- When blood first leaves the heart it is travelling in the arteries.
- Arteries divide to form arterioles.
- These divide to make capillaries
- These join up to form venules.
- And these join to form veins.
- Veins carry blood back to the heart.
Comparison of Artery, Vein and Capillary
Elastic tissues in wall
- Artery: Large amount, especially in arteries close to the heart. This allows the wall to stretch and recoil as high pressure blood pulses through.
- Vein: Small amount. Blood in veins is at low pressure, so there is no need for the walls to be elastic.
- Capillary: None.
Smooth muscle in wall
- Artery:Relatively large amount in small arteries and arterioles. Contraction of this muscle reduces the size of the lumen, which can divert blood from one area to another.
- Vein: Small amount. All blood in veins is travelling back to the heart, so there is no advantage in being able to divert it to different tissues.
- Capillary: None.
Thickness of wall
- Artery:Relatively thick. Artery walls must be strong enough to withstand the high pressure of the blood flowing inside them.
- Vein: Relatively thin. The blood in veins is at a low pressure, so there is no need for a thick wall.
- Capillary: The wall is only one cell thick. Moreover, these cells are thin and flattened, so the wall is as thin as possible. This allows rapid transfer of substances by diffusion between blood and tissue fluid.
Endothelium (inner lining)
- Artery:Very smooth. This allows blood to flow freely and quickly. A rough wall would present more resistance to blood flow. Intact endothelium decreases the likelihood of a thrombus (blood clot) forming.
- Vein: As arteries
- Capillary: The wall of the capillary is made of endothelium only, with no other layers of tissue. The thin endothelium and pores speed up exchange of substances with the tissues.
Presence of valves
- Artery:There are no valves in the arteries, except those in the aorta and pulmonary artery as they leave the heart.
- Vein: Veins have valves, which allows blood to flow towards the heart but not away from it. They are necessary because of the low pressure of blood in the veins.
- Capillary: There are no veins in the capillaries.
Diameter of lumen
- Artery:Relatively small compared with the veins
- Vein: Relatively large. The wide lumen of a vein provides less resistance to blood flow then narrow lumen of arteries. It allows blood through at a low pressure easily.
- Capillary: Tiny. This brings the blood as close as possible to the cells in the tissues with which it is exchanging materials such as oxygen and carbon dioxide.
- Its wall is made up of three layers;
- An inner endothelium- made up of a layer of flat cells. It’s called the Squamous epithelium and is very smooth.
- Tunica media- contains smooth muscle, collagen and elastic fibres. (the thickest part)
- Tunica externa- contains elastic fibres and collagen fibres.
- •Arterioles are smaller vessels which much more smooth muscle which helps them contract.
- As blood leaves a capillary bed, the capillaries join together to make venules and then veins.
- The blood which enters veins is at a much lower pressure then arteries.
- No need for thick elastic walls. The tunica media is much thinner with far fewer elastic fibres and muscle fibres.
- Veins contain semilunar valves, formed from their endothelium. Allow blood to move towards the heart but not away from it.
- Networks of capillaries are sometimes known as capillary beds.
- They are about the same width as a red blood cell. This means they can only fit through in single file.
- This means every red blood cell is brought as close as it can to the cells of the surrounding tissues.
- Speeds up the transfer of oxygen and CO2
- Capillaries are very thin, one cell think. Also speeds up diffusion.
- Tiny gaps in between the cells that form the endothelium. This allows an easy transfer of substances dissolved in the plasma out of the capillary to surrounding cells.
This is the end of the topic
Drafted by Eva (Biology)