Make your own free website on Tripod.com

Physiology II
Cardiovascular Physiology

Hemodynamics

Key words: 

Systemic or peripheral circulation: The function of the circulation is to service the needs of the tissues. It transports nutrients to the tissues, transports waste products away, and conducts hormones from one part of the body to another, and in general maintains an appropriate environment in all the tissue fluids for optimal survival and function of the cells. The peripheral circulation supplies all the tissues of the body except the lungs with blood flow.

Large elastic arteries: The arteries transport blood under high pressure to the tissues. They have strong vascular walls and blood flows rapidly through them.

Small muscular arterioles: These are the last small branches of the arterial system and they act as control valves through which blood is released into the capillaries. They have a strong muscular wall that is capable of closing the arteriole completely or allowing it to be dilated several fold, thus having the capability of vastly altering blood flow to the capillaries in response to the needs of the tissues.

Capillaries: They function of capillaries is to exchange fluid, nutrients, electrolytes, hormones, and other substances between the blood and the interstitial fluid. For this role, the capillary walls are very thin and permeable to small molecular substances.

Venules: These collect blood from the capillaries and they gradually coalesce into progressively larger veins.

Veins: The veins function as conduits for transport of blood from the tissues back to the heart, but equally important, they serve as a major reservoir of blood. Because the pressure in the venous system is very low, the venous walls are thin. Even so, they are muscular, and this allows then to contract or expand and thereby act as a controllable reservoir for extra blood, either a small or a large amount, depending on the needs of the body.

Intravascular pressure: Blood pressure is the force exerted by the blood against any unit area of the vessel wall. It is usually measured in mmHg although one mmHg = 1.36 cm H2O. The tissues must have flow through them to satisfy their metabolic needs. The blood vessel system is a tubular one and you need to have a pressure difference through the system in order to have flow. There needs to be a pressure gradient for the heart to generate flow and it must contribute to the maintenance of the pressure gradient. Sources of pressure in the blood vessels are volume and gravity. Volume deals with the stretching of a container with filling. The stiffness of the container determines the amount of pressure inside for that volume. This is also termed compliance of walls and the volume in it. If the walls are stiff, the pressure inside will go up. If the volume decreases the pressure will fall and vice versa. The lymphatics and fluids in and out comprise the blood volume. Gravity is discussed below.

Transmural pressure: This is another source of pressure for the blood vessels. This refers to the wall tension of the vessels or of the heart. The myocardial cells and the vascular smooth muscles are contractile units that can shorten for contraction. A pressure is exerted by squeezing the contents within. This is the wall tension. LaPlace’s theory comes in to play here. He found that spherical or cylindrical objects that have pressure in them also have a tension in their walls that oppose this pressure. This tension keeps the walls together and if pressure inside is too high the walls will split. This is the basis for presuming that the length tension curve has a relationship of tension to pressure. In the heart the contraction generates a tension in its wall and is translated into an increase in pressure inside the chambers. In veins the contraction squeezes down on the vessel and increases the pressure.

Velocity: The speed with which the blood flows through the vessels.

Flow: Blood flow means the quantity of blood that passes a given point in the circulation in a given period. The overall blood flow in the circulation is called the cardiac output, which is the amount of blood pumped by the heart in a unit period.

Hydrostatic pressure (pgh): This is the gravitational component to sources of pressure. Archimedes first discovered gravity. In a column of liquid the liquid weighs down upon itself so the pressure at the bottom of the liquid is higher than at the top. This pressure contribution from the weight of gravity is expressed in the equation – pgh where p (is actually the Greek letter roe) = density of blood, g = the gravitational constant or the density of gravity, and h = the height of the column. When a person stands up the pressure in the legs is much higher than that in the head due to the gravitational pressure exerted on the feet.

Kinetic Energy of flow (1/2 pv2): The total energy of flowing blood = the energy described as pressure from the heart and vessels and gravity and a kinetic energy component which is the blood flowing – the velocity of flow. Bernoulli’s Principle states that in areas of a vessel narrowing, the velocity increases and when the velocity increases the kinetic energy increase and pressure falls. When flow gets back to a large area the velocity and energy will fall and pressure will go back up. Kinetic energy is important during the peak of systolic ejection when a large stroke volume is pumped out of a small opening. Velocity of flow may be so high that kinetic energy is lost. Normally the vessels grade down gradually so kinetic energy isn’t a factor except at areas of acute opening or enlargement. This concept holds important for the insertion of arterial catheters that have a blunt tip with the opening on the side so as to avoid reading any kinetic energy pressure and reading only true arterial flow pressure.

