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Blood Pressure
measured as mmHg
Main factors affecting blood pressure:
**1. cardiac output
- peripheral resistance
- blood volume**
a change in any of these could cause a corresponding change in blood pressure
blood circulates by going down a pressure gradient:
presssure gradient depends on:
- force of heart beat
- peripheral resistance - counteracts pressure
Blood Flow = Difference in pressure Peripheral resistance
Measuring Blood Pressure
use sphygmomanometer
usually use brachial artery
procedure: a. increase pressure above systolic to completely cut off blood flow in artery b. gradually release pressure until 1 st^ spurt (pulse) passes through cuff = systolic pressure c. continue to release until there is no obstruction of flow sounds disappear = diastolic pressure
normal BP = 110-140 / 75-80 [mm Hg]
top number = systolic pressure force of ventricular contraction
bottom number = resistance of blood flow may be more important indicates strain to which vessels are continuously subjected also reflects condition of peripheral vessels
Pulse Pressure:
Pulse Pressure = Systolic Pr – Diastolic Pr
eg. 120/80; then PP = 40 mmHg
Mean Arterial Pressure:
represents the average of Sys & Diast BP’s MAP = Diastolic Pr + 1/3 rd^ x Pulse Pressure
Variations in Blood Pressure
heart pumps ~5L (4.8 )of blood per minute 5 liters in systemic circuit 5 liters in pulmonary circuit
Systemic Circuit averages 100 0 difference = 100 mmHg
high resistance: 62000 miles of vessels from L ventricle to R atrium especially in arterioles
arteries variable systolic = 130-5 mmHg diastolic = 85-90 mmHg capillaries relatively constant 35 – 15 mmHg veins relatively constant 6 – 1 mmHg larger veins near 0 mmHg
Pulmonary Circuit much fewer and shorter vessels much lower resistance averages 15 5 difference = 10 mmHg
low resistance no pulmonary edema
numerous blood-borne chemicals influence short term control of blood pressure
act directly on vascular smooth muscle or on vasomotor system
eg. NO secreted by endothelial cells localized vasodilation lowers BP very brief effect, quickly destroyed is the major antagonist to sympathetic vasoconstriction
eg. viagra stim production of NO
eg. inflammatory chemicals histamines, kinins, etc potent vasodilators lower BP increase capillary permeability
eg. nicotine intense vasoconstriction raise BP simulates sympathetic release of NE
eg. alcohol depresses VMC inhibits ADH release vasodilation (esp of skin vessels) lowers BP (also flushing of face)
3. Blood Volume a. Short Term Control of Blood volume blood pressure is directly affected by the volume of fluids retained or removed from body: greater bld volume increases BP eg. excessive salts promote water retention lower bld volume decreases BP eg. dehydration eg. internal bleeding
kidneys act both directly and indirectly to help regulate arterial pressure:
short term renal control = Direct Control
high BP triggers more filtration from kidney greater urine output
lowers BP
low BP triggers more reabsorption of water by kidney lower urine output raises BP
b. Long Term Renal Control of BP = Indirect Renal Control
are slower to change
can act as a system-wide control over whole body blood pressure baroreceptors quickly adapt to long term (chronic) changes in blood pressure stop “trying” to return to “normal”
renin-angiotensis mechanism if lower BP: kidneys release enzyme = renin renin triggers production of angiotensin II angiotensin causes: vasoconstriction raises BP release of ADH conserves water to raise BP
Abnormal Blood Pressure
Hypotension low BP systolic < usually not a cause for concern often associated with long healthy life but. in some may produce dizziness when standing up too quickly (esp in older patients) may be due to severe bleeding and lead to circulatory shock may hint at poor nutrition eg. <blood proteins
Hypertension if transient is normal: adaptation during fever, exercise, strong emotions if persistent is a cause for concern
