Flashcards in 11/20/2014 Medical Physiology: Hemodynamics Barry Knox Deck (52):
Write flow equation
Q = A x v
Write Ohm's Law of Fluids
Q = change in P/R
Why can blood flow through an individual organ be considered to be flow through a series network?
- consecutive vascular network directs blood in the organ from arteries to veins
- there can be dramatic differences in resistance in different organs --> allows changes in control of blood flow even though the mean arterial blood pressure is relatively constant
Who determines the overall resistance of any organ and why?
Arterioles resistance (controlled by arteriolar radius) --> they have the largest resistance compared to other vessels
Cite a major control point in cardiovascular physiology
Contraction or relaxation of the smooth muscles in the arteriolar wall
Define total peripheral resistance (TPR)
Overall resistance to flow through the circulation
In what kind of network are organs arranged?
*the resistance of each organ contributes to the TPR according to the inverse relationship
Why does any pressure difference comes only from frictional losses from circulation?
Because the aortic valve and the right atrium are at the same height
What is total flow equal to (approximately, if we talk about the entire circulatory system)?
*assuming central venous pressure = 0
What is mean arterial pressure equal to?
CO (cardiac output) X TPR (total peripheral resistance)
What is the normal value of TPR at rest?
What does the slope of P vs. Q graph represent?
How does R behave in exercise and hypertension compared to rest?
R (hypertension) > R (rest) > R (exercise)
A measure of intermolecular attractions in the liquid, which determines the steepness of the velocity gradient
*different from density
Also called specific gravity, and which is defined as the mass per unit volume (g/ml)
Differentiate between laminar and turbulent flow
*Laminar (silent) flow --> flow is proportional to ∆P --> breaks down when velocity reaches a critical point --> Re flow above critical point of velocity --> effective resistance increases --> causes significant losses of kinetic energy --> Q proportional to the square root of ∆P --> Re>3000
*For the same ∆P, there is less flow when flow is turbulent than when laminar
What is the Reynolds number?
Critical parameter that characterizes flow (Re)=disruptive forces/cohesive forