Ch 10 - Blood Vessels and Blood Pressure
Bulk Flow Forces
-Slightly more fluid is filtered out of the capillaries into the interstitial fluid than is reabsorbed from the interstitial fluid back into the plasma -Excess fluid is picked up by the lymphatic system and returned to general circulation
Blood Pressure
-Systolic pressure: =peak pressure exerted by ejected blood against vessel walls during cardiac systole =averages 120 mm Hg -Diastolic pressure: =minimum pressure in arteries when blood is draining off into vessels downstream =averages 80 mm Hg
Mean Arterial Pressure
-TPR is determined by the degree of arteriolar vasoconstriction -Blood volume is controlled by kidneys
Lymphatic System
-The extra fluid, any leaked proteins, and bacteria in the tissue are picked up by the lymphatic system -Bacteria are destroyed as lymph passes through lymph nodes on the way to being returned to the venous system -Transports absorbed digestive fats
Bulk Flow
-Ultrafiltration occurs when pressure inside the capillary exceeds pressure outside and fluid is pushed out through the pores -Reabsorption occurs when inward driving pressures excedd outward pressures and net movement of fluid back into the capillaries occurs
Vascular Tone
-Vascular tone is the state of partial constriction that estbalishes a baseline of arteriorlar resistance -Arteriolar vasodilation decreases resistance and increases blood flow through the vessel
Venous return
-Venous return is enhance by: =venous vasoconstriction by sympathetic system =external compression of the veins from contraction of surrounding skeletal muscles =one way venous valves -These actions help counter the effects of gravity on the venous system
Veins
-Venules communicate chemically with nearby arterioles to control capillary flow -Veins serve as a blood reservoir containing 60% of blood volume -Veins return blood back to the heart
Vascular Tone
-Arteriole vascoconstriction increases resistance and decreases flow -Arteriolar tone is controlled by local (intrinsic) controls and extrinsic controls
Baroreceptor Reflex
-Baroreceptor reflex is short term mechanism for regulating blood pressure -Acts on heart and vessels to regulate CO and TPR -Baroreceptors in carotid sinus and aortic arch detect blood pressure changes
Arteries
-Because of their large radius, arteries have low resistance(rapid-transit passageways) -Due to elastic properties they act as pressure reservoir to provide driving force for blood when ventricle is in diastole
Blood Flow
-Blood flow to other organs can be adjusted according to metabolic needs -Brain function requires constant blood flow -Maintaining adequate flow to the brain is a priority of the circulatory system
Vascular Tree
-Blood flows in a closed loop between the heart and the organs -Arteries transport blood from the heart throughout the body -Arterioles control the amount of blood that flows through each organ
Blood Flow
-Blood is transported to all parts of the body through a system of vessels -Reconditioning organs(digestive organs, kidneys, skin) receive a disproportionally high blood flow -This allows them to replenish nutrient supplies and remove metabolic wastes to help maintain homeostasis
Bulk Flow Forces
-Bulk flow occurs because of differences in the hydrostatic and colloid osmotic pressures between plasma and interstitial fluid -Capillary hydrostatic pressure pushes fluid out of the capillary bed -Plasma colloid osmotic pressure draws fluid back into the capillary bed
Bulk Flow
-Bulk flow occurs when protein free plasma filters out of the capillary, mixes with the interstitial fluid and then is reabsorbed -Important in regulating the distribution of ECF between plasma and interstitial fluid to help maintain arterial blood pressure
Capillaries
-Capillaries are thin-walled, small-radius, extensively branched vessels -Surface area for exchange is maximized
Vascular Tree
-Capillaries are vessels where materials are exchanged between blood and surrounding tissue cells -Veins return blood from the tissue back to the heart
Extrinsic Control
-Changes in arteriolar resistance bring about changes in mean arterial pressure -NE released from sympathetic nerves causes vasoconstriction -Skeletal and cardiac muscles have local control mechanisms to override sympathetic vasoconstirction during exercise -The cardiovascular control center in the medulla adjusts sympathetic output to the arterioles
Circulatory Shock
-Circulatory shock occurs when blood pressure falls so low that adequate blood flow to tissues can no longer be maintained
Blood Pressure Adjustments
-Constantly monitored by baroreceptors(pressure sensors) within circulatory system -Short term(within seconds) adjustments are made by neural control of cardiac output and total peripheral resistance -Long term(requiring minutes to days) adjustments are made by controlling total blood volume by adjusting urine output and thirst
Bulk Flow
-Depends on two processes: =ultrafiltration =reabsorption
Capillaries
-Diffusion distance is minimized -Large cross-sectional area results in slow blood velocity to maximize time for exchange
Capillary Exchange