Poiseuille’s Law: He determined that a container filled with a fluid of some kind will have a varied flow rate of the fluid dripping out of the bottom if the length of the tube from the bottom changed or if the radius of this tube changed or if he used things more viscous than others or changed the height of the column (pressure gradient). The most important and powerful factor was the radius so that if you doubled the radius you would increase the flow by 16 times. His equation is the following:

Flow = Delta P x pie x r4 or R = 8ln

8 x l x n pie r4

l = length and you can’t change this of a limb or organ so can’t use this to regulate the resistance. N is the viscosity coefficient and you can’t change this to affect the resistance although changes in hematocrit will affect resistance. Thus r= radius and changes in this are the body’s primary way of changing the resistance. Contraction of the smooth muscular walls will change the radius.

Viscosity: This is the thickness of the blood or the stickiness of it. It is a source of resistance because if the Hct is high the blood will flow slower and with more friction which will increase cardiac work. Hematocrit is the % of blood made of RBC’s, which determined by the O2 transport means of a person. Since red cells take several days to be made and last for several months, it is not a means of changing the body’s resistance quickly like the radius is.

Resistance: This is the impediment to blood flow in a vessel. Sources of resistance are the geometry of the vessels – the physical properties of the vessels such as radius and length, bifurcations, obstructions, valves and the viscosity of the blood.

Conductance: This is the measure of the blood flow through a vessel for a given pressure difference. Very slight changes in the diameter of the vessel cause tremendous changes in its ability to conduct blood when the blood flow is streamline.

Total peripheral resistance: This is the measurement of all the resistance in the periphery added together – all the pressure differences from the systemic arteries to the systemic veins.

Laminar flow: Laminar flow is streamlined, smooth, and quiet. This is the typical pattern of flow in the blood vessels. Flow here usually flows in the shape of a parabolic curve with the fastest velocity in the center of the tube and slowest velocity along the edges due to the friction of the blood with the sides of the walls.

Turbulent flow: Flow that is turbulent is characterized by eddies and currents and is no longer smooth and streamlined. It is very noisy due to the vibrations of the currents hitting the walls of the vessels.

Reynold’s number (Re): This is the number that velocity of flow must exceed so that laminar flow changes to turbulent flow. Reynolds worked with the London sewer system and explored the flow of sludgy fluids. The physical conditions of density and viscosity contribute to the flow patterns. Velocity is the most important though. If flow is forced through a narrow opening, it can exceed critical flow rate and change from laminar to turbulent flow.

Critical velocity: If this is exceeded then flow converts from laminar to turbulent.

Murmur: If critical velocity is exceeded and the flow converts to turbulent through a narrowed opening then this turbulent flow bumps against the walls and you can hear a murmur. If you heard this over a stenotic artery it would be called a bruit.

Hematocrit: As described above – the % of RBC’s in the blood.

Ery-throcyte deformability:

Rheology: The science of the deformation and flow of matter, such as the flow of blood through the heart and blood vessels

Learning Objectives:

Write a general equation relating flow, pressure gradient, and resistance, and apply this equation to the relationships among cardiac output, total peripheral resistance, mean systemic arterial pressure, and right atrial (central venous) pressure: Delta P = MSAP-CVP and Delta P/Flow = R. So flow or cardiac output = the pressure changes from one end to the other MSAP – CVP divided by the TPR or resistance. There must be a pressure gradient in order to have flow and the pressure is lost as it travels through the system. The resistance is the total of the sources of energy lost through the system.

List and describe the three major components of the total fluid energy of flowing blood as described in class: Viscosity will affect the flow of blood. The more viscous it is the slower the flow.

Describe the effects of flow rate and vessel cross-sectional area on the linear velocity of flow within the circulation: Poiseuille found that the vessel cross-sectional area or the radius would have the greatest effect on the flow rate. If you doubled the radius, the flow rate will increase 16fold. Linear velocity can increase through narrowed openings thus increase the flow rate. Laminar flow will cause the blood to flow smoothly with the highest velocity being in the center of the parabolic curve. When the rate of flow becomes too great, when it passes an obstruction in a vessel, when it makes a sharp turn, or when it passes over a rough surface, the flow may become turbulent rather than streamline. Blood will flow with much greater resistance when flow is turbulent. The tendency for turbulent flow increases in direct proportion to the velocity of blood flow, in direct proportion to the diameter of the blood vessel, and inversely proportional to the viscosity of the blood.

