Arterioles and Capillaries Flashcards
What 2 factors influence Arteriolar Radius?
- Local metabolic changes
- Extrinsic control of arteriolar radius
What local metabolic changes influence arteriolar radius?
Active Hyperemia
What happens in active hyperemia (local metabolic change)
- During increased metabolic activity like exercise, O2 concentration decreases as the actively metabolising cells use up more O2 to support oxidative phosphorylation to produce ATP. This causes local arteriolar dilation by triggering relaxation of arteriolar smooth muscle in the vicinity, increasing blood flow to the area.
What chemical changes result from Active Hyperemia (local metabolic change)
- Decreased O2
- Increased CO2 as a waste product from increased oxidative phosphorylation
- Increased acid due to increased O2
- Increased K+ as repeated action potentials outpace the ability of Na-K pump to restore concentration gradients, resulting in increased K+ in interstitial fluid
- Increased osmolarity due to elevated cellular metabolism
- Adenosine released in response to increased metabolic activity
What are the EXTRINSIC controls of arteriolar radius?
- Sympathetic Nervous system
- Hormones and cardiovascular control center
How does sympathetic nervous system (extrinsic control) affect arteriolar radius?
Norepinephrine released from sympathetic nerve endings combine with a1 adrenergic receptors on arteriolar smooth muscle to cause vasoconstriction
How do hormones and the cardiovascular control center (Extrinsic control) affect arteriolar radius?
- Cardiovascular control center located in the medulla of the brainstem
- Adrenal medullary horones like epinephrine (more abundant in skeletal muscles and heart, has greater affinity for B2 receptors that cause vasodilation) and norepinephrine (combines with a1)
- Vasopressin (maintain water balance by regulating amount of water kidneys retain during urine formation) and angiotensin II (regulate body’s salt balance) as potent vasoconstrictors
What are the functions of capillaries?
- Sites for exchange of materials between blood and tissue walls
- Branch extensively to bring blood within reach of every cell
What are the factors that enhance diffusion across capillaries?
- Thin capillary wall and small capillary diameter, coupled with proximity of every cell to a capillary
- Large surface area available for exchange
- Blood flows slower in the capillaries (velocity of flow)
How does the thin capillary wall enhance diffusion?
- Diffusing molecules only need to travel short distances
- Capillary walls are composed of simple squamous epithelium, supported by a thin basement membrane. Materials entering or leaving the capillaries pass freely through the basement membrane, There are pores that allow passage of water-soluble material
- Narrow capillary diameter allows plasma contents to either be in direct contact with the inside of the capillary wall, or are diffusing across a short distance
How does large surface area in capillaries facilitate diffusion?
A small volume of blood is exposed to an extensive surface area
How does velocity of blood flow affect diffusion across capillaries?
Slow velocity allows adequate time for exchange of nutrients and metabolic end products between blood and tissue cells.
How does local metabolic changes in capillaries affect precapillary sphincters?
- When tissue metabolic activity increases, precapillary sphincters relax, increasing the number of open capillaries. Arteriolar vasodilation increases total flow to organ. This results in increased total volume and surface area available for exchange and the diffusion distance between cells decrease.
- When tissue metabolic activity decreases, precapillary sphincters contract. Blood will bypass the capillary bed and flow through the metarteriole.
What is the function of interstitial fluid?
Cells exchange materials directly with interstitial fluid, mainly by diffusion
Explain how interstitial fluid is used as a passive medium between blood and cells
- Reconditioning organs continuously add nutrients and oxygen and remove CO2 and other wastes as blood passes through them
- As cells use up oxygen and glucose, blood constantly brings in fresh supplies of these materials, maintaining concentration gradients that favour the net diffusion of these substances from blood to cells
- Ongoing net diffusion of CO2 and metabolic wastes from cells to blood is maintained by continuous production and removal of these wastes
- Extent of exchange is determined by magnitude of concentration gradient. As cells increase level of activity, they use up more O2, produce more CO2, creating a larger concentration gradient, increasing the rate of diffusion
