The rate and distribution of blood flow through the circulatory system are variable and related to several factors including physical activity, cardiac output (the amount of blood pumped out of the heart) and venous return or the blood returning to the heart through the veins.
What is blood pressure?
Blood pressure (BP) is defined as the force exerted by the blood against the vessel wall. It is highest in arteries as these are the first blood vessels it flows into from the heart and gradually decreases as it passes through arterioles, capillaries, venules and finally, veins.
It is also variable and can increase due to exercise where the cardiac output increases thus forcing more blood through the arteries or by altering the peripheral resistance (resistance caused by the blood vessels). This can occur in a number of ways; by vasoconstriction where the blood vessels contract, increases in blood viscosity (thickness) and changes in shape or size of the vessels. The regulation of blood pressure is the responsibility of the sympathetic and parasympathetic nervous systems.
When BP falls sensors called baroreceptors are stimulated which causes a nervous impulse to be transmitted to the arterioles, causing them to vasoconstrict, which results in an increased BP due to the smaller cross-sectional area through which the blood can pass.
In reverse, raised BP also stimulates baroreceptors which cause impulses directing the arterioles to vasodilate, increasing the area through which blood can pass and consequently reducing pressure.
Veins too can alter their shape in response to stimuli received from the sympathetic and parasympathetic nervous systems. They do not have such a thick muscular wall as arteries although they are capable of increasing the venomotor tone of their walls to alter their shape to increase or decrease blood pressure, although this is not as effective as the vasodilation/constriction which occurs in arteries. For this reason, veins require some extra help to increase their pressure and return the blood to the heart.
- With exercise, metabolism speeds up and because of this the muscles require more oxygen
- So the heart beats faster to supply the muscles with more oxygen-rich blood
- In turn, the speed of blood flow increases.
- Due to an increase in heart rate (and stroke volume) to meet demands, cardiac output (the volume of blood pumped out of the heart in one minute) automatically increases
- The faster and harder the heart pumps, the higher the rate of blood circulation.
- Venous return is the return of blood to the heart via venules and veins
- If this is slow, the volume of blood pumped from the heart with each beat (stroke volume) is lower
- This lowers cardiac output and reduces BP and flow rate.
Venous return (blood returning to the heart) must constitute three-fifths of the blood circulating the body at any time in order to maintain steady blood flow. At rest this is not a problem, however, during exercise, the blood pressure in the veins is not high enough to increase the level of venous return and so maintain the higher stroke volume and cardiac output which exercise requires. A number of mechanisms are used which help to increase venous return:
- Pocket Valves: located within the veins prevent the backflow of blood and help it towards the heart
- Muscle Pump: Many veins are situated between skeletal muscles, which when they contract and relax, squeeze on the veins and help push the blood back towards the heart.
- Smooth Muscle: The wall of each vein contains smooth muscle which contracts to help push the blood back towards the heart
- Respiratory Pump: The respiratory pump helps return blood in the thoracic cavity and abdomen back to the heart. Whilst exercising we breathe faster and deeper which rapidly changes the pressure within the thorax between high and low to help to squeeze the blood in the area back to the heart.
- Gravity: Veins in the upper body are aided by gravity in order to return blood to the heart.