Control of the cardiovascular system:

Blood enters and flows from the right ventricle to the lungs to the left ventricles and then to the various organs of the body. The heart needs to regulate blood flow through certain arteries which it does by regulating the diameter of small diameter (changing the radius of the lumen) arterioles. These contain a lot of smooth muscle which can exert a contractile force and modify the diameter, thereby altering the flow of blood to different arterioles. In order to control pressure, heart rate and stroke volume must be controlled.

How to control cardiac output?

The Frank Starling mechanism explains this using stroke volume. Blood enters the heart, which causes an increase in contraction. The sinoatrial node is the intrinsic pacemaker of the heart. The sympathetic innervations (stimulation) of the heart will speed the heart up by the action of the SA node, and it can increase contraction due to its actions of ventricular cells. The parasympathetic system will slow the heart rate but doesn’t have much of an impact on ventricular tissue, although having said that, there is some vagal innervation of ventricular tissue. Innervation of the heart by the autonomic nervous system is the main neuronal way of regulating the heart.

How does the body know how to speed up or slow down the heart?

There are cardiac muscle receptors, chemoreceptors and baroreceptors that detect stretch. If they detect stretch, it’s telling the heart that pressure has increased, and will signal it to slow down. The brainstem will send an autonomic outflow to the heart and vessels which will slow down the heart. Chemoreceptors affect the respiratory system more than the cardiac system. They act by decreasing the oxygen content in the blood and increasing the carbon dioxide content.

Autonomic control of blood vessels:

Sympathetic vasoconstrictors will release noradrenalin and ATP in the arteries in the heart which will cause vasoconstriction via Beta 1 receptors. Alpha 1 receptors are Gq-linked which causes an increase in calcium release. The parasympathetic system on the other hand will cause a vasodilation which is dependent on the epithelium, important in regulating vessel diameter.

The Renin-angiotensin system is also very important in regulating blood pressure and volume. An increase in blood pressure can be detected by the kidneys which will cause the juxtaglomerular cells to release renin into the bloodstream which will cleave off a protein called angiotensinogen to form angiotensin 1. This will be acted on by an angiotensin converting enzyme, present on epithelial cells and will produce angiotensin 2 which is a very powerful vasoconstrictor.

So a decrease in volume or blood pressure will lead to sympathetic activity and cause the kidneys’ juxtaglomerular cells to release renin into the bloodstream. This will again cleave off a protein called angiotensinogen to form angiotensin 1 which will be acted on by an angiotensin converting enzyme present on epithelial cells which will produce angiotensin 2. This can cause vasoconstriction of blood vessels, it can act on the kidneys, causing an increase in sodium reabsorption while the brain releases an anti-diuretic hormone which will act on the kidneys to increase water reabsorption (making you thirsty and causing you to have a drink of water). There will be an increase in blood pressure and this will have a negative feedback effect on the kidneys, reducing the amount of renin release from the juxtaglomerular cells.

All of these intricate control mechanisms are vitally important in the smooth-running of the cardiovascular system, which in turn, effects all the vital organs and physiological processes in the body.