Describe the contribution of hydrostatic pressure gradients (pressure gradients produced by gravitational forces acting on the blood) to the distribution of transmural pressures and blood volume in the circulatory system of a healthy human standing upright: The pressure in the legs of an upright person is greater than the pressure in the heart of the same person. The differences are due to the gravitational forces exerted on the legs. When the person stands up the leg veins swell and fill. This is the reservoir effect. Once the vessels are filled then this filling creates a wall tension or pressure on the walls of the vessels, which causes the pressure inside to elevate and begin squeezing the blood back up to the heart. The normal response to an upright posture is to stiffen the veins to reduce the pooling effect. If the person lacked the ability to change the passive properties of the veins then he would faint every time he stood up as the blood pooled in the veins and cause a transient lack or reduced blood flow until the blood could be returned to the heart.

Describe the physiological advantages in having the vascular beds supplying major organs arranged in parallel rather than in series: A parallel arrangement allows for individual differences in resistance in individual tissues or organs. Each tissue is exposed to the same pressure gradient, which is centrally controlled but each tissue, and organ must have its own set of flow requirements. The total flow requirements are a sum of flow through all organs and at the same time there is preservation of a pressure gradient for each organ to receive flow. In a series arrangement the resistance is added up as you go across the field. The last organ would face very high resistance and would have minimal flow – little O2 left and the pressure would be very low. To add another organ here would only increase the total resistance and would impair flow to that organ. If you added another organ on to the parallel model then the total resistance would fall. The analogy of a barrel filled with water was used. If there were several holes in the barrel, the water would leak out. If you poked one more hole in regardless of size then the water would leak out faster and the resistance would then be lower. Maintaining adequate pressure gradients will provide continuous flow. However, the body can not maintain maximal flow to all areas at the same time. CO would increase but it can only do so by a certain amount. During exercise typically flow is decreased to the splancnic, GI and renal systems and is increased to the skeletal muscles.

Write from memory an equation (Poiseuille’s equation) relating the flow rate through a vessel to pressure gradient, vessel radius, vessel length, and blood viscosity, and demonstrate an understanding of the physiological relationships among these factors: See the key words for the actual equation. Flow is directly proportional to the height of the column (pressure gradient) so that the higher the pressure, the faster the flow. Flow is inversely related to the length of the tube so that if you increase length you will decrease flow. Flow is directly proportional to the radius to the 4th power so that if you increase the radius you will tremendously increase flow. Flow is inversely proportional to the viscosity of the liquid so that if you increase viscosity you will decrease flow. The pie component of the equation is due to the fact that the vessels flow through a cylinder.

List and describe the factors that will determine the critical velocity at which flow will change from laminar to turbulent: When flow rate becomes too great, when it passes an obstruction in a vessel, when it makes a sharp turn, or when it passes over a rough surface, the flow may become turbulent rather than laminar. Flow moves with greater resistance when it is turbulent. When flow reaches Reynold’s number which is a reflection of the velocity, the viscosity, the diameter of the blood vessel and the density, the flow will become turbulent even if it occurs in a straight, smooth vessel.

Describe, using words or a simple graph, the change in blood viscosity that occurs as blood flow velocity increases, and relate this change to erythrocyte deformability. Blood viscosity is due mainly to the large number of suspended red cells in the blood, each of which exerts frictional drag against adjacent cells and against the wall of the blood vessel. This friction determines viscosity. Therefore, the viscosity of the blood increases drastically as the hematocrit increases. Another factor that affects blood viscosity is the concentration and types of proteins in the plasma. Blood viscosity in the microcirculation is affected by 3 additional factors. 1.Blood flow in minute tubes exhibit far less viscous effect than it does in large vessels. This is called the Fahraeus-Lindquist effect. In tubes as small as capillaries, the viscosity of whole blood is as little as one half that in large vessels. This effect is caused by the alignment of red cells as they pass through the vessels, which is to line up single file. 2.The viscosity of the blood increases tremendously as its velocity of flow decreases. Velocity in the small vessels is extremely slow and so the viscosity can increase as much as 10-fold from this factor alone. 3. Cells often become stuck at constrictions in small blood vessels; this happens especially in capillaries where the nuclei of endothelial cells protrude into the capillary lumen. Flow can be blocked for a fraction of a second or for longer periods, thus giving an apparent effect of greatly increased viscosity. I am assuming that a deformity in the red cell would further complicate this flow through the capillaries causing more red cells and nuclei to get caught up in the capillary lumens and cause or more likely cause obstructions to flow.


Return To The MNA 2001 Homepage