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Blood Flow

الكلية كلية طب حمورابي     القسم الكلية ذات القسم الواحد     المرحلة 2
أستاذ المادة شهلاء خزعل عبد محمد جابك       14/11/2016 06:40:58
A- Regulation of local blood flow to tissues:
Blood flow in different organs is physiologically maintained by adjusting the diameter of their arterioles. The mechanisms of regulation of regional: local blood flow can be classified into; Short-term and long tent regulation mechanisms.
Short-tern regulation mechanisms:
These mechanisms adjust the minute-to-minute flow to the organs according to their metabolic needs. Four mechanisms are involved:
a- Metabolic auto regulation. c- Hormonal regulation.
b- Nervous regulation. d- Myogenic auto regulation.
Metabolic autoregulation:
Increased metabolic activity dilates the blood vessels. This effect is mediated by:
A- Hypoxia, The increased O2 utilization by the tissues produces local hypoxia. The degree of vascular dilation is directly proportionate to the degree of hypoxia in arteriolar blood. in contrast, an increase in O2, level (hyperoxia) produce local vasoconstriction and decrease in blood flow.
B- Vasodilator metabolites, hid metabolism with hypoxia produces a number of vasodilator metabolites, which include:
1- Adenosine; which is the most important vasodilator substance released from active tissues. An ischernic or hypoxic heart releases adenosine which dilates the coronaries and corrects the ischemia or hypoxia. Adenosine is also released by skeletal muscles and other tissues.
2. ADP and AMP which are produced by hydrolysis of AT?.
3. CO2 and H+ ions which act directly on the vascular smooth muscles and relax them. CO2 is a very powerful dilator of the cerebral blood vessels
4. Lactic acid which is produced by anaerobicglycolysis. it has no direct vasodilator effect, hut acts through elevation of H ion concentration.
5. IC released from active cells. It relaxes the vascular smooth muscles.
C- The endothelium produces several molecules that promote smooth muscle relaxation (vasodilators), including nitric oxide (NO), bradykinin, and prostacyclin.
D- The Endothelium-Derived Relaxing Factor (EDRE): This is a powerful vasodilator substance secreted by the vascular endothelium. This substance chemically was found to he nitric oxide. it is released from the arterial endothelium when stimulated by bradykinin, VIP or ACH (acetylcholine). i.e. these vasodilator substances act through releasing EDRF and it is the EDRF which mediates the vasodilatoreffect or these substances. in the absenceof EDRE, bradykinins and VIP are ineffective and ACH produces vasoconstriction not dilation, EDRE is also produced when the blood flow to a tissue is increased as a result of arteriolar dilatation, thus further increasing the blood flow to that tissue.


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