7.6 Blood vessels and their functions

Question 1

What are the 4 different types of blood vessels and what do they do

Answer 1

Arteries: Carry blood away from the heart, and into arterioles

Arterioles: Smaller arteries that control blood flow from arteries into capillaries

Capillaries: Tiny vessels that link arterioles to veins

Veins: Carry blood from capillaries to the heart

Question 2

Arteries, arterioles, capillaries and veins all have the same 5 layers

Describe these
. Outer layer
.Muscle layer
. Elastic layer
. Inner lining
. Lumen

Answer 2

. Tough fibrous outer layer: Resists pressure changes from within and outside

. Muscle layer: It can contract and relax and so control the flow of blood

. Elastic layer: Helps to maintain blood pressure by stretching then springing back (recoiling)

. Thin inner lining (endothelium): that is smooth to reduce friction and thin to allow diffusion

. Lumen: A central cavity of the blood vessel in which blood flows

Question 3

How is the structure of an artery related to its function

Thick outer muscle layer

Answer 3

Arteries have to transport blood rapidly under high pressure from the heart to tissues

. They have a really thick muscle layer compared to veins
. This means smaller arteries can be constricted and dilated to control the volume of blood passing through them

Question 4

How is the structure of an artery related to its function

Thick elastic layer compared to veins

Answer 4

Arteries have to transport blood rapidly under high pressure from the heart to tissues

. Blood pressure in arteries needs to be kept high in order for blood to reach all body parts

. The elastic wall is stretched at each beat of the heart (systole), and then it springs back when the heart relaxes (diastole)

. This stretching and recoil action helps to maintain high pressure and smooth pressure surges created by the heart

Question 5

How is the structure of an artery related to its function

Thick wall

No valves

Answer 5

Arteries have to transport blood rapidly under high pressure from the heart to tissues

The thick wall is very thick which can help the vessel resist bursting under pressure

There are no valves (except in arteries leaving heart eg aorta and pulmonary artery) because blood is kept under constant high pressure due to the heart pumping blood into the arteries.
Therefore it tends to not flow backwards so there is no need for valves

Question 6

compare the structure of arterioles and arteries

Answer 6

. Arterioles carry blood under lower pressure than arteries , from arteries to capillaries, and they control the flow of blood between the two

• They have a thicker muscle layer than in arteries because it allows for the constriction of the lumen of the arteriole.
  This constricts the flow of blood so can control the movement of it into capillaries
• The elastic layer of them is thinner than in arteries because blood pressure is lower as they are further from the heart

Question 7

Describe the difference in muscle layer between veins and arteries

Answer 7

. Muscle layer is relatively thin in veins compared to arteries
. This is because veins carry blood away from tissues, towards the heart so if they constrict and dilate, it won’t control the flow of blood to the tissues

Question 8

Describe difference in elastic layer between veins and arteries

Answer 8

. It is thinner in veins compared to arteries
. This is because the blood pressure is low in veins so they won’t burst, and they won’t recoil

Question 9

Describe difference in thickness of the wall for veins compared to arteries

Answer 9

. The wall is much thinner in veins than in arteries
. Because the pressure in veins is much lower so there is no need for a thick wall to prevent bursting
. It also allows them to be flattened easily which helps blood flow in them

Question 10

Describe valves in veins
What happens to veins when body muscles contract

Answer 10

. Ensure that blood flows in the right direction and not backwards, which it might do because the pressure is so low

. When body muscles contract, veins are compressed which pressurises the blood within them

. Valves ensure that this pressure directs blood in one direction, towards the heart

Question 11

What is the function of capillaries

Answer 11

. To exchange metabolic materials eg oxygen, carbon dioxide, and glucose between the blood and cells of the body

. The flow of blood in the capillaries is slow which allows more time for exchanging materials

