| What are arteries | - they carry blood from the heart to the rest of the body.
- their walls are thick, muscular and have elastic tissue to stretch and recoil as the heart beats, which helps maintain the high pressure |
| What are arterioles | - they have a layer of smooth muscle and less elastic tissue
- this allows them to expand or contract, controlling the amount of blood flowing |
| What are capillaries | - they are the smallest of the blood vessels
- substances like glucose and oxygen are exchanged between cells and capillaries
- their walls are one cell thick |
| What are venules | - they join together to form veins |
| What are veins | - they take blood back to the heart under low pressure
- they have a wider lumen, less elastic and muscle tissue
- they contain valves, which prevent back flow |
| How does blood flow through veins | - blood flows with the help of contractions of the body muscles surrounding them |
| What is tissue fluid | - the fluid that surrounds cells in tissue
- it's made from substances that leaves the blood, for example: oxygen, water and nutrients |
| What does tissue fluid not contain | - red blood cells and big proteins |
| How is tissue fluid formed from plasma | - at the start of the capillary bed, the hydrostatic pressure inside the capillaries is greater than the hydrostatic pressure in the tissue fluid
- this difference in hydrostatic pressure forces fluid out of the capillaries and into the spaces around the cells, forming tissue fluid
- after tissue fluid leaves, the hydrostatic pressure reduces in the capillaries (being much lower at the end nearest to the venules) |
| Describe pressure filtration | - as water leaves the capillaries, the concentration of plasma proteins in the capillaries increases, and the water potential decreases
- plasma proteins in the capillaries generate a form of pressure called oncotic pressure
- as water potential in the capillaries is lower than the tissue fluid, some water re-enters the capillaries from the tissue fluid at the venule end by osmosis |
| What are lymph vessels | excess tissue fluid |
