Circulation 6 Flashcards
What are the resistance vessels?
Arterioles
Precapillary sphincters
Metarterioles
Venous resistance
For what aspects of circulation is vascular smooth muscle responsible?
Control of total peripheral resistance
Arterial and venous tone
Distribution of blood flow through the body
Global control mechanisms
Neural and hormonal
Local control mechanisms
Myogenic
Metabolic
Endothelial
Mechanical
Autoregulation
The intrinsic property of an organ or tissue to maintain constant blood flow deespite changes in arterial perfusion pressure (due to gravity)
When does autoregulation fail?
At very high or very low pressures
Myogenic hypothesis
Vascular smooth muscle contracts in response to stretch and relaxes in response to a reduction in stretch
Result of the myogenic response during increase in pressure
Causes a stretch of the vascular wall, which causes the vascular smooth muscle to contract, which results in an increase in resistance and a decrease in flow
Result of myogenic response during a decrease in pressure
Reduction of stretch of the vessel wall, allowing the muscles to relax, reducing resistance and increasing flow
Metabolic hypothesis
Metabolic activity produces substances that relax vascular smooth muscle (CO2, H+, adenosine)
What happens with the metabolites as pressure increases?
There is a brief increase in blood flow when there is increased pressure, which picks up the metabolites and then allows the vessels to constrict, which then decreases flow and increases resistance
What happens with metabolites as pressure decreases?
The metabolites build up due to decreased blood flow, which then causes vessels to dilate, resistance to decrease, and flow to increase
Organs with strong autoregulation
Heart, brain, kidney, skeletal muscle
Organs with weak autoregulation
Splanchnic circulation
Organs with little autoregulation
Skin, lungs
Is autoregulation dependent upon the endothelium?
No
For what response is the endothelium responsible?
Endothelial-mediated mechanism (release of EDRF or NO)
Endothelial-derived agents
Vasodilators: EDRF, NO, prostacyclin
Vasoconstrictors: ET
Active (functional) hyperemia
Increased blood flow caused by enhanced tissue activity
Possible factors in active hyperemia
Potassium ion, inorganic phosphate, interstitial osmolarity
Reactive hyperemia
Transient increase of blood flow that follows a brief arterial occlusion
Mechanical augmentation
An increase in tissue pressure can mechanically compress small vessels and alter blood flow
Possible causes of mechanical augmentation
Muscular contraction (heart, muscle) Alveolar pressure Tumors (venous constriction/obstruction)
