Regulation of Organ Blood Flow and Special Circulations

Question 1

organ blood flow rate

Answer 1

Qi = ∆P/Ri

• Qi = volumetric Blood flow rate to organ i
• ∆P = perfusion pressure
• Ri = vascular resistance for organ i

Question 2

organ blood flow

Answer 2

• parallel arrangement of the organ vasculatures, therefore the perfusion pressure for organ blood flow is the same for all organs (except the liver)
• blood flow to each organ is adjusted by changing the specific vascular resistance of that organ by contraction of the SMCs surrounding the arterioles of that organ –vasoconstriction or vasodilation

Question 3

ways in which arteriole SMC tone is modulatd

Answer 3

1. direct autonomic control (extrinsic)
2. local myogenic and metabolic factors (intrinsic)
3. humoral factors

Question 4

direct autonomic neural control of organ blood flow

Answer 4

• extrinsic

* parasympathetic or sympathetic

Question 5

direct autonomic neural control of organ blood flow- sympathetic

Answer 5

• sympathetic postganglionic vasoconstrictor fibers are widely disseminated throughout the blood vessels of the body - most abundant in the kidneys and the skin, relatively sparse in the coronary and cerebral resistance vessels
• norepinephrine - alpha 1 and alpha 2
• sympathetic postganglionic vasodilator fibers found on arterioles in skeletal muscle - Beta 2

Question 6

direct autonomic neural control of organ blood flow- parasympathetic

Answer 6

• innervation of blood vessels by parasympathetic fibers far less common than sympathetic vasoconstrictor fibers
• found in erectile blood vessels and blood vessels in salivary glands
• acetylcholine - vasodilation

Question 7

local myogenic factors that regulate organ blood flow

Answer 7

• affect arteriole vascular tone
• in some organs, like the heart and brain, local factors are the prime regulators of blood flow
• two theories - myogenic and metabolic

Question 8

local myogenic factors that regulate organ blood flow - myogenic theory

Answer 8

• autoregulation
• kidneys, brain, skeletal muscle
• if perfusion pressure falls, vascular resistance must also fall for flow to be maintained
• if perfusion pressure increases, the resistance must increase to maintain flow

Qi = ∆P/Ri

•vascular smooth muscle tends to contract when it is stretched (increased pressure, contraction) and relax when tension is reduced (decreased pressure, dilation)

Question 9

local myogenic factors that regulate organ blood flow -metabolic theory

Answer 9

• local generation of metabolites affects vascular smooth muscle tone
• K+, H+, lactate, CO2, adenosine
• these metabolites tend to cause smooth muscle relaxation –> vasodilation –> increases blood flow
• this increased blood flow tends to wash out the metabolites, returning vascular tone and lowering blood flow back to normal

Question 10

humoral factors that regulate organ blood flow

Answer 10

1. circulating catelcholamines
2. nitric oxide
3. vasopressin
4. angiotensin II
5. other vasoactive substances

Question 11

humoral factors that regulate organ blood flow - circulating catelcholamines

Answer 11

• ANS indirect sympathetic
• sympathetic postganglionic fibers innervate the adrenal medulla –> release norepinephrine and epinephrine
• norepinephrine - beta 1 receptors in the heart to cause an increase in heart rate and contractility
• circulating catecholamines can also activate alpha 1 receptors causing vasoconstriction
• coronary, pulmonary, skeletal muscle, abdominal and renal vasculatures contain beta 2 receptors —> vasodilation (high levels of epinephrine can cause vasodilation and higher doses cause vasoconstriction - beta 2 receptors are more sensitive than alpha 1)

Question 12

humoral factors that regulate organ blood flow - nitric oxide

Answer 12

• endothelial derived relaxing factor (EDRF)
• produced and released by endothelial cells
• formation stimulated by thrombin, bradykinin, substance P, ADP and acetylcholine
• shear stress (activates Ca++ channels, increasing NO production) can also stimulate EDRF - flow dependent Induced) vasodilation

Question 13

humoral factors that regulate organ blood flow - vasopressin

Answer 13

• ADH - polypeptide released from posterior pituitary gland in response to low extracellular volume and high extracellular fluid osmolarity
• acts on kidney ducts to decrease renal excretion of water
• potent vasoconstrictor
• normally not a physiologically significant vasoconstrictor, but under extreme conditions (hemorrhage)

Question 14

humoral factors that regulate organ blood flow - angiotensin II

Answer 14

• polypeptide
• important in maintaining sodium balance and blood volume regulation
• potent vasoconstrictor
• normally not a physiologically significant vasoconstrictor, but under extreme conditions (hemorrhage)
• renin-angiotensin system thought to be a cause of hypertension

