8.1.48 · Transport in Mammals
Blood Vessels- Structures and Functions — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
5 min read 12 parts Lesson 6 of 21 1 quiz
Lesson summary
8.1.4 Blood Vessels: Structures and Functions — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Relate the structure of arteries, arterioles, capillaries, venules and veins to their functions.Role — arteries carry blood away from the heart at high pressure.Walls are t…
- Wall layers
- Elastic recoil
- Vasoconstriction
- Vasodilation
- Distribution of blood
- Site of exchange
Part 1 of 10Arteries
Role
- Role arteries carry blood away from the heart at high pressure.
- Walls are thick and surround a narrow lumen, which helps maintain pressure.
- Peak pressure in the aorta is about 120 mmHg, so a thick wall is needed to resist bursting.
Wall layers
- Wall layers an inner endothelium, a middle layer of elastic fibres and smooth muscle, and an outer layer of collagen.
Elastic recoil
- Elastic recoil the pressure surge from ventricular contraction stretches the elastic tissue, which recoils during diastole.
- This smooths out the pulsing flow and keeps blood moving.
- Large arteries such as the aorta have more elastic fibres and a pulse that can be felt.
Part 2 of 10Elastic and muscular arteries
| Feature | Elastic artery (near heart) | Muscular artery (further away) |
|---|---|---|
| Tunica media | Thinner, mainly elastin and collagen | Thicker, mainly smooth muscle |
| Smooth muscle | Relatively few fibres, so no vasoconstriction or vasodilation | Much more, so it can perform vasoconstriction and vasodilation |
| Elastin | Plenty, so the wall stretches and recoils with each pulse | Much less, so it cannot stretch and recoil |
| Lumen | Narrow, blood under high pressure | Narrow, blood under high pressure |
| Function | Withstand and smooth the pressure surge | Adjust the blood supply to different tissues |
Part 3 of 10Arterioles
Arterioles
Structure
- Structure arterioles are smaller vessels leading from arteries to capillaries, with more smooth muscle and less elastic tissue.
Vasoconstriction
- Vasoconstriction contraction of the smooth muscle narrows the lumen and reduces blood flow to an organ.
Vasodilation
- Vasodilation relaxation of the muscle widens the lumen and increases blood flow.
Distribution of blood
- Distribution of blood arterioles direct blood to where it is needed.
- Skeletal muscle receives more blood during exercise.
- Skin receives more blood when the body is hot.
Pressure
- Pressure pressure falls as blood passes through the arterioles.
Part 4 of 10Capillaries
1Site of exchange
- Site of exchange capillaries form dense capillary beds in all tissues, where blood is brought close to the cells.
2One-cell-thick wall
- One-cell-thick wall the wall is a single layer of flattened endothelial cells, so the diffusion distance is very short.
3Very narrow lumen
- Very narrow lumen the lumen is about 7 micrometres, the same as a red blood cell.
- Red cells are squeezed through in single file, giving a long contact time.
- This brings haemoglobin close to the wall.
4Large total surface area
- Large total surface area there are millions of capillaries, so blood flows slowly and there is time for exchange.
5Pores
- Pores gaps between endothelial cells let fluid and small solutes leak out to form tissue fluid.
6No elastic, smooth muscle or collagen
- No elastic, smooth muscle or collagen the thin wall lets capillaries fit between cells so diffusion is rapid.
Part 5 of 10Venules and veins
1Role
- Role veins return blood to the heart at low pressure, and venules are small veins that collect blood from capillaries.
2Thin wall
- Thin wall the middle layer has little smooth muscle and elastin because low-pressure blood needs no stretch and recoil.
3Collagen
- Collagen a tough outer layer of collagen gives strength and shape.
4Wide lumen
- Wide lumen it offers little resistance to flow, so pressure is reduced and there are no surges.
5Semilunar (pocket) valves
- Semilunar (pocket) valves they close to prevent backflow of blood.
6Movement of blood
- Movement of blood it is pushed along by skeletal muscles squeezing the veins and by pressure changes in the thorax during breathing.
