44. Control mechanisms of the circulatory system. General considerations

The circulatory system is regulated by many different mechanisms:

Neuronal mechanisms
Cardiovascular reflexes
Humoral mechanisms
Local mechanisms

Neuronal mechanisms

As already described, many parts of the circulatory system are innervated by the parasympathetic and, more importantly, the sympathetic nervous system. This includes the heart and blood vessels. Generally, sympathetic activation causes:

Increased heart rate
Increased myocardial contractility
Vasoconstriction of veins -> increased venous return -> increased cardiac output
Vasoconstriction of splanchnic arteries and arterioles -> blood is redirected from the splanchnic organs to the skeletal muscles and heart
Vasoconstriction of arteries in the skin -> decreased heat loss

Generally, parasympathetic activation causes:

Decreased heart rate
Vasodilation of splanchnic arteries and arterioles -> blood is redirected back to splanchnic organs

Cardiovascular reflexes

Cardiovascular reflexes are subconscious nervous control mechanisms which are constantly working to maintain normal blood pressure. Most of these reflexes are negative feedback reflex mechanisms, meaning that they respond to increased blood pressure by activating mechanisms which decrease the blood pressure.

Local mechanisms

The local mechanisms are those which act locally on the circulation in a specific area or tissue rather than in the whole body.

More about local mechanisms in topic 45.

Humoral mechanisms

Humoral refers to anything which relates to compounds found in the blood. So, humoral mechanisms of circulatory regulation are those which are mediated by compounds in the blood, often hormones.

There are many compounds in the blood which affect the circulatory system. The most important compounds are:

Catecholamines
- Epinephrine
- Norepinephrine
Vasopressin (ADH)
Components of the renin-angiotensin-aldosterone system – described in topic 58
- Angiotensin II
- Aldosterone

Catecholamines:

Catecholamines are synthesized in the medulla of the adrenal gland. They’re then released into the circulation. The catecholamines bind to adrenergic receptors all over the body, which causes the same effects as sympathetic activation. The adrenal medulla releases catecholamines in response to sympathetic stimulation. As such, we should think of the circulating catecholamines as an extension of the sympathetic nervous system.

There are two main types of adrenergic receptors, alpha adrenergic and beta adrenergic receptors. Each main type has multiple subtypes. Each subtype has different functions and each subtype has different affinity for the two catecholamines norepinephrine and epinephrine.

Property	Alpha adrenergic receptors		Beta adrenergic receptors
Property	Alpha-1 receptors	Alpha-2 receptors	Beta-1 receptors	Beta-2 receptors	Beta-3 receptors
Location in the cardiovascular system	Vessels in skin and splanchnic circulation, veins	Not found	SA node, AV node, atria, ventricles	Vessels in skeletal muscle	Not found
Effect	Vasoconstriction	–	Positive heart effects	Vasodilation	–
Affinity to epinephrine	Lower	–	Equal	Higher	–
Affinity to norepinephrine	Higher	–	Equal	Lower	–

When the sympathetic nervous system is activated the adrenal medulla releases epinephrine and norepinephrine into the blood. These hormones will bind to alpha-1, beta-1 and beta-2 receptors in the circulatory system, which will increase cardiac output and redirect blood from the splanchnic circulation to the skeletal muscle.

Vasopressin:

Vasopressin (AVP), also called anti-diuretic hormone (ADH) is a hormone with two effects:

It is a vasoconstrictor
It increases water reabsorption in the kidney, concentrating the urine
- Hence the alternative name anti-diuretic hormone

Vasopressin is synthesized in the hypothalamus and released by the posterior lobe of the pituitary gland into the blood. This hormone is not very important in the physiological regulation of blood pressure, but it can be important in case of severe blood loss.