Lecture 24 Flashcards
What is your blood flow determined by?
Your metabolic needs of your tissue. You exercise thus increase your metabolic needs and thus increase cardiac output. We very carefully need to regulate the blood flow to the organs.
Describe the pressure in the small arteries/arterioles?
The greatest change in resistance thus results in a big pressure drop. The drop in pressure occurs of the large resistance vessels, so we have an inverse relationship between velocity and total cross-sectional area. Flow is constant throughout the circuit, but due to the change in the cross-sectional area the velocity changes. As we move down from the aorta, but due to the big increase in surface area there is a drop in velocity. The flow through your organs is the same through your heart.
Describe Ohm’s law and the circulation?
Flow is the change in pressure over resistance (hydraulic resistance). This is the difference between MAP - Central Venous pressure (normally this is zero). In a 70kg male blood flow is 5L/min at rest - SV x HR (70ml x 70bpmin) so it is 4.9L/min. In heart failure there are higher pressures.
Describe resistance?
The CVS is arranged almost in a parallel arrangement. Resistance in one organ changing without the other. Any change in resistance in one organ will have small affect on total peripheral resistance. Blood flow quickly divides up and goes to the relative organs. CO is 7% of body weight. If we increase the resistance to one vessel, it will reduce the flow in the organ but it won’t have much effect on total flow - as blood will go down through the other organs (i.e. increase their flow to compensate).
Describe Poiseuille law?
Total flow is dependent on the pressure gradient, pi, radius to the power of four (the bigger the diameter of the tube the more flow you’ll have in it), viscosity of the blood and the length of the tube (doesn’t change on moment to moment basis). Resistance is proportional to: 1) the tube length and 2) the viscosity of the fluid AND resistance is inversely proportional to 3) the radius raised to the fourth power (r4). The smaller the radius the bigger the resistance as well as the higher the viscosity.
Describe the radius factor?
This is the biggest determinant of flow (or resistance in a blood vessel). Only need to decreases the radius of a blood vessel by 19% to double the pressure for blood to drive the same flow through. When trying to improve blood flow you should dilate the blood vessels (increase radius) - it is exponential.
Describe viscosity?
This is the friction between the different molecules in the fluid.
What are the determinants of blood viscosity?
- Temperature - cooler temperature will make the blood more thicker.
- Haematocrit - Normal is 45% (45% is RBC); if you increase the proportion of RBC your viscosity will become higher thus your blood flow will be slower. With anaemia; low RBC -> low viscosity so low blood flow.
- Shear rate (velocity) - the faster your blood is flowing there is a decrease in viscosity (as the RBC tend to go in the middle of the fluid and you have water against the surface of the blood cells). As you slow your flow rate the RBC are spread out so an increase in viscosity.
- Vessel diameter - in small capillaries (less than 0.3mm) the viscosity tends to decrease as it is harder for RBC to get through. In a side branch there is a lower haemtocrit.
Describe distensible vessels?
Our vessels are distensible - if there is a high pressure they stretch; increase in the radius. If your pressure is low the veins will collapse and the flow has stopped (critical closing pressure).
Describe the contractile state of vascular smooth muscle?
There are smooth muscle cells that line the blood vessel as well as nerves and endothelial cells. So the blood vessels are exposed to mediators in the blood, nervous activity and what is excreted by the endothelial cells.
Describe shear stress?
As we get the blood flow moving past the endothelial cells we get the defamation of the endothelial cells and this will cause the release of nitric oxide. It quickly diffuses across and affects your smooth muscle cell (causes relaxation). As your blood flow increases, you will get relaxation of your smooth muscle cells, this will cause an increase in radius so this will help the blood flow to that region.
Describe turbulent flow?
High fluid densities, large tube diameters (hear turbulent flow in the large arteries i.e. carotid artery), high flow velocities (stenosis of the mitral valve, same volume has to get through the narrow valve so there is an increase in velocity of the blood - after the narrowing is the widening so then you will hear the turbulence), low fluid viscosities, abrupt variations in tube dimensions (branching), and irregularities in the tube walls. The flow is proportional to the square root of the pressure.
How can we predict turbulent flow?
Reynold’s number is an arbitrary number that we can use to predict whether there will be turbulence (>3000). Re is dependent on (directly proportional to) the density of the fluid, the mean velocity and the diameter; it is inversely proportional to the viscosity. We very rarely know the velocity of the flow, so they rearrange the formula to use cardiac output (Q - flow). An increase in diameter will increase reynolds number. The idea that a big diameter tube you are more likely to have turbulent flow.
Describe how you can get turbulent flow in stenosis?
After the narrowing, there is turbulent flow due to the high velocity in the narrowed part (stenosed part) - high velocity causes turbulence. This will cause murmurs.
Describe bernoulli’s principle?
Energy must always be conserved. Total energy is constant and is determined by the pressure energy, the potential gravitational energy and the kinetic energy (inertia - which is determined by velocity). If you get an increase in velocity so there is an increase in kinetic energy, because total energy must maintain the same, so the pressure in the narrow area must decrease. That is why after a stenosed part of the vessel/widening area, due to turbulent flow and pressure increase so the area is susceptible to blowing out - NO release, dilation of the vessel i.e. aortic aneurysm. This principle also helps the valves shut. There is a drop in pressure also due to the heat production.
