44. Control mechanisms of the circulatory system. General considerations

Last updated on October 6, 2020 at 12:45

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.

More about these reflexes in topic 38.

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 35.

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

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.


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43. Nervous control of the heart

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45. Local control of the vascular smooth muscle

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