They form the link between arteries and veins, which are on each end of it

Question 12

Describe the walls of the capillaries and why this is important

Answer 12

. Thin walls of lining, only one cell thick

. This means the diffusion distance is much shorter (short diffusion pathway) which allows for more rapid diffusion between blood and cells

Question 13

Why are capillaries numerous and highly branches

Answer 13

. To provide a large surface area for exchanging substances

Question 14

Why do capillaries have a narrow diameter

Answer 14

. They can permeate tissues, so no cell is far from a capillary so there is a
short diffusion pathway

Question 15

Why is the lumen of capillaries so narrow

Answer 15

. So red blood cells are squeezed flat against the walls of the capillary
. This brings them even closer to the cells that they supply oxygen to

. So makes a short diffusion pathway

Question 16

Why are there spaces between the lining (endothelial) cells in the capillaries

Answer 16

It allows substances such as white blood cells to pass through and escape to deal with infections in tissues

Question 17

What is the purpose of tissue fluid

Answer 17

Capillaries can’t reach every cell directly so there is a liquid solution that bathes the tissues called tissue fluid, which lets metabolic materials through to reach the cells

It is the means by which materials are exchanged between blood and cells

Question 18

What is tissue fluid

What goes into it from the tissues

Answer 18

. Watery liquid that contains glucose, amino acids, ions, oxygen etc
. Tissue fluid supplies all the tissues and cells with these substances

In return it receives carbon dioxide and waste materials from the cells

Question 19

Where is tissue fluid formed

What is it controlled by

Answer 19

It is formed in blood plasma

The composition of blood plasma is controlled by homeostatic systems

Question 20

How is tissue fluid formed

Answer 20

1) Blood pumped away from heart passes along arteries, then narrow arterioles, then narrower capillaries

2) This pumping from the heart creates hydrostatic pressure at arterial end of the capillary (not the venule /vein end)
This pressure forces tissue fluid to move out of blood plasma capillaries, towards the tissues

Question 21

The outward hydrostatic pressure that forces blood plasma out of the capillary is opposed by 2 forces:

Answer 21

. The blood has a lower water potential than the cells due to the plasma proteins and red blood cells etc, so this causes water to move into the capillaries to the blood, from the tissue

. The hydrostatic pressure of the tissue fluid outside the capillaries, this can resist the outward movement of liquid

However a combination of these forces creates a pressure that forces tissue fluid out of capillaries at the arterial end

Question 22

Describe ultrafiltration

Answer 22

When the hydrostatic pressure is high at the arteriole end of capillary, and forces tissue fluid out

The pressure is only strong enough to force small molecules out eg water, amino acids, glucose

This leaves white blood cells and plasma proteins etc behind in the blood as they’re too big to cross the membrane

Question 23

How does tissue fluid return to the circulatory system

Answer 23

. Once tissue fluid has exchanged metabolic materials with the cells it bathes, most of it returns to blood via the capillaries

1) Loss of tissue fluid from capillaries reduces the hydrostatic pressure inside capillaries
2) So by the time the blood has reached the venule end of capillary network it has a lower hydrostatic pressure than the tissue fluid around it

3) So the tissue fluid is forced back in by the higher hydrostatic pressure outside the capillary

4) Also, blood plasma has lost lots of water but still has proteins etc in it so has a lower water potential than outside in the tissue fluid

5) As a result, water leaves tissues by osmosis and enters blood down a water potential gradient

Question 24

What happens to the tissue fluid that doesn’t return to the capillary

Answer 24

. The remainder is carried back via the lymphatic system
. This is lots of vessels that begin in the tissues

These vessels originally resemble capillaries (but have dead ends) and gradually merge into larger vessels forming a network around the body

These larger vessels drain their contents back to the heart via 2 ducts joining veins close to heart

Question 25

What moves the contents of lymphatic system (not the pumping of heart)

Answer 25

• Hydrostatic pressure of tissue fluid that has left the capillaries

-Contraction of body muscles that squeeze the lymph vessels valves to ensure the fluid is moving in the direction of the heart