What Is the Structure of the Circulatory System?

The circulatory system is a continuous closed duct system distributed throughout the body, which includes the cardiovascular system and the lymphatic system. Blood circulates through the cardiovascular system. Lymph fluid flows through the lymphatic system. Lymphatic fluid flows along a series of lymphatic ducts to the heart and eventually flows into the vein, so the lymphatic system can also be considered as an auxiliary part of the venous system. ^[1]

Circulatory system

The circulatory system is a continuous closed duct system distributed throughout the body, which includes the cardiovascular system and the lymphatic system. Blood circulates through the cardiovascular system. Lymph fluid flows through the lymphatic system. Lymphatic fluid flows along a series of lymphatic ducts to the heart and eventually flows into the vein, so the lymphatic system can also be considered as an auxiliary part of the venous system. ^[1]

The simplest form of circulation can be seen in unicellular and multicellular organisms, including plant cells-

Vascular wall

With abundant elastic fibers and

The contraction and relaxation of blood vessels is called vasomotor movement, and the nerves that govern vasoconstriction are called

Cerebellum, midbrain and hypothalamus regulate vascular movement

Cerebellum and midbrain can cause vascular motor response when stimulated, which can be inhibited by stimulating the anterior cerebellar cortex.

The chemicals released by some tissues and organs into the blood of animals have a regulating effect on the functional state of the vascular system. Some of them work in synergy with vascular reflexes under neural control and become a part of the regulation of the entire circulatory system. In addition, some humoral factors are not controlled by the nerve and are important factors for local blood flow regulation. They can be grouped into three categories:

hormones secreted by endocrine glands, such as epinephrine and norepinephrine;

Some chemicals that can affect vascular movement, such as bradykinin, renin, serotonin, histamine, etc., released by tissues during certain special activities;

General metabolites of tissues, such as carbon dioxide, lactic acid, adenine triphosphate decomposition products, adenine acid, etc.

The first is controlled by the nerves. The second and third categories have little or no relationship with the nerves (Table 3).

Epinephrine and norepinephrine

Both are secreted by the adrenal medulla, and their effects are similar to when the sympathetic nerves are excited. Both hormones can increase the heart's metabolic rate; speed up, strengthen, and increase cardiac output. Adrenaline has a stronger effect on the heart. Norepinephrine has a stronger effect on blood vessels. The combined effect of the two hormones on the heart and blood vessels is increased heart rate, cardiac output, and systemic blood pressure.

Acetylcholine

Can make small blood vessels relax and increase blood flow to local tissues. Because it is easily destroyed by cholinesterase, it is unlikely that a large amount of acetylcholine will appear in the blood under normal circumstances. A small amount of acetylcholine has a transient hypotensive effect. Its physiological significance is that it is a transmitter of cholinergic vasodilator fibers. When the vagus nerve and other cholinergic vasodilator fibers are excited, acetylcholine is released to cause local vasodilation and cardiac arrest.

Pituitary vasopressin

Vasopressin secreted from the posterior pituitary gland causes small blood vessels to contract, including coronary vessels. The action time is longer, and the endocrine function of the posterior pituitary is controlled by the nerve. Stimulating the central end of the nerve increases secretion, and the secretion of vasopressin from the posterior pituitary in pressor reflex caused by painful stimulation also plays a certain role.

Renin and angiotensin

Partial blocking of the renal arteries results in inadequate renal blood supply, which can cause renal hypertension in animals. The reason for this is that blood sodium decreases when renal blood supply is insufficient and stimulates the cells next to the glomerulus to release an enzyme called renin (angiotensin Proenzyme), which can hydrolyze angiotensinogen (in 2 globulin) in plasma into a decapeptide called angiotensin . When it passes through the pulmonary circulation, it is stripped of two amino acids by its converting enzyme, which becomes angiotensin II. Under the action of aminopeptidase, angiotensin II is hydrolyzed into a heptapeptide, angiotensin III. Angiotensin and both have high biological activity. In particular, angiotensin is the strongest vasoconstrictor substance found. Angiotensin mainly stimulates the adrenal cortex to secrete aldosterone, thereby strengthening the renal tubules to sodium and Reabsorption of water, and both have the effect of increasing blood pressure.

Local humoral regulators

Most of the tissue metabolites, such as carbon dioxide, lactic acid, hydrogen ions, potassium ions, and adenosine triphosphate decomposition products, such as adenine acid, generally have local vasorelaxant effects and help increase blood supply to active organs. Histamine is a decarboxylated product of histidine. Many tissues, especially the skin, lungs, and intestinal mucosa, contain more mast cells. They are released during tissue inflammation, injury, and allergic reactions, contracting smooth muscles, but making capillaries strongly relax Causes damage, leading to increased permeability of small blood vessels, a large amount of exudation of plasma, thereby reducing circulating blood volume, lowering arterial blood pressure, these reactions have a damaging effect on circulation. Tryptophan derivatives such as serotonin (5-HT) in the digestive tract, brain tissue, and platelets generally have vasoconstrictive effects, but a small amount relaxes the muscle blood vessels. Prostaglandin is widely present in various tissues and can be released under physiological and pathological conditions. It first comes to interstitial fluid and then to circulating blood. Its composition is complex, and some components have local vasoconstrictive effects, but the main component of prostaglandin Causes vasodilation.