Biology 20: Circulatory System Cheat Sheet (DRAFT) by dianadavis

Arteries carry blood away from the heart
Most arteries carry oxygenated blood (except the pulmonary artery that goes from heart to lungs)
They have thick walls with 3 layers:
Inner and outer walls: connective tissue
Middle layers: made of muscle fibres and elastic connective tissue
The walls stretch with every heart contra­ction and recoil after the blood has passed through

Arteri­oles:
- Middle layer is smooth muscle and elastic fibre
- Pressure is still high but less than arteries
- The diameter is controlled by the autonomic nervous system, messages from the ANS can cause vasoco­nst­riction or vasodi­lation
- Pre-ca­pillary sphincters regulate blood flow, so not all arterioles are open at any given time

Venules:
- Lined with smooth muscle, but not 3 layers (like seen in arteries)
- Not enough blood pressure to return blood to heart

- Mainly carry deoxyg­enated blood (exception of the pulmonary vein which brings blood from the lungs to the heart)
- Blood pressure in the veins is much lower than in the arteries therefore, veins are lined with valves to prevent backflow
- If blood pools, the vein will swell in front of the valve
- Blood moves through veins as the vein is squeezed by skeletal muscles
- Act as blood reservoirs (65% of total blood volume can be found in the veins)

- Composed of a single layer of cells
- Small diameter, slows the flow of red blood cells
- Thin wall ideal for gas and fluid exchange
- Due to high surface area, the pressure drops signif­icantly
- 85% of fluid returns to capill­aries (remaining 15% returns via lymph)