Review of Microvascular Anatomy and Physiology Where gas and nutrient exchange occurs

Microvascular anatomy Analogy with freeways and highways: The large arteries and veins are like the freeways and highways for the peripheral system while the arterioles, venules and capillary beds are like the smaller streets and alley ways of a city An arteriole is a small diameter blood vessel in the microcirculation that branches out from an artery that leads into capillaries. Arterioles have muscular walls and are the primary sites of vascular resistance* Venule is a very small vessel that allows blood to return from the capillaries back into the venous circulation Capillaries are the smallest blood vessel in the body, they have a wall that is one cell thick and are the connection between the arterioles and venules. These microvessels are the sites of gas and nutrient exchange in the body. The greatest change in blood pressure and velocity of blood flow occurs at the transition of arterioles to capillaries The decrease in velocity in the capillaries increases blood pressure (Bernoulli’s principle) This induces oxygen and nutrients to move from inside the capillary to the interstitial space (where oncotic pressure is lower) and into cell tissues The opposite occurs in the venule side where blood pressure is lower and oncotic pressure is lower The major regulator of vascular resistance in the body is regulation of vessel radius. In humans, there is very little pressure change as blood flows from the aorta to the large arteries, but the small arteries and arterioles are the site of about 70% of the pressure drop, and are the main regulators of SVR. When environmental changes occur (e.g. exercise, immersion in water), neuronal and hormonal signals, including binding of norepinephrine and epinephrine to the α1 receptor on vascular smooth muscles, cause either vasoconstriction or vasodilation. Because resistance is inversely proportional to the fourth power of vessel radius, changes to arteriole diameter can result in large increases or decreases in vascular resistance.

Arterioles and Peripheral Vascular Resistance Arterioles are the main regulators of peripheral vascular resistance Changes in vessel diameter (vasoconstriction or vasodilatation) most important factors for blood flow regulation within an organ and to regulate arterial pressure Vasoactive substances mediate constriction or vasodilatation depending upon environmental triggers (for example exercise) Resistance to blood flow within a vascular network is determined by the size of individual vessels (length and diameter), the organization of the vascular network (series and parallel arrangements), physical characteristics of the blood (viscosity, laminar flow versus turbulent flow), and extravascular mechanical forces acting upon the vasculature. Of the above factors, changes in vessel diameter are most important quantitatively for regulating blood flow within an organ, as well as for regulating arterial pressure. Changes in vessel diameter, particularly in small arteries and arterioles, enable organs to adjust their own blood flow to meet the metabolic requirements of the tissue. Therefore, if an organ needs to adjust its blood flow (and therefore, oxygen delivery), cells surrounding these blood vessels release vasoactive substances that can either constrict or dilate the resistance vessels. The ability of an organ to regulate its own blood flow is termed local regulation of blood flow and is mediated by vasoconstrictor and vasodilator substances released by the tissue surrounding blood vessels (vasoactive metabolites) and by the vascular endothelium. There is also a mechanism intrinsic to the vascular smooth muscle (myogenic mechanism) that is involved in local blood flow regulation. 

Vasodilatation and vasoconstriction https://www.youtube.com/watch?v=M- vNa3NdGmA

Capillary Gas and Nutrient Exchange https://www.youtube.com/watch?v=- qY17G_n-WA

Gas exchange in the lungs

Summary The components of the microvascular anatomy include arterioles, venules, capillaries Arterioles are the main regulators of peripheral vascular resistance Capillaries are small, thin wall micro-vessels were gas and nutrient exchange occurs Venules are low pressure vessels that pick up de- oxygenated blood to carry back to the lungs In the lungs oxygen is collected at the interphase between capillaries and the alveoli