Human Anatomy and Physiology - N. I. Fedyukovich 2003

The Cardiovascular System
Regulation of the Cardiovascular System

Regulation of the Circulatory system is primarily achieved through changes in Cardiac Output and the resistance of regional parts of The Vascular System. The mechanisms regulating Blood Circulation are conventionally divided into local (peripheral or regional) and central (neurohumoral). The former regulate BLOOD FLOW IN Organs and Tissues according to their Functions and METABOLISM, while the latter regulate systemic hemodynamics during the body's adaptive responses.

Local mechanisms are based on the fact that metabolic byproducts are capable of dilating precapillary arterioles and increasing the number of open, functioning sphincters in accordance with organ activity.

Neural and humoral factors play a major role in adapting the Cardiovascular system to provide an optimal Blood supply to organs and tissues. This regulation is carried out by a complex mechanism that includes sensory, central, and efferent pathways.

Sensory innervation of Blood Vessels is mainly represented by branched nerve endings (angioreceptors). Functionally, the latter are divided into baroreceptors and chemoreceptors. The former respond to changes in arterial pressure, as well as the rate and degree of vessel wall stretch caused by pulsatile fluctuations in blood pressure, while the latter respond to changes in The chemical composition of the blood.

Angioreceptors are located throughout the vascular system, forming a single receptive field. However, they are most abundant in the primary reflexogenic zones (aortic and carotid sinus) and in the Vessels of the Pulmonary Circulation. Stimulation of the aortic zone not only leads to a decrease in aortic pressure but also causes vasoconstriction, stimulates Cardiac Activity, and increases systemic arterial pressure. Maintaining constant pressure in the aorta is achieved through autoregulatory mechanisms based on the feedback principle.

Chemoreceptors respond to Changes in the blood concentration of O₂, CO₂, and H⁺. Their excitation can be triggered by certain organic and inorganic substances.

Central mechanisms regulating the maintenance of arterial pressure are mediated by a collection of neural structures known as the vasomotor center. The structures belonging to the vasomotor center are localized in the Spinal Cord, Medulla Oblongata, Hypothalamus, and Cerebral Cortex.

Neural mechanisms are the first component of regulation, involving sympathetic Neurons located in the thoracic and lumbar segments of the spinal cord and in the paravertebral ganglia. The second component consists of parasympathetic neurons of the Vagus nerve Nucleus, which is located in the medulla oblongata. The endocrine mechanism of cardiovascular regulation includes the Adrenal Medulla and Cortex, the Pituitary Gland, and the juxtaglomerular apparatus of the Kidneys.

Epinephrine (an adrenal hormone) has the most pronounced vascular effect of all Hormones. It constricts blood vessels in the Skin, digestive organs, kidneys, and Lungs, but dilates the vessels of skeletal Muscles and the smooth Muscle of the Bronchi; it helps increase Blood flow through skeletal muscles, the Brain, and The Heart during physical exertion and emotional stress.

Aldosterone has a strong ability to enhance sodium reabsorption in the kidneys, Salivary Glands, and Digestive System, thereby altering vascular sensitivity to the effects of epinephrine and norepinephrine.

Vasopressin is a hormone of the posterior pituitary gland. It constricts the Arteries and arterioles of the abdominal organs and lungs, but dilates the vessels of the brain and heart, which helps improve the nourishment of both brain tissue and the myocardium, stimulates uterine Muscle contraction, regulates Water-salt balance, and more.

Renin, an enzyme of the juxtaglomerular apparatus of the kidneys, is converted with the participation of blood globulins into angiotensin II, which has a potent vasoconstrictive effect—greater than that of norepinephrine—but does not cause the release of blood from reservoirs. Renin and angiotensin are considered to constitute the so-called Renin-Angiotensin System.

Histamine dilates the Blood vessels of the Liver, heart, and intestines, increases capillary filling, and also decreases the circulating blood volume.

Prostaglandins are a large group of BIOLOGICALLY ACTIVE SUBSTANCES produced in all organs and tissues. Some prostaglandins constrict blood vessel walls and increase arterial pressure, while others have a vasodilating effect, causing a hypotensive response. Biological substances such as serotonin and bradykinin also influence The activity of the cardiovascular system.

In neural and endocrine regulation, a distinction is made between short-acting, intermediate-acting, and long-acting hemodynamic mechanisms. Short-acting mechanisms (by duration of action) include circulatory responses of neural origin, such as baroreceptor and chemoreceptor Reflexes, and the CNS ischemic response. They develop within a few seconds. Intermediate mechanisms encompass changes in capillary exchange, stress relaxation of the vessel wall, and the response of the renin-angiotensin system. These mechanisms require minutes to initiate and hours to fully develop. Long-acting mechanisms influence the relationship between intravascular blood volume and vascular capacity, operating through transcapillary fluid exchange. This process involves the hormones vasopressin and aldosterone, as well as renal regulation of fluid volume. Mechanical, or hemodynamic, regulation (Frank–Starling law) is expressed in the fact that the force of contraction is directly proportional to the degree of initial stretching of the right heart chambers by venous blood. This type of regulation ensures the maintenance of constants such as stroke volume and cardiac output.