Cardiovascular mechanics 3 Flashcards
What is the point of circulation?
Circulation is designed to transport blood around the body and to regulate temperature.
How do materials move out of blood vessels?
Diffusion (out of the capillaries).
Diffusion is only effective over short distances so the capillary needs to be around 10 micrometers from each cell. This necessitates a highly branched structure.
Describe the 2 circuits of the circulatory system
Each circuit has its own pump (left and right ventricles). They are coupled together.
Pulmonary circulation- right ventricle sending blood to the lungs so that the blood returning from the body can be oxygenated.
Systemic circulation- pumps blood to the rest of the body (through the aorta).
The heart is a muscular pump which generates a pressure gradient that propels blood through a network of tubes (blood vessels)
What are the properties of the ALL blood vessels?
Arteries- muscular lots of elastic fibres for recoil and smooth muscle. Smooth muscle.
Arterioles- smaller than arteries in diameter. Responsible for decreasing pressure of the blood. Have smooth muscle to regulate diameter side
Capillaries- one cell thick endothelium. LARGEST total surface area.
Venules- thin walls, smaller SA than capillaries, blood at low pressure larger diameter
Veins- BIG diameter can hold a lot of blood at low pressure. Small total surface area.
Largest proportion/ reservoir of blood is held in the veins and venules. This is so that they can push the blood back to the heart in times of stress and exercise. They also have valves to prevent backflow of blood.
What is blood pressure?
It is the force that drives circulation
What is the importance of pressure difference?
The pressure difference between 2 locations drives the direction of flow. You can therefore alter the profusion of blood to a particular capillary bed depending on the demands required of your body. (E.g exercise- diversion of blood to your muscles).
What is resistance in terms of the circulatory system
Resistance is impedence to blood flow.
In a circuit (V=Ir)
So in a fluid circuit, we can say that the:
Pressure difference= volumetric flow x resistance
This is known as Darcy’s law
Pressure difference between the arteries and veins
The pressure falls across the circulation due to viscous (frictional) pressure losses (against the vessel walls).
The biggest drop is in the arterioles- this is important because blood under high pressure would damage the capillaries.
Small arteris and arteriole therefore have the most resistance to flow.
There is virtually no BP when the blood returns to the RA. The mean bp remains relatively constant.
With Darcy’s Law, we can determine the blood pressure (mean arterial pressure)- what is the equation?
Blood Pressure (MAP) = Cardiac Output x Resistance (PVR)
This is an approximation because it assumes:
- Steady flow
- Rigid vessels
- Right atrial pressure is negligible
The regulation of flow is achieved by variation in resistance in the vessels whil blood pressure remains relatively constant.
What factors affect resistance to blood flow?
- Fluid viscosity
- The length of the tube (L)
- Inner radius of the tube (r)
How do you work out resistance to blood flow?
Poiseuille’s Equation shows that when you change the radius, you change the flow that is flowing thorugh it. Realistically, the viscosity of the blood and the length of the blood vessel will not change.
Since radius is on the bottom, small changes in radius (vascular tone) will cause a large change in flow. If you were to half the radius, the resistance would increase 16 fold.
Since all of the other variables are constant, you can say that:
Resistance = 1/r^4
Changing resistance can change blood flow to different organs (changing the pressure difference)
At rest, our cardiac output is 5L per minute
However, when we are exercising, the cardiac output is 20L/min with 16L diverted to the muscles
What are the 2 different blood flow profiles?
1) LAMINAR FLOW- healthy blood flow and velocity is constant at any one point and flows in layers. The fastest blood flow is in the centre of the vessel, the flow close to the walls of the vessel is slower because of frictional resistance.
2) TURBULENT FLOW- not so healthy- they can form eddy currents and blood flows erratically. Prone to pooling in some areas. Associated with pathopathological changes to endothelial lining.
What is shear rate?
The gradient of velocity profiles in one vessel at one point
What is shear stress?
Shear rate x viscosity
Shear stress governs how well the endothelium cells of the vessel work. Endothelium release molecules the govern laminar flow which are important.
Describe the 2 different types of shear stress
1) High shear stress (laminar shear stress) - good laminar flow and cells align themselves with the direction of flow. It promotes endothelial cell survival and quiescence. Secretions from the endothelium promote vasodilation and anticoagulation.
2) Low shear stress (disturbed shear stress)- Promotes endothelium proliferation, apoptosis and cell change. Secretions promote vasoconstriction, coagulation and platelet aggregation.
How to measure and calculate blood pressure?
Measured on the upper arm and is accessed at the at heart level.
Slow deflation of cuff causes turbulent flow which can be heard with a stethoscope.
You need the pressure of the cuff to be similar to the pressure of the artery.
When vessel is open, you cannot hear it cos its laminar flow and this is diastolic pressure.
The mean arterial pressure isn’t the mean between systolic and diastolic
1/3 in ejection (systolic) and 2/3 relaxation (diastolic) so need to adjust- use the equation above.
What is the windkessel effect?
Pulsatile pushing of the blood is sustained due to the expansion of the aorta- not just spurts its constanly releasing blood and maintaining diastolic pressure
If the compliance of the aorta begins to decrease (i.e. with age), the windkessel effect is reduced and pulse pressure increases.
The pressure effect on vessels
The pressure gradient will determine the direction of flow of blood.
Transmural pressure causes a tension force (T) in the wall that can be described by the LaPlace law. (i.e. due to the pressure inside, radius and thickeness of the vessel too).
It is a circumferential stress pushing out in all directions.
Constant high circumferential stress causes vessel distension.
What are aneurysms?
If vessels weakes, balloon like extensions can form.
Vascular aneurysms can increase the radius of the vessel. Therefore the tension pushing out increases. If the vessel has the same thickness, the circumferential stress (the inward force) must also increase.
If the muscle fibres are also weak, the force needed cannot be produced and so the aneurysm will continue to expand until it ruptures.
This is seen in the gut- Diverticulitis- pockets forming in the get wall getting bigger and bigger over time until they burst.
How does compliance vary between arteries and the veins?
Veins are very compliant and can dilate a lot. Can store a lot of blood.
Compliance depends on vessel elasticity. Venous compliance is 10 to 20 times great than arterial compliance. So the less elastic, the more compliant.
You can change the amount of blood volume that the veins hold by increasing smooth muscle contraction, decreasing the radius and decreasing the volume it holds. Small changes in pressure will distend the veins. Increasing smooth muscle will decrease compliance
The importance of gavity in all this :)
Standing up will increase hydrostatic pressure in legs due to gravity. Blood can pool in veins due to their high compliance, and this would reduce caridac output and blood pressure. This is why you can feel light headed if you get up really quickly after being sat after a long time.
Muscle and respiratory pumps
The pumps push blood back through the veins up to the heart.
The skeletal muscle will pump more blood back to the heart.
The respiratory pump also allows blood to the heart. Diaphragm drop, negative thoracic pressure.
