Flashcards in CVS- peripheral circulation Deck (45)
What is the function of arteries?
Gross conduction and distribution of blood supply
What is the function of arterioles?
Local distribution and fine control to defined tissue volume.
What is the function of capillaries?
Micro-diffusion and filtration.
What is the function of veins?
Collection, return and capacitance (reservoir).
What is compliance?
Ability of vessels to distend and increase volume due to pressure increase.
What is capacitance?
Effectively the same as compliance. It is a measure of relative volume increase per unit increase in pressure.
In which component of the CVS is the most blood found by volume at rest?
What is cardiac output?
Stroke volume x heart rate
How much is the normal cardiac output?
What is total peripheral resistance?
The sum of all arteriolar resistance.
What is the effect of compliance on the pulsatile pressure changes of systole and diastole?
Compliance acts to store mechanical energy or rising pressure wave in systole (by distension of elastic walls) and dissipates it more gradually over diastole )by recoiling of elastic walls). It therefore 'smooths out' the pressure wave.
Which arteries have the most compliance?
The aorta and elastic arteries with less smooth muscle/more elastin.
What is the Windkessel effect?
A term used to account for the shape of the arterial blood pressure waveform in terms of the interaction between the stroke volume and the compliance of the aorta and large elastic arteries (Windkessel vessels).
What is the systolic pressure?
The maximum major arterial pressure that occurs in systole.
What is diastolic pressure?
The minimum major arterial pressure that occurs in diastole.
Which vessels can be palpated for a pulse?
Arteries and large arterioles.
What factors affect systolic and diastolic pressure?
1. Cardiac output (SVxHR)
2. Arterial compliance (no SNS input to elastic arteries)
3. TPR (increase in TPR -> increase in arterial pressure)
What is the effect of ateriosclerosis in the elderly?
It decreases the compliance of arteries and therefore increases blood pressure, causing hypertension.
What is the pulse pressure?
Systolic pressure - diastolic pressure
How would you calculate average pressure?
Diastolic pressure + 1/3 of pulse pressure
How much time on average is spent in diastole compared to systole?
0.55s compared to 0.3s
What causes the decrease in pulse pressure?
Summated resistance and capacitance of arteriolar network.
What happens if there is not enough pressure for blood to flow through the capillaries?
List the resistance vessels of the circulatory system:
Arterioles and pre-capillary sphincters
How do arterioles and pre-capillary sphincters create high resistance?
They narrow the lumen by contraction of the proportionally large amount of smooth muscle in their tunica media.
What governs the blood flow to capillary beds?
Arteriolar vasomotor tone
What two opposing control elements work together to finely regulate vasomotor tone, to very small tissue volumes?
Regulation of vasoconstriction and vasodilatation.
Why is it important to regulate blood flow to capillary beds?
In order to precisely match substrate supply to metabolic demand.
What factors govern the high arteriolar vasomotor tone at rest?
Mostly centrally controlled by autonomic SNS (circulating hormones modulate):
1. Release of NA acts on alpha-1-GPCRs -> [Ca2+]i contraction.
Why is there a high vasomotor tone at rest?
Becuase there is only a modest resource demand and therefore only a low blood flow is needed. There is no need to employ large functional reserve.
What is the difference between vasodilation and vasodilatation?
Vasodilation is when there is no tonic constriction whereas vasodilatation is when there is an offset/reduction of vasoconstriction, in the presence of ongoing casoconstrictive signals.
What substances cause vasodilatation?
Local vasodilator metabolites, produced by metabolically active tissues, such as:
H+, CO2, K+, Adenosine, lactate
What is the time frame of local metabolite vasodilators?
Acute - they are released locally by actively metabolising tissue and then the increased blood supply (from vasodilation) removes these factors and there is a gradual return to vasomotor tone dominated by the SNS.
What happens to the sympathetic vasoconstrictive signals to arterioles during vasodilatation governed by local metabolite vasodilators?
It remains unchanged, therefore when the local metabolites are washed away by blood flow, the sympathetic vasomotor tone returns to its basal levels.
Reactive hyperemia occurs when blood flow to a tissue has been cut off for a short period of time and is then returned. What happens in reactive hyperemia?
The hyperemia occurs because during the period of occlusion, tissue hypoxia and a build up of vasodilator metabolites (e.g., adenosine) dilate arterioles and decrease vascular resistance. Then when perfusion pressure is restored (i.e., occlusion released), flow becomes elevated because of the reduced vascular resistance. During the hyperemia, the tissue becomes reoxygenated and vasodilator metabolites are washed out of the tissue. This causes the resistance vessels to regain their normal vascular tone, thereby returning flow to control.
Apart from the ANS sympathetic contribution and local metbolite vasodilators, what other factors can affect vasomotor tone?
1. Hormonal - systemic contribution (e.g. adrenaline)
2. Myogenic - local
3. Endothelial factors - local
How can myogenic factors affect local vasomotor tone?
Arteriolar smooth muscle can contract acutely to protect against rapid intraluminal pressure rises (pressure spikes (>200mmHg))e.g. caused by violent coughing.
How do endothelial factors affect local vasomotor tone?
Factors released from arteriolar endothelium (autacoids) can modulate (+/-) vasomotor tone.
What is the relationship between total peripheral resistance and blood flow demand?
TPR is inversely proportional to the total body demand for blood flow. As demand goes up, TPR decreases.
How high is vein compliance?
Very high - 10%+ increase in volume/mmHg.
What determines vein pressure?
The volume of blood they contain. This depends on the balance between flow in from the body and flow out via the heart. This is affected by:
1. Cardiac output
2. Body metabolic demand
3. 'Back pressure' from pressure waves
What is central venous pressure?
is the pressure of blood in the thoracic vena cava (great veins), near the right atrium of the heart. CVP reflects the amount of blood returning to the heart and the ability of the heart to pump the blood into the arterial system.
What is the range of central venous pressure?
-10 to +10mmHg
What determines the filling of the right atrium during diastole?
CVP - there are no valves between the veins and atrium.