ARTERIOLES: CHARACTERISTICS, HISTOLOGY, FUNCTIONS - ANATOMY AND PHYSIOLOGY - 2024

The arterioles are small blood vessels that are part of the arterial system and that act as control conduits through which the blood from the arteries is carried to the capillaries. The arterioles have strong walls of smooth muscle, which allow vasoconstriction (closure) and vasodilation (opening or relaxation).

The ability of the arterioles to close or dilate multiple times is important because it allows them to respond to heat, cold, stress, and hormones, as well as local chemical factors in the tissue, such as the absence of oxygen. In this way, the blood flow to the tissue is altered according to its need.

Source: Kelvinsong

characteristics

Blood is pumped from the heart to the arteries, which branch into small arteries, then arterioles, and finally an intricate system of capillaries, in which it is balanced by interstitial fluid.

During this journey, fluctuations in blood pressure between systole and diastole are dampened by the small arteries and arterioles. The speed of blood flow and blood pressure decrease progressively.

The speed of blood flow decreases because: 1) the diameter of the arterioles (0.01–0.20 mm) and capillaries (0.006–0.010 mm) is much smaller than that of the arteries (25 mm), causing them to offer more resistance to said flow; 2) the further from the heart, there are more branches of the arterial system, increasing its cross-sectional area.

The arterioles play a critical role in regulating blood pressure. When arterioles increase in diameter, vasodilation and blood pressure decrease. When they decrease in diameter, vasoconstriction blood pressure increases. For this reason, the arterioles are called resistance vessels.

The vasoconstriction of the arterioles in an organ decreases blood flow to that organ. Vasodilation has the opposite effect.

Histology

The diameter of the lumen of the arterioles equals the thickness of their walls, which consist of three layers, or tunics: 1) intima (or internal); 2) mean; 3) adventitia (or external).

The intimate tunic is the innermost layer. It consists of an endothelium (made up of epithelial cells), a subendothelial layer (made up of fibroblast-like cells that synthesize collagen and elastin), and a basal lamina (or internal elastic lamina). This last lamina is present in the large arterioles and absent in the small arterioles.

The tunica media consists of one or more layers of smooth muscle reinforced with elastic tissue, which form an elastic layer called the external elastic lamina. This lamina separates the tunica media from the tunica adventitia.

The tunica adventitia is the outermost layer. It is usually a thin layer made up of connective tissue, nerve fibers, and collagen fibrils. This layer fuses with the connective tissue of the surrounding organ.

The microvasculature begins at the level of the arterioles. It consists of small arterioles (metarterioles) that guide blood into the capillary system. Venule-arteriole anastomosis allows direct flow from the arterioles to the venules.

Features

Changes in diameter in the resistance vessels (small arteries and arterioles) represent the most important mechanism for regulating the resistance of the vascular system. Normally, these resistance vessels are partially constricted, which is called the vascular tone of the vessels.

Vascular tone is produced by the contraction of smooth muscle within the wall of the blood vessel.

Starting from this state, the blood vessel can become more constricted or dilated, thus changing its resistance. This mechanism responds to extrinsic, neuronal or humoral factors, or to intrinsic factors such as hormones or local metabolites.

Vasoconstriction is stimulated by the nerve fibers of the sympathetic system and hormones that travel in the bloodstream. For example, norepinephrine, a neurotransmitter, diffuses through the muscle layer and induces the contraction of cells.

Vasodilation is activated by nerve fibers of the parasympathetic system. For example, the release of acetylcholine from nerve endings stimulates the endothelium to release nitric oxide, which causes vasodilation.

Changes in the resistance of the arterioles are important for the functioning of all organs and tissues, especially the kidneys, skin and skeletal muscle.

Function of arterioles in the kidneys

Systemic blood pressure is regulated by intrinsic or extrinsic mechanisms. In the latter are involved, firstly, the heart, and secondly, the kidneys. The latter control blood pressure through the renin-angiotensin system.

When the kidneys detect a drop in blood pressure, they secrete the enzyme renin, which clears angiotensinogen, a plasma protein, and initiates a series of reactions that culminate in the synthesis of angiotensin II. This hormone causes vasoconstriction and increases aldosterone secretion.

Aldosterone is a hormone that promotes the reabsorption of salt. This effect worsens existing hypertension. If the diastolic pressure rises above 120 mm Hg, bleeding of the blood vessels occurs, while the kidneys and heart deteriorate rapidly, leading to death.

Angiotensin converting enzyme inhibitor drugs dilate the efferent arterioles of the renal cortex, causing a decrease in the glomerular filtration rate. These drugs reduce hyperfiltration and the occurrence of nephropathy in diabetes mellitus.

The prostaglandins E ₂ and I ₂, bradykinin, nitric oxide and dopamine produce vasodilation of the renal arterioles, increasing renal blood flow.

Function of arterioles in the skin

Regulation of the diameter of the arterioles in the skin in response to changes in temperature is controlled by the nervous system.

In hot weather, the arterioles dilate, which increases blood flow through the dermis. Consequently, the excess heat radiates from the body surface to the environment.

When it is cold, the arterioles contract, which allows the conservation of heat. By decreasing blood flow through the dermis, heat is kept within the body.

Function of arterioles in skeletal muscle

Unlike the brain, which receives a constant blood flow, skeletal muscle receives a variable blood flow that depends on the level of activity. At rest, the arterioles contract, so blood flow in most capillaries is very low. The total blood flow through the muscular system is 1 L / min.

During exercise, arterioles dilate in response to epinephrine and norepinephrine from the adrenal medulla and sympathetic nerves.

The precapillary sphincters dilate in response to muscle metabolites, such as lactic acid, CO _2, and adenosine. Blood flow increases more than 20 times during extreme exercise.

References

1. Aaronson, PI, Ward, JPT, Wiener, CM, Schulman, SP, Gill, JS 1999. The cardiovascular system at a glance Blackwell, Oxford.
2. Barrett, KE, Brooks, HL, Barman, SM, Yuan, JX-J. 2019. Ganong's review of medical physiology. McGraw-Hill, New York.
3. Gartner, LP, Hiatt, JL, Strum, JM 2011. Cell biology and histology. Wolters Kluwer-Lippincott William and Wilkins, Baltimore.
4. Gaze, DC 2012. The cardiovascular system: physiology, diagnostics and clinical implications. InTech, Rijeka.
5. Hall, JE 2016. Guyton and Hall textbook of medical physiology. Elsevier, Philadelphia.
6. Johnson, KE 1991. Histology and Cell Biology. Williams and Wilkins. Baltimore.
7. Kraemer, WJ, Rogol, AD 2005. The endocrine system in sports and exercise. Blackwell, Malden.
8. Lowe, JS and Anderson, PG 2015. Human Histology. Elsevier. Philadelphia.
9. Rogers, K. 2011. The cardiovascular system. Britannica Educational Publishing, New York.
10. Taylor, RB 2005. Taylor's Cardiovascular Diseases: A Handbook. Springer, New York.
11. Topol, EJ, et al. 2002. Textbook of Cardiovascular Medicine. Lippincott Williams & Wilkins, Philadelphia.
12. Whittemore, S., Cooley, DA 2004. The circulatory system. Chelsea House, New York.
13. Willerson, JT, Cohn, JN, Wellens, HJJ, Holmes, DR, Jr. 2007. Cardiovascular medicine. Springer, London.