Lecture 12 - Control of Blood Pressure Flashcards Preview

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Flashcards in Lecture 12 - Control of Blood Pressure Deck (19):

Why does pressure in the blood vessel drop as it goes from

arteries to arterioles to capillaries?


Because as the number of branches increases, flow and therefore pressure is distributed between more and more parallel vessels



This does not mean that total or mean pressure changes; what changes is the pressure in individual vessels as the blood passes through arteries to arterioles to capillaries




What is the ultimate function of the circulatory system?


To ensure adequate blood flow through the capillaries of various organs.



Why does arterial pressure fluctuate between systole and diastole?


Expansion of elastic arteries during systole reduces systolic pressure

Elastic recoil during diastole generates diastolic pressure while maintaining flow and preventing vessel collapse




What is darcy's law ?



Mean Arterial Blood Pressure (MABP)= cardiac output(CO) x Total Peripheral Resistance (TPR)




Remember: Cardiac output is the same 120 from both left and right ventricles, so total flow must be the same in the systemic and pulmonary circulations 

But pressures are very different because TPR is very different: Much lower in pulmonary vs. systemic 


Put another way:
Contraction of the heart provides the driving force behind blood flow (cardiac output)

So, control of cardiac output regulates total blood flow through the body

However, TPR (and therefore MABP) is regulated by the vascular system 



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What is the driving force behind blood flow (CO)?


Contraction of the heart

So, control of cardiac output regulates total blood flow through the body



What is TPR regulated by?


The vascular system - especially the arterioles



Function of arterioles? (2)


Arterioles are the resistance vessels of the cardiovascular system

This gives them 2 important functions:

1)  Main site of control of total peripheral resistance (TPR) and therefore a major determinant (together with cardiac output) of mean arterial blood pressure (MABP)

2)  Site of control of relative blood flow through different vascular beds at any given MABP


On the one hand
MABP must be maintained within normal limits in order to ensure adequate perfusion of all parts of the body: 

Not too high risking tissue damage or vessel rupture

Not too low risking vessels collapse 


On the other hand
regional changes in metabolic demand require regional changes in blood flow to meet that demand: (L13) 




How do arterioles alter TPR and MABP?


1) They are at the point on the pressure graph where changes in diameter relate to the biggest changes in

pressure. Since R is proportional to r4, very small changes in radius will cause relatively large changes in vascular resistance

2) Arterioles can actively alter their radius through constriction

or dilation of the smooth muscle layer




Laplace law?


Tube radius (r) = Wall tension (T) / Pressure in the tube (P)


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Vasoconstriction stimulators?


noradrenaline, angiotensin II, endothelin and others



Vasodilator stimulators?


Adrenaline  (depending  on  vascular  bed),  acetylcholine, histamine, nitric oxide and others




Why is blood flow velocity important?


Low capillary flow velocity maximizes time available for exchange of substances between blood and interstitial fluid



Will vasoconstriction increase or decrease MABP and why?


It will increase pressure.

Smooth muscle contraction causes increased wall tension - this results in reduced vessel radius and increased pressure until tension once again balances pressure



Mean arterial blood pressure (MABP) ??

= diastolic pressure + pulse pressure/3 


Typically at rest, diastolic = 80 mmHg, Systolic = 120 mmHg, measured from any large artery, typically the brachial artery, using a sphygmomanometer: 


giving MABP = 80 +(120-80)/3 = ~93 mmHg 


Reducing radius will decrease pressure



Reducing radius increases pressure !!



Mechanism of contraction/relaxation ??


1 ) Smooth muscle contraction is induced by an increase in intracellular Ca2+ ([Ca2+]i)

[Ca2+]i is increased primarily via activation of phospholipase C, which triggers both release of stored Ca2+ and influx from outside the cell

Ca2+ enters via non-selective cation channels, which cause depolarisation, thus also opening voltage-gated Ca2+ channels

Depolarisation may spread through gap junctions to other muscle cells more distant from the source of the original stimulus

2) Smooth muscle relaxation is induced by an decrease in intracellular Ca2+ ([Ca2+]i)

[Ca2+]i  is decreased primarily via opening of K+  channels, which causes hyperpolarisation. Also via production of cAMP and cGMP

Hyperpolarisation  closes  voltage-gated  Ca2+    channels,  thus  reducing [Ca2+]i





What is the microcirculation?


The collective term for the arterioles, capillaries and venules



Difference between systolic and diastolic pressure ?

pulse pressure