The circulatory system is regulated by many different mechanisms:
- Neuronal mechanisms
- Cardiovascular reflexes
- Humoral mechanisms
- Local 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 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.
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 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:
- Vasopressin (ADH)
- Components of the renin-angiotensin-aldosterone system – described in topic 58
- Angiotensin II
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.
|Alpha adrenergic receptors||Beta adrenergic 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 (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.
33. Nervous control of the heart
35. Local control of the vascular smooth muscle