-Exchanges between blood and tissues across the capillary are accomplished in two ways: =passive diffusion down concentration gradients is the primary mechanism for exchanging solutes =bulk flow determines the distribution of the ECF volume between the vascular and the interstitial fluid compartments
Extrinsic Control
-Extrinsic sympathetic control of arteriolar radius is important in regulating blood pressure -Increased sympathetic activity produces generalied arteriolar vasoconstriction -Decreased sympathetic activity leads to generalize arteriolar vasodilation
Resistance
-Factors affecting resistance include: =blood viscosity - friction between molecules of a fluid during flow =vessel length - the longer the vessel the greater the resistance to flow =vessel radius - the smaller the radius the greater the resistance
Flow Rate
-Flow rate through a vessel (volume of blood passing through per unit of time): =directly proportional to the pressure gradient =inversely proportional to vascular resistance (F = triangle P / R) (F = flow rate of blood through a vessel) (triangle P = pressure gradient) (R = resistance of blood vessels)
Blood Pressure
-Force exerted by blood against a vessel wall -Pressure depends on: =the volume of blood contained within the vessel =compliance of the vessel walls; how easily the vessel distends when it fills with blood
Circulatory Shock
-Four types: =hypovolemic shock due to extensive loss of blood =cardiogenic shock due to failure of heart =vasogenic shock due to widespread vasodilation =neurogenic shock due to neurally defective vasoconstrictor tone
Arterioles
-Have a thick layer of circular smooth muscle -The radius of arterioles can be adjusted to accomplish two functions: =to variably distribute cardiac output among the organs depending on body needs =to help regulate arterial blood pressure
Extrinsic Control
-Hormones that influence blood pressure: =adrenal medulla hormones epinephrine and norepinephrine reinforce sympathetic tone =vasopressin and angiotensin II are also important in controlling fluid balance
Blood Pressure Abnormalities
-Hypertension is blood pressure above 140/90 mm Hg -Two classes of hypertension: =primary hypertension: unknown cause, accounts for 90% of cases =secondary: occurs secondary to another known primary problem, accounts for 10% of cases
Blood Pressure Abnormalities
-Hypotension defined as blood pressure below 100/60 mm Hg -Orthorstatic hypotension results from insufficient sympathetic compensation when a person moves from a horizontal to a vertical position
Capillary Exchange
-Individual solutes are exchanged primarily by diffusion down concentration gradients -Lipid soluble substances pass directly through endothelial cells lining a capillary -Water soluble substances pass through water filled pores between the endothelial cells -Plasma proteins generally do not escape
Intrinsic Control
-Local chemical changes associated with changes in the level of metabolic activity affect arteriole resistance -Increased blood flow in response to enhance tissue activity is called active hyperemia
Intrinsic Control
-Local influences associated with increased metabolic activity include: =decreased O2 =increased CO2 =increased acid =increased K =increased osmolarity =adenosine release
Arterioles
-Major resistance vessels -High resistance produces a large drop in mean pressure between the arteries and capillaries -This decline enhances blood flow by contributing to the pressure gradient between the heart and organs
Mean Arterial Pressure
-Mean arterial pressure is the blood pressure that is monitored and regulated in the body -Regulated by controlling cardiac output (CO), total peripheral resistance (TPR), and blood volume -Control of CO depends on regulation of heart rate and stroke volume
Blood Pressure Regulation
-Others factors that influence blood pressure: =left atrial volume receptors =hypothalamic osmoreceptors =behaviors and emotions =hypothalamic control for temperature regulation =vasoactive substances from endothelial cells
Pressure Gradient
-Pressure gradient is the difference in pressure between the beginning and end of a vessel -Blood flows from an area of higher pressure to an area of lower pressure down a pressure gradient -The greater the pressure gradient the greater the flow rate through that vessel
Blood Pressure
-Pulse Pressure: the difference between systolic and diastolic pressures -Mean Arterial Pressure(MAP): the average driving pressure throughout the cardiac cycle =diastolic pressure + 1/3 (pulse pressure) -Mean arterial pressure is monitored and regulated by blood pressure reflexes
Resistance
-R is proportional to 1/r^4 -Doubling the radius reduces resistance to 1/16th original value and increases flow 16-fold
Resistance
-Resistance is the hindrance to blood flow through a vessel -Major determinant of resistance ti flow is vessel radius -Small change in radius produces significant change in blood flow
Baroreceptor Reflex
-Send signals into cardiovascular control center in the medulla -The cardiovascular control center alters the activity of the sympathetic and parasympathetic systems to return MAP to normal
