I. Vasodilation– relaxation of smooth muscle in wall of vessel – especially arteriole
II. Vasoconstriction– contraction of smooth muscle in wall of vessel – especially arteriole
III. Smooth muscle responds to:
A. Local factors
1. Hyperemia –active metabolism leading to decreased oxygen levels and increased metabolic byproduct levels cause vasodilation and increased blood flow to area
2. Flow autoregulation –if pressure decreases to a certain organ, vasodilation occurs and therefore restores blood flow
B. Extrinsic factors
1. Neural control via Sympathetic nerves –peripheral and visceral vasoconstriction
2. Epinephrine –hormone released from adrenal gland as a result of a fight or flight response resulting. It acts as a neurotransmitter in peripheral and visceral vasoconstriction but vasodilation to skeletal muscle blood vessels
3. Angiotensin II –potent vasoconstrictor (pathway starts with decreased blood flow/pressure to kidney causing renin release. Renin, an enzyme, then stimulates the activation of angiotensinogen to angiotensin in the bloodstream. In addition to angiotensin II being a vasoconstrictor, it also causes the release of aldosterone from the adrenal gland. Aldosterone causes sodium reabsorption from urine to bloodstream in the kidney, and water passively follows. This increase in blood volume results in increase in blood pressure.)
IV. Capillaries –tiny vessels with thin walls in through which exchange of nutrients and waste occurs primarily by diffusion.
A. Continuous capillaries –most common type in which the endothelial cells are connected via tight junctions; areas of unjoined membranes exist through which fluids and small molecules can pass from blood to interstitial fluid ( e.g. skin capillary).
B. Fenestrated capillaries –endothelial cells have many pores or fenestrations that allow the regulated movement of solutes (e.g. kidney).
C. Sinusoidal capillaries –endothelial cells are loosely linked allowing large molecules to move between them (e.g. liver & bone marrow).
D. Movement across capillaries is by diffusion (primary mechanism), vesicle transport (endocytosis and exocytosis), bulk flow (fluid movement as a result of a pressure gradient) and mediated transport (via a carrier protein as in active and passive transport).
E. Capillary exchange of water
1. Depends on hydrostatic pressure (ie blood pressure) which is a “pushing” pressure based on a fluid pushing on a membrane; Osmotic pressure (also known as colloid osmotic pressure) is a “pulling” pressure based on the particles in a fluid. Normally hydrostatic pressure of blood (ie blood pressure) favors fluid movement out of the capillary and colloid osmotic pressure of the blood favors movement of fluid into the capillary from the interstitial fluid.
2. Lymphatics can remove excess interstitial fluid and return it to the bloodstream. These are vessels that are anatomically similar to veins and can move fluids from the interstitial regions back into the veins.