Part 6 of 10Comparing artery, capillary and vein
| Feature | Artery | Capillary | Vein |
|---|---|---|---|
| Wall | Thick; elastic, muscle, collagen | One cell thick | Thin |
| Lumen | Narrow | Very narrow (red cell diameter) | Wide |
| Pressure | High, pulsing | Falling | Low, smooth |
| Valves | Only at heart (semilunar) | None | Present |
| Function | Carry blood from heart | Exchange | Return blood to heart |
Part 7 of 10Pressure and speed of blood flow
1
Pressure
- Pressure it is highest in the aorta and falls with distance from the heart.
- Friction between blood and vessel walls causes the fall.
- The large combined cross-sectional area of the capillaries also lowers it.
- Pulsing is damped out by the time blood reaches the capillaries.
2
Velocity
- Velocity it depends on total cross-sectional area.
- Flow is fast in the aorta, slowest in the capillaries (largest total area) and increases again in the veins.
- Slow flow in the capillaries allows time for exchange.
Part 8 of 10Control of flow and disease
1
Control of arterioles
- Control of arterioles smooth muscle is controlled by the autonomic nervous system and hormones such as adrenaline.
- In exercise, arterioles in skeletal muscle dilate and those to the gut constrict.
- When hot, skin arterioles dilate and lose more heat; when cold they constrict.
2
Large arteries
- Large arteries their smooth muscle responds to signals from the brain, and elastic recoil maintains diastolic pressure.
3
Atherosclerosis
- Atherosclerosis fatty deposits (atheroma) and fibrous tissue build up under the endothelium.
- This narrows the lumen, reduces elasticity and raises blood pressure.
- A clot may form over a damaged area (thrombosis), leading to a heart attack or stroke.
Part 9 of 10More from the specification
More from the specification
Closed Double Circulatory System
- Mammals possess a closed circulatory system where blood remains enclosed in vessels and passes twice through the heart per complete circuit, comprising the pulmonary circulation to the lungs and systemic circulation to tissues.
Capillary Microcirculation
- Capillary walls are composed of a single layer of squamous endothelium, facilitating rapid diffusion and exchange of dissolved substances including oxygen, glucose,
- and metabolic wastes between blood plasma and tissue cells.
Blood Plasma Composition
- Blood plasma is an aqueous transport medium composed predominantly of water carrying dissolved mineral ions, respiratory gases,
- and nutrient molecules throughout the body.
Closed Double Circulatory System
- Mammals possess a closed circulatory system where blood remains enclosed in vessels and passes twice through the heart per complete circuit, comprising the pulmonary circulation to the lungs and systemic circulation to tissues.
Capillary Microcirculation
- Capillary walls are composed of a single layer of squamous endothelium, facilitating rapid diffusion and exchange of dissolved substances including oxygen, glucose,
- and metabolic wastes between blood plasma and tissue cells.
Blood Plasma Composition
- Blood plasma is an aqueous transport medium composed predominantly of water carrying dissolved mineral ions, respiratory gases,
- and nutrient molecules throughout the body.
Plasma
- The liquid part of blood.
Circulatory system
- Blood vessels, a pump and valves transporting blood around the body.
Hydrostatic pressure
- The pressure a fluid exerts on the walls of its container.
Water potential
- A measure of the tendency of water to move; pure water has the highest.
Concentration gradient
- The difference in concentration of a substance between two regions.
Part 10 of 10More on this topic
- Ventricles
- thicker, more muscular walls squeeze the blood, raising its pressure and pushing it out through the semilunar valves.
Key terms
| Term | Meaning |
|---|---|
| Artery | A vessel that carries blood away from the heart at high pressure. |
| Arteriole | A small vessel with muscular walls that controls blood flow into capillaries. |
| Capillary | A tiny vessel with a one-cell-thick wall where exchange occurs. |
| Vein | A vessel that returns blood to the heart at low pressure. |
| Vasoconstriction | Narrowing of a vessel as smooth muscle contracts. |
| Vasodilation | Widening of a vessel as smooth muscle relaxes. |
| Elastic recoil | Return of a stretched vessel wall to its original size, pushing blood on. |
Avoid these mistakes
1
Saying the capillary wall has no cells — it is a single layer of endothelial cells.
2
Saying arteries have valves along their length — valves are in veins (and semilunar valves at the heart).
3
Saying arteries pump blood — the heart pumps; arteries stretch and recoil.
4
Saying blood flows fastest in capillaries — it is slowest there because the total cross-sectional area is largest.
5
Saying veins have thin walls only because they are small — the wall is thin because blood pressure is low.
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