Pressure and flow in the systemic circulation Flashcards
Where do you measure total peripheral resistance?
In the aorta
What determines mean arterial pressure?
Flow generated by the heart (cardic output)
Resistance generated by the arterioles (systemic vascular resistance aka total peripheral resistance)
What is pulse pressure?
The difference between the systolic and diastolic blood pressure
What is the clinical significance of pulse pressure?
Diastolic pressure is affected by the resistance in arterioles.
In conditions which cause vasodilation (e.g. sepsis) results in a drop in diastolic pressure, so the pulse pressure widens.
In haemorrhage, diastolic pressure rises so the pulse pressure narrows
What proportion of the cardiac cycle is spent in systole and diastole
Systole: 1/3 (fixed)
Diastole: 2/3 (decreases as heart rate increases)
Describe the structure of the veins
Thin walled
Highly compliant
Pressure determined by volume of blood contained
Contains 66% of blood volume
Factors controlling blood pressure
Heart rate
Stroke volume
Changes in compliance
Arteriolar resistance
Mean arterial pressure = CO x TPR
How is cardiac output affected by changes in heart rate?
CO - HR x SV
If heart rate is doubled and the stroke volume is unchanged, then cardiac output.
However if the heart rate increases, eventually the stroke volume will increase, meaning that cardiac output can increase by 4x.
How does distensibility affect pulsatile flow?
In blood vessels, 1/3 of the stroke volume leaves the arteries during systole. The remaining 2/3 stretches the artereis and they become distended. During diastole, the elastic arteries recoil which drives blood into the arterioles.
Pulsatile flow produces a square pulse pressure (on-off) as the valve is opened and closed repeatedly. In a rigid tube, the output flow mimics this pulsatile flow.
In distensile (elastic) tubes, some of the force from the pulsation is absorbed, which means that the elastic stretches to accommodate changes in volume. This converts the pulsatile flow into more of a constant sigmoid flow. There is always a low level out output.
Explain the changes between in pressure, flow and velocity between the left ventricle and the right atrium
Flow of blood is dependent on a pressure gradient. If there is no pressure gradient, there is no flow.
In blood leaving the left ventricle via the aorta, blood leaves at a high pressure. The aorta is a large muscular vessel and has a low cross sectional area so velocity is high. As the blood passes from large arteries to small arteries the vessel diameter is reduced and pressure begins to decrease, which reduces the velocity.
As blood passes from small arteries to arterioles, there is a large drop in pressure due to the large incerase in cross-sectional area. Velocity is reduced.
Lowest blood flow velocity is through the capillaries, which allows more time for exchange of material.
More blood ls found in the veins because they are more compliant. Venous valves produce a flow velocity which transports blood back to the heart.
Describe the changes in pressure in the systemic circulation
Pressure = flow x resistance
The total blood flow through the systemic blood vessels is equal to the cardiac output. Changes in pressure in the systemic circulation is proportional to the resistance of the vessel.
The pressure in the aorta and large arteries is high and pulsatile, and there is only a small drop in pressure as blood passes between them, as blood reaches the small arteries the pulse pressure decreases slightly.
The largest drop in pressure is within the arterioles. These have the largest contribution to peripheral resistance.
Pressure continues to decrease in the capillaries and veins.
What is vessel compliance?
Describes the flexibility of the vessels in response to pressure.
The greateer the compliance, the more flexible the vessel is. Vessels which are rigid are less compliant.
Compliance decreases with age
What is vasomotor tone?
The degree of tension of the smooth muscle within the walls of blood vessels, particularly in the arterial tree, which is controlled by the sympathetic nervous system.
(Resting level of contraction)
Name 4 metabolites which cause vasodilation
K+
H+
adenosine
CO2
hypoxia in the systemic circulation
Cause vascular smooth muscle relaxaion - vasodilation
How does metabolism affect local blood flow?
Increased metabolism increases the concentration of vasodilator metabolites. These cause smooth muscle relaxation and vasodilation, overriding vasomotor tone from the sympathetic nervous system.
Vasodilation causes increased supply pressure - thus increasing blood flow. This removes the vasodilator metabolites (reduces concentration). Reduced metabolite concentration restores vasomotor tone causing vasoconstriction.
If there is less metabolism, there are less metabolites, and therefore less flow.
This resposnse allows blood to be directed to tissues that are more metabolically active.
Describe how transmural pressure is controlled
Transmural pressure is the pressure between two sides of the vessel (or organ) wall.
It can be controlled by altering the volume of the vessel (vasoconstriction or vasodilation) - compliance
Describe how hormones affect vasomotor tone
Adrenaline and noradrenaline released from sympathetic nerves bind to a1-adrenergic receptors on smooth muscle in the arteriole and cause vasoconstriction, binding to b2-receptors causes vasodilation
Angiontensin II produced by ACE enzyme causes vasoconstriction which increases peripheral resistance
ANP released in response to atrial stretch causes vasodilation by regulating Na+ balance and blood volume
What is myogenic contraction?
Response of blood vessels to changes in blood pressure to keep the blood flow within the blood vessel constant.
Passive stretching of arterioles by a sudden rise in bp stimulates contraction of smooth muscle cells leading to vasoconstriction (increased resistance to flow).
The smooth muscle of the blood vessels reacts to the stretching of the muscle by opening Ca2+ channels, which cause the muscle to depolarize, leading to muscle contraction. This reduces the volume of blood able to pass through the lumen, reducing blood flow through the blood vessel.
Reduction in pressure favours relacation of the smooth muscle causing vasodilation and decreased resistance to flow
Describe the effects of chemicals released by endothelial cells on blood vessels
Endothelial cells secrete chemicals which both vasoconstrict and vasodilate blood vessels back acting on smooth muscle cells.
Nitric oxide is a vasodilator
Endothelin I is a vasoconstrictor.
How do you calculate mean arterial pressue?
Why is it important?
diastolic pressure + 1/3 of the pulse pressure
The mean arterial pressure is the average pressure driving the blood into the tissues over the entire cardiac cycle.
Describe the neural control of vasomotor tone
Vasomotor tone is an autonomic reflex regulated from the medulla oblongata.
The vasomotor centre activates sympathetic nerves which can raise blood pressure by releasing NorA.
Acts on arterioles causing smooth muscle contraction which increases resistance to flow. As a result pressure in the capilaries is reduced so there is more reabsorption of fluid into the circulation
This binds to b-adrenergic receptors in the heart to stimulate the heart rate and contractility, increasing cardiac output
Explain the pattern of the systemic ventricular and arterial waveforms
The systemic arterial pressure waveform results from ejection of blood from the left ventricle into the aorta during systole
Systolic peak, dicrotic notch as aortic valve closes, diastolic drop
What is autoregulation of blood flow?
Where blood flow is adjusted to the existing metabolic activity in the tissues.
Metabolic activity releases vasodilator metabolites into the tissue to increase O2 delivery from the blood. An increase in blood flow decreases the the tissue concentration of metabolites.
What determines the pressure in veins?
Volume of blood contained within them
Veins are thin walled and highly compliant
Name four factors that affect regional blood flow
Vasomotor tone
Local metabolites
Transmural pressure
Myogenic contraction
Vasoactive agents
Hormones
Autonomic nervous activity
Why is autoregulation important?
Local blood flow control mechanisms help keep blood flow constant over a wide range of arterial pressure. Autoregulation ensures that critical organs (kidneys, brain) have an adequate blood flow and oxygen delivery.
A change in mean arterial pressure, e.g. hypovolemia or circulatory shock, In athersclerosis an increase in pressure decreases flow, this causes the vessels to dilate.
At extremes of pressure autoregulation breaks down.
What is perfusion pressure?
The pressure gradient required to perfuse the tissue with blood (equal to the difference between the arterial and venous pressure)
Name four factors that affect venous return
Muscle contraction. Contraction of limb muscles during activity promotes venous return
Decreased venous compliance. Sympathetic activation of veins decreases venous compliance, increases central venous pressure and promotes venous return
Respiratory activity. venous return increases because of a decrease in right atrial pressure.
Vena cava compression. An increase in the resistance of the vena cava decreases return . e.g. thoracic vena cava becomes compressed during a Valsalva maneuver or during late pregnancy Gravity. decreases venous return
What is the value of mean arterial pressure
93 mmHg
Diastolic pressure + 1/3 pulse pressure
