Question

Arterioles present the greatest resistance to blood flow. Explain blood flow mechanism and resistance occurring in arterioles. use pressure and resistance in explanation

Answer 1

The constriction of arterioles increases resistance, which causes a decrease in blood flow to downstream capillaries and a larger decrease in blood pressure. Dilation of arterioles causes a decrease in resistance, increasing blood flow to downstream capillaries, and a smaller decrease in blood pressure.

An arteriole is a small-diameter blood vessel in the microcirculation that extends and branches out from an artery and leads to capillaries

Microanatomy

In a healthy vascular system the endothelium lines all blood-contacting surfaces, including arteries, arterioles, veins, venules, capillaries, and heart chambers. This healthy condition is promoted by the ample production of nitric oxide by the endothelium, which requires a biochemical reaction regulated by a complex balance of polyphenols, various nitric oxide synthase enzymes and L-arginine. In addition there is direct electrical and chemical communication via gap junctions between the endothelial cells and the vascular smooth muscle.

Blood pressure

Blood pressure in the arteries supplying the body is a result of the work needed to pump the cardiac output (the flow of blood pumped by the heart) through the vascular resistance, usually termed total peripheral resistance by physicians and researchers. An increase in the media to lumenal diameter ratio has been observed in hypertensive arterioles (arteriolosclerosis) as the vascular wall thickens and/or lumenal diameter decreases.

The up and down fluctuation of the arterial blood pressure is due to the pulsatile nature of the cardiac output and determined by the interaction of the stroke volume versus the volume and elasticity of the major arteries.

The decreased velocity of flow in the capillaries increases the blood pressure, due to Bernoulli's principle. This induces gas and nutrients to move from the blood to the cells, due to the lower osmotic pressure outside the capillary. The opposite process occurs when the blood leaves the capillaries and enters the venules, where the blood pressure drops due to an increase in flow rate. Arterioles receive autonomic nervous system innervation and respond to various circulating hormones in order to regulate their diameter. Retinal vessels lack a functional sympathetic innervation.

Further local responses to stretch, carbon dioxide, pH, and oxygen also influence arteriolar tone. Generally, norepinephrine and epinephrine (hormones produced by sympathetic nerves and the adrenal gland medulla) are vasoconstrictive acting on alpha 1-adrenergic receptors. However, the arterioles of skeletal muscle, cardiac muscle, and pulmonary circulation vasodilate in response to these hormones when they act on beta-adrenergic receptors. Generally, stretch and high oxygen tension increase tone, and carbon dioxide and low pH promote vasodilation. Pulmonary arterioles are a noteworthy exception as they vasodilate in response to high oxygen. Brain arterioles are particularly sensitive to pH with reduced pH promoting vasodilation. A number of hormones influence arteriole tone such as angiotensin II (vasoconstrictive), endothelin (vasoconstrictive), bradykinin (vasodilation), atrial natriuretic peptide (vasodilation), and prostacyclin (vasodilation).