12.7 A Flashcards
What three organs get most of the resting blood flow?
liver, kidney, muscle
Why does resting blood flow differ between organs?
because although MAP is the same throughout the system, the resistance in each organ is different
Describe the pressure gradient that pushes blood across the various organs.
each has an equal pressure gradient since they’re in parallel
The heart and brain are unique in that their vascular tone is primarily controlled by what?
vasodilator metabolites whereas other organs are more dependent on sympathetic innervation
Is the basal tone for arteries constriction or dilation?
their basal tone leans more towards constriction
Oxygen uptake into an organ depends on what factors?
- rate of blood flow
- ability to extract oxygen
Oxygen uptake by an organ can be calculated with what equation?
O2 uptake = blood flow x [(A-V)O2 Difference]
What determines the ability of an organ to extract oxygen from blood?
the total surface area of perfused capillaries, which is controlled by precapillary sphincters
How do precapillary sphincters affect organ oxygen extraction?
they alter the total surface area of perfused capillaries to alter the organs ability to extract oxygen
How does basal oxygen extraction differ in the heart from other organs like the kidney?
there are no precapillary sphincters in the heart, so all capillary beds are perfused and oxygen extraction is always maximal
Although kidneys have precapillary sphincters, they are unable to significantly alter what in order to increase oxygen uptake.
the kidney is limited in its ability to increase blood flow
What is autoregulation?
the ability of the heart and brain to maintain relatively constant blood flow in response to changes in systemic arterial pressure
What is the autoregulatory range?
the range of MAPs across which organs are capable of maintaining steady state organ blood flow
The limits of autoregulation represent what quality of the organ’s arteries?
maximal dilation or constriction
When arterial pressure drops, what happens to the arterioles of systemic organs? What about to the arterioles of the brain and heart?
- systemic arterioles will contract
- brain and heart arterioles will dilate
Which organs are capable of autoregulation?
the brain and heart
Autoregulation involves what two mechanisms?
- myogenic response where changes in vascular pressure alter stretch of the vessel wall, resulting in constriction or dilation
- changes in the level of metabolic vasodilators in interstitial fluid of the organ
Describe the myogenic response that underlies changes in regional blood flow.
- increase in arteriolar pressure
- increased stretch of the wall
- increased frequency of APs in smooth muscle
- opening of calcium channels
- calcium influx
- contraction/constriction
The interstitial concentration of vasodilator metabolites is controlled by what factors?
- rate of formation (proportional to metabolic rate)
- rate of removal (proportional to organ blood flow)
What are some vasodilator metabolites?
- adenosine
- potassium
- lactic acid
- carbon dioxide
When ATP formation is impaired, what vasodilator metabolite level rises?
adenosine
How does potassium serve as a vasodilator metabolite?
- when frequency of APs increases interstitial concentration of potassium rises
- local hyperkalemia causes hyperpolarization of arterioles which decreases calcium influx to smooth muscle
- causes relaxation
Decreased ATP levels have what effect on interstitial potassium levels?
decreased ATP levels activate K/ATP channels which increases potassium efflux and results in hyperkalemia which decreases calcium influx to smooth muscle, leading to relaxation
All vasodilators do what on an ionic level?
decrease calcium levels within vascular smooth muscle
What is active hyperemia?
increased blood flow to an organ due to an increase in metabolic activity of the organ
Which vasodilator metabolites are responsible for mediating active hyperemia?
- adenosine
- potassium
- lactic acid
- carbon dioxide
Give an example of active hyperemia?
exercise causes active hyperemia in skeletal muscle
What is reactive hyperemia?
a phenomenon where blood flow is transiently increased following a brief period of total ischemia
What is a good example of reactive hyperemia?
use of a blood pressure cuff
Describe whats happening during reactive hyperemia due to a blood pressure cuff.
- cuff is inflated and blood flow is cut off
- vasodilator metabolites start to form and dilate the vessels and there is a myogenic response
- when cuff is released, blood flow is immediately higher than it was before the cuff went on
- then it gradually drops back to normal
How much must arterial blood partial pressure of oxygen drop before cerebral blood flow increases?
partial pressure must drop roughly 50%
Why does it take a 50% drop in arterial blood partial pressure of oxygen before cerebral blood flow increases?
due to the nature of hemoglobin, oxygen content isn’t really reduced until the partial pressure of oxygen is cut in half
What is hypercapnia?
increased arterial partial pressure of carbon dioxide
When carbon dioxide levels in the brain drop, what happens to arterioles?
they constrict in an effort to build up CO2 and maintain a stable pH
Why does the brain attempt to maintain a constant pCO2?
because it needs to maintain a stable pH
Oxygen delivery to the heart is mainly controlled by changes in what factor?
rate of blood flow with little change in oxygen extraction
MIs are most common where in the heart muscle?
LV subendocardium
Why are MIs more common in the subendocardium?
because during systole, vessels in the left ventricle are compressed due to the high pressure developed within the ventricle and this decreases coronary blood flow
Left coronary blood flow mainly occurs when during the cardiac cycle?
ventricular diastole
What are the four determinants of myocardial oxygen demand?
- inotropic state
- intraventricular pressure
- heart rate
- end diastolic volume
What is the most costly determinant of cardiac oxygen consumption?
inotropic state because a 100% increase in inotropic state increases oxygen consumption by 200%
What factor is capable of increasing cardiac output without significantly increasing the myocardial oxygen demand?
increasing end diastolic volume
Decrease in tissue oxygen levels within the lung cause what arteriolar change?
vasoconstriction so that blood flows to the better oxygenated areas of tissue
If hypoxia occurs throughout the lungs, this causes what problem?
widespread pulmonary arteriolar constriction, increasing pulmonary resistance and elevating pulmonary artery pressure causing right ventricular hypertorphy
What is the role of portal circulation?
transoprt of blood from one capillary bed in a splanchic organ to another capillary bed in the liver
The liver receives a blood supply from what two sources?
the heart and from splanchic organs via the portal vein
Edema can arise in splanchnic organs due to what?
portal hypertension which increases venous pressure in splanchnic veins
What is a major cause of portal hypertension?
cirrhosis
