| Hydrostatic pressure | - The pressure created by the pumping of the heart
- At arterial end of capilliaries |
| Different types of blood vessels | - Arteries
- Arterioles
- Capillaries
- Veins |
| Arteries | - Carry blood under high pressure away from the heart, into arterioles, to tissues |
| Arterioles | - Small arteries
- Controls blood flow from arteries to capillaries
- Under lower pressure than arteries |
| Capillaries | - Tiny vessels that link arterioles to veins
- Exchanges metabolic material, e.g. O2, between blood + cells
- Flow of blood slow |
| Veins | - Carry blood slowly, under low pressure, from capillaries in tissues to heart |
| The layered structure of blood vessels | - Tough fibrous outer layer
- Muscle layer
- Elastic layer
- Endothelium (inner lining)
- Lumen
- Each vessel has different proportions of layers depending on its function |
| Tough fibrous outer layer | - Resists pressure changes from inside + outside |
| Muscle layer | - Can contract to control flow of blood |
| Elastic layer | - Maintain blood pressure by stretching + springing back (recoiling) |
| Endothelium (inner lining) | - Smooth to reduce friction
- Thin shorter diffusion path |
| Lumen | - Not a layer
- The central cavity of the vessel
- Blood flows through |
| Artery structure related to function | - Muscle + elastic layer is thicker than veins
- Overall thickness is great
- No valves |
| Muscle layer is thick compared to veins | - Has high blood pressure form heart |
| Elastic layer is relatively thick compared to veins | - Important to keep high pressure in arteries so blood can reach extremities of body
- Elastic wall stretched at each heart beat (systole)
- Springs back when heart relaxes (diastole)
- This recoiling helps maintain high + smooth pressure surges created by heart beats |
| Overall thickness is great | - Resists the vessel bursting under pressure |
| No valves | - Except in arteries leaving heart
- Because blood is under constant high pressure due to heart pumping blood into arteries
- Therefore tends to flow backwards |
| Arteriole structure related to function | - Muscle layer is thicker than in arteries
- Elastic layer is thinner than in arteries |
| Muscle layer is relatively thicker than in arteries | - So they can constrict + dilate to control the flow of blood in them |
| Eastic layer is relatively thinner than in arteries | - Because blood pressure is lower |
| Capillary structure related to function | - Walls mostly consist of endothelium
- Highly branched
- Narrow diameter + lumen
- Space between lining (endothelial) |
| Walls consist mostly of endothelium | - Thin, shorter diffusion path
- Rapid diffusion between cells + blood |
| Highly branched | - LSA for exchange |
| Narrow diameter | - So can permeate tissues, so cells not far from capillary
- Shorter diffusion path |
| Lumen is narrow | - Red blood cells squeeze flat against side of capillary
- So they are even closer to cells to supply O2
- Shorter diffusion path |
| Spaces between lining (endothelial) | - Allows white blood cells to escape in order to deal with infections within tissues |
| Vein structure related to function | - Muscle elastic layer thin compared to arteries
- Overall thickness of the wall is small
- Valves |
| Muscle layer is relatively thin compared to arteries | - Carry blood away from tissues
- So constriction + dilation cannot control flow of blood |
| Elastic layer is relatively thin compared to arteries | - Lower blood pressure, so wont burst
- Too low to create recoil action |
| Overall thickness of the wall is small | - Lower blood pressure, so wont burst
- Easily flattened, aiding flow within them |
| Valves | - Prevents backflow of blood, as pressure is low so its more likely
- When muscles contract, veins compress, pressurising the blood in them
- Valves ensure that this pressure directs the blood in 1 direction only, towards the heart |
| Tissue fluid | - Supplies cells with glucose, amino acids, fatty acids, ions + O2
- Receives CO2 + other waste products from cells
- Provides constant environment for cells, bathes tissues in final journey of metabolic materials |
| Formation of tissue fluid | - Formed from blood plasma
- Blood plasma controlled by various homeostatic systems |
| Hydrostatic pressure | - The pressure created by the pumping of the heart
- At arterial end of capillaries
- This causes tissue fluid to move out of the blood plasma |
| Outward pressure is opposed by two factors | - Hydrostatic pressure of tissue fluid outside capillaries, which resists the outward movement of liquid
- The low water potential of blood, due to plasma proteins, that causes water to move back into blood in capillaries |
| The combined effect of these forces | - Creates overall pressure that pushes tissue fluid out of capillaries at arterial end
- This pressure can only force small molecules out of capillaries, leaving large molecules- cells + proteins, ultrafiltration |
| Return of the tissue fluid to the circulatory system (1) | - The loss of tissue fluid from capillaries reduces hydrostatic pressure inside them |
| Return of the tissue fluid to the circulatory system (2) | - As a result, by the time the blood has reached the venous end of the capillary network its HSP is lower than tissue fluid outside it |
| Return of the tissue fluid to the circulatory system (3) | - Therefore tissue fluid is forced back into the capillaries by the higher HSP outside them |
| Return of the tissue fluid to the circulatory system (4) | - The plasma has lost water, still contains proteins
- So has lower water potential than tissue fluid |
| Return of the tissue fluid to the circulatory system (5) | - Water leaves tissue by osmosis down water potential gradient |
| The lymphatic system | - System of vessels that begin in tissues
- Resemble capillaries, accept they have dead ends
- Merge into large vessels forming network
- Large vessels drain their contents back into bloodstream via 2 ducts that join veins close to heart |
| What the lymphatic system is moved by | - Not by pumping of heart
- HSP of tissue fluid that has left capillaries
- Contraction of body muscles |
| Contraction of body muscles in lymphatic system | - Squeezes lymph vessels
- Valves in lymph vessels ensure fluid inside them moves away from tissues in direction of heart |
