Blood Vessels Part 4: Exam 2 Flashcards
Control of Blood Flow
- Tissue Perfusion: blood flow through body body tissues involved in:
1) delivery of O2 and nutrients to and removal of wastes from tissue cells
2) gas exchange (lungs)
3) absorption of nutrients (digestive tract)
4) urine formation (kidneys) - rate of flow is right amount to provide proper function to tissue or organ
Rate of Blood Flow is Controlled by Extrinsic and Intrinsic Factors
Extrensic Control:
- sympathetic nervous system and hormones control blood flow throughout body
- act on arteriolar smooth muscle to reduce flow to regions that need it least
- control is from outside of tissue or organ
- uses nerves or hormones
Intrinsic Control (autoregulation or local control):
- blood flow is adjusted locally to meet specific tissues requirements
- local arterioles that feed capillaries can undergo modification of their diameters
- organs regulate own blood flow by varying resistance of their own arterioles
- control is from within tissue or organ
- uses paracrine or properties of muscle tissue
Distribution of Blood Flow at Rest and During Strenuous Exercise
- at rest, skeletal muscles receive about 20% of total blood in body but during exercise skeletal muscles can receive over 70% of blood
- intrinsic controls: skeletal muscle arterioles dilate, increasing blood flow to muscle
- extrinsic controls: decrease blood flow to other organs like kidneys and digestive organs
- MAP is maintained despite dilation of skeletal muscle arterioles
Autoregulation
local (intrinsic) conditions that regulate blood flow to that area
2 types of intrinsic mechanisms both determine final autoregulatory response
1) Metabolic Controls
- increase in tissue metabolic activities result in:
- decline levels of O2
- increase levels of metabolic products (H+, K+, adenosine, prostaglandins)
- effects of change in levels of local chemicals
- cause direct relaxation of arterioles and relaxation and precapillary sphincter
- cause release of nitric oxide (NO): a powerful vasodilator by endothelial cells
2) Myogenic Controls
- Myogenic Responses: local vascular smooth muscle responds to changes in MAP to keep perfusion constant to avoid damage to tissue
- Passive Stretch: increased MAP stretches vessel walls more than normal (smooth muscle responds by constricting, cause decrease blood flow to tissue)
- Reduced Stretch: decreased MAP causes less stretch than normal (smooth muscle responds by dilating, cause increase blood flow to tissue)
Long-Term Autoregulation
- occurs when short term autoregulation cannot meet tissue nutrient requirements
- long term may take weeks or months to increase blood supply
- # of vessels to region increases (angiogenesis), and existing vessels enlarge
Blood Flow in Skeletal Muscles
- blood flow varies with fiber type and activity
- at rest, myogenic and neural mechanisms predominate; maintain flow at about 1 L/min
- Active or Exercise Hyperemia: during muscle activity, blood flow increases in direct proportion to metabolic activity
- local controls override sympathetic vasoconstriction; flow can increase 10x
Blood Flow in Brain
- blood flow to brain must be constant because neurons are intolerant of ischemia
- flow averages about 750 ml/min
- brain vulnerable under extreme systemic pressure changes
- MAP below 60 mm Hg can cause syncope (fainting)
- MAP about 160 mm Hg can result in cerebral edema
Control Mechanisms
- Metabolic Controls
- decreased pH or increased CO cause marked vasodilation
- very high CO2 levels depress autoregulatory mechanisms
- Myogenic Controls
- decreased MAP causes cerebral vessels to dilate
- increased MAP causes cerebral vessels to constrict
Blood Flow in Skin
- functions of blood flow through skin
1) supplies nutrients to cells
- autoregulatory in response to O2 needs
2) helps regulate body temp
- neurally controlled
- important function of skin
3) provides blood reservoir
- neurally controlled
Blood Flow in Heart
- blood flow through heart is influenced by aortic pressures and ventricular pumping
- during ventricular systole, coronary vessels are compressed
- myocardial blood flow ceases
- stored myoglobin supplies O2
- during diastole, high aortic pressure forces blood through coronary circulation
- at rest, coronary blood flow is about 250 ml/min
- control is via myogenic mechanisms
- during strenuous exercise, coronary vessels dilate in response to local accumulation of vasodilators
- blood flow may increase 3-4 times
- important because cardiac cells use 65% of O2 delivered
- other cells use 25% of delivered O2
- increasing coronary blood flow is only way to provide more O2
Velocity of Blood Flow
- velocity of flow changes as blood travels through systemic circulation
- fastest in aorta, slowest in capillaries, then increase again in veins
- speed is related to total cross-sectional area
- capillaries have largest area so slowest flow
- slow capillary flow allows adequate time for exchange between blood and tissues
Vasomotion
intermittent flow of blood through capillaries
- due to on//off opening and closing of precapillary sphincters
Capillary Exchange of Respiratory Gases and Nutrients
- many molecules pass by diffusion between blood and interstitial fluid
- move down concentration gradients
- molecules use four different routes to cross capillary:
1) diffuse directly through endothelial membranes
- ex: lipid-soluble molecules such as respiratory gases
2) pass through clefts
- ex: water-soluble solutes
3) pass through fenestrations
- ex: water-soluble solutes
4) active transport by pinocytotic vesicles
- ex: larger molecules, like proteins
Fluid Movements: Bulk Flow
- fluid is forced out clefts of capillaries at arterial end, and most returns to blood at venous end
- extremely important in determining relative fluid volumes in blood and interstitial space
- bulk fluid across capillary walls causes continuous mixing of fluid between plasma and interstitial fluid; maintains interstitial environment
- direction
Direction and Amount of Fluid Depend on 2 opposing forces
- Hydrostatic Pressures (HP)
- force exerted by fluid pressing against wall
- capillary hydrostatic pressure (HPc): capillary blood pressure that force fluids through capillary walls (greater at arterial end (35 mm Hg) of bed than at venule end (17 mm Hg))
- intersitial fluid hydrostatic pressure (HPif): pressure pushing fluid back into vessel; 0 because lymphatic vessels drain interstitial fluid
- force exerted by fluid pressing against wall
- Colloid Osmotic Pressures
- capillary colloid osmotic pressure (oncotic pressure (OPc)
- “sucking” pressure created by nondiffusible plasma proteins pulling water back into capillary
- OPc is about 26 mm Hg - intersitial fluid colloid osmotic pressure (OPif)
- pressure is inconsequential because intersitial fluid have low protein content
- OPif around 1 mm Hg
- capillary colloid osmotic pressure (oncotic pressure (OPc)
- Hydrostatic-Osmotic Pressure Interactions
- net filtration (NFP): compromises all forces acting on capillary bed
- NFP = (HPc + OPif) - (HPif + OPc) - net fluid flow out of arterial end (filtration)
- net fluid flow in at venous end (reabsorption)
- more fluid leaves at arterial end than is returned at venous end
- excess intersitial fluid is returned to blood by lymphatic system
- net filtration (NFP): compromises all forces acting on capillary bed