Question 15

humoral factors that regulate organ blood flow - other vasoactive substances

Answer 15

• histamine, bradykinin, serotonin and prostaglandins all have vasoactive properties
• histamine and bradykinin: allergic responses, trauma - arteriolar dilation, venular constriction and an increase in microvascular permeability, enhance extravasation of fluid and plasma proteins into tissue causing edema
• serotonin is stored in platelets and upon release causes vasoconstriction - migraine headaches?
• prostaglandins produce various effects - prostacyclin and prostaglandin E cause vasodilation, thromboxane causes vasoconstriction

Question 16

distribution of cardiac output

Answer 16

•the blood flow to an organ is regulated by controlling the vascular resistance
•vascular resistance is regulated by altering arteriolar tone
-local (intrinsic) and central (extrinsic) factors

Question 17

factors that regulate coronary circulation

Answer 17

1. coronary architecture
2. coronary exchange vessels
3. the effect of cardia contraction on coronary blood flow
4. control of coronary blood flow

Question 18

Fick equation

Answer 18

CO = VO2/Ca-Cv

• VO2 = steady state O2 consumption
• Ca = O2 content of arterial blood
• Cv = O2 content of venous blood

Question 19

factors that regulate coronary circulation - coronary architecture

Answer 19

• coronary arteries arise from sinuses behind two cusps of the aortic valve - open throughout cardiac cycle
• right serves right heart, left serves left heart
• most coronary venous blood drains into the coronary sinus in the right atrium and the rest drains directly into the cardiac chambers
• metabolic demands are great, at rest it is 20x that of skeletal muscle at rest
• average blood flow through coronaries is 80-100ml/min/100g

Question 20

factors that regulate coronary circulation - coronary exchange vessels

Answer 20

• capillary density is much greater than he capillary density of skeletal muscle
• cardiac muscle fibers are smaller than skeletal muscle fibers
• one capillary per fiber for skeletal and cardiac muscle
• only a fraction of skeletal muscle capillaries are perfused at rest
• most cardiac capillaries are perfused at rest
• smaller myocytes and greater capillary density = reduction in diffusion distance
• at rest, about 75% of the O2 delivered to the heart via the coronary arteries is extracted (can increase to 90% when cardiac workload is substantially increased)

Question 21

factors that regulate coronary circulation - cardiac contraction

Answer 21

• because of cardiac contraction, the perfusion to the heart muscle is cyclical
• during ventricular ejection, the heart muscle is contracted, compressing the coronary arteries, increasing coronary vascular resistance and decreasing flow
• because the contractile forces developed are greatest in the subendocardium, the flow “deficit” is greatest in this region - flow to the more superficial parts of the heart are not as compromised
• the subendocardium is most prone to ischemic injury

Question 22

factors that regulate coronary circulation - control of coronary blood flow

Answer 22

• because myocardial O2 extraction from the blood is quite high and can only increase by 15%, increased O2 delivery is achieved by an increase in blood flow
• local metabolic control - hypoxia and adenosine
• nitric oxide
• sympathetic alpha (vasoconstriction) and beta (vasodilation) adrenergic fibers innervate the coronary arteries –> net result is vasodilation (metabolites produced by positive inotropic and chronotropic effects of sympathetic stimulation combined with beta dilation overwhelm the alpha receptor response)

Question 23

extravascular compression - control of organ blood flow

Answer 23

• mechanical compressive forces can affect vascular resistance and blood flow within organs
• physiological (flexing muscle) or pathological (neoplasm, compartment syndrome, edema)

Question 24

pulmonary circulation

Answer 24

• because the entire cardiac output perfuses the lung, the blood flow greatly exceeds the nutritional needs of the pulmonary tissue
• little moment to moment control of the blood flow, though pulmonary flow will be shunted away from poorly ventilated areas of the lung
• larger surface area, small diffusion distances
• macrophages

Question 25

skeletal muscle

Answer 25

• largest “organ system” in the body
• like the heart, the capillaries run parallel to the muscle fibers
• intercapillary anastomoses (heart has a lot more), frequency depends on oxidative nature of the muscle (slow muscles have more)
• 16% CO at rest, increases to 60-70% during exercise

Question 26

regulation of skeletal muscle blood flow

Answer 26

• at rest, sympathetic alpha constrictor and sympathetic beta 2 vasodilators fibers dominate blood flow (constriction more)
• intrinsic - metabolic hyperemia - during exercise - increase metabolites —> vasodilation —> increased flow + capillarity increase O2 and glucose extraction during exercise
• extrinsic - sympathetic - at rest