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Pressure, Flow and Resistance Learning Objectives:
- 1. What is pulse pressure and how is it regulated?
- 2. Know the relationship between MAP and blood flow
- 3. Understand the various regulators of arterial tone
- 4. Describe the mechanisms underlying hyperemia
Pressure Throughout Systemic Circulation
- highest in arteries and decreases as blood flows through.
- Pulse Pressure = Systolic P – Diastolic P
- proportional to the strength of ventricular contraction
- magnitude determined by: 1. stroke volume 2. distensibility of the arteries
Pressure Flow and Resistance in the Cardio Vascular System
- flow is inversly related to change in pressure /resistance
- 1: blood flows if a pressure gradient is present
- 2: goes from high to low pressure
- 3: flow is opposed by resistance
- 4: 3 factors affecting resistance are radius, viscosity and length
- 5: flow expressed in litres per minute
- 6: velocity of flow expressed in cm per minute
- 7: primary determinant of velocity is cross sectional area
Mean Arterial Pressure (MAP)
- approximation of mean artery pressure
- regulating MAP and not pulse pressure allows continuous blood flow
- MAP = 1/3 pulse pressure + diastolic pressure
- Flow (cardiac output) = arterial pressure/R = MAP/R Mean arterial pressure
- cardiac output × resistance
Factors that Influence Mean Arterial Pressure:
- blood volume
- effectiveness of heart as a pump
- resistance of the system
- distrobution of blood between arterial and venous blood vessels.
- influenced by local control, sympathetic reflexes, hormones
- resistance and pressure gradient determine flow rate.
- Determined by 3 factors: viscosity (determined by concentration of red blood cells and plasma proteins), vessel length, and radius.
- Resistance is regulated by smooth muscle cells.
- R = 8ηL/πr4
- vasoconstriction caused by: myogenic activity, oxygen, carbon dioxide, endothelin, sympathetic stimulation, vasopressin, angiotensin2, and the cold
- vasodialation caused by: decreased myogenic activity oxygen, carbon dioxide, decreased sympathetic stimulation, histamine release, and heat.
Hypermia: (a Locally Mediated Increase in Blood Flow)
- Active hyperemia: increased tissue metaoblism causes increased release of metabolic vasodilators into the ECF, arterioles dilate, decreased resistance creates increased blood flow, oxygen and nutrient supply increases as long as metabolism is increased
- Reactive hyperemia: decreased tissue blood flow due to an occlusion, metabolic vasodilators accumulate in the ECF, arterioles dilate but occlusion prevents blood flow, leads to removal of the occlusion thus decreased resistance increases blood flow, as vasodilators wash away, arterioles constrict and blood flow returns to normal.