Control of veins in animals
On sympathetic stimulation adrenaline causes venous constriction but has no influence on peripheral resistance to increase venous return to the heart to maintain normal cardiac output and BP.
In dog and cat, the sympathetic vasoconstrictor fibres (spinal nerves from lumbar 2-4) control the vasomotion in the veins of the hind legs. The splanchnic nerve supplies both vasoconstrictor and vasodilator fibres to the portal veins.
Factors causing venous blood flow
- Low pressure in the veins
- Unidirectional blood flow by valves
- Higher intra- abdominal pressure
- Tonicity of the skeletal muscles
Venous pump/muscle pump
The veins contain valves to direct the blood flow only towards the heart. Movement of the legs or tension on the leg muscles causes the propulsion of the blood towards the heart and the venous pressure is lowered. This pumping mechanism is referred as venous pump.