small lumens, while veins have relatively large lumens.

Compare artery and vein wall thickness.

Arteries have thick walls, while veins have thinner walls.

Compare elastic tissue in arteries and veins.

Arteries contain much more elastic tissue than veins.

Compare smooth muscle in arteries and veins.

Arteries generally contain more smooth muscle than veins.

Compare blood pressure in arteries and veins.

Arteries carry blood at high pressure, while veins carry blood at low pressure.

Compare valves in arteries and veins.

Veins contain one-way valves to prevent backflow; arteries generally do not require valves.

Compare the functions of arteries and veins.

Arteries carry blood away from the heart at high pressure, whereas veins return blood to the heart at low pressure.