Control of capillary blood flow Flashcards
On the simplest level how is capillary flow regulated?
Arteriolar resistance
What are the 3 factors that regulate arteriolar resistance?
- Nerves
- Hormones and other vasoactive substances
- Local tissue metabolism
CO is proportional to what other measure?
Why is this?
VO2 (Volume of oxygen used per minute)
Blood flow is very well matched to metabolic demand
Why is VO2 used as a proxy for CO in studies of athletes?
It is proportional to CO
What equation is used to calculate flow in a capillary?
What is this simplified to?
Why can it be simplified?
Q = (PA - PV) / (Rpre + Rcap + Rpost)
Q α 1/Ra
Pressure gradient is fairly constant
Arteriolr resistance makes up 70% of total series and is the only directly regulated resistance
What is the purpose of LOCAL control of arteriolar resistance?
Matches local blood flow to local metabolici demand
What is the purpose of central autonomic contol of arteriolar resistance?
Controls TPR to maintain a constant ABP
What mechanisms of regulation are used in central control of arteriolar resistance?
Neuronal and endocrine
How is arteriolar smooth muscle arranged?
Radially
How is tension of vascular smooth muscle modulated?
Intracellular Ca2+ concentration
Phosphorylation of myosin light chain kinase
How is functional hyperaemia meausured?
Inflated cuff above arterial pressure around arm for 10 minutes
Cuff removed and an increase in blood flow through the arm is recorded
What metabolic factors promote vasodilatation of systemic arterioles?
Changes accompanying increased metabolism or normal metabolism with reduced blood flow:
-Reduced PO2
-Increased CO2
- Decreased pH
- Increased adenosine
- Increased extracellular K+
Why does a large increase in pressure only result in a small increase in flow to areas such as the brain, heart and kidney?
Resistance increases as vessels vasoconstrict
What are the direct and indirect effects of an increased ABP on vasoconstriction?
DIRECT:
vasoconstriction via the mygogenic mechanism (increased pressure = increased resistance to prevent rapid rise in flow)
INDIRECT:
increased perfusion washes out local metabolites responsible for vasodilatation
How was the role of the endothelium in regulating vasuclar responses first observed?
ACh could only dilate arteries when endothelium was intact
(whereas noradrenaline constricted them even if the endothelium had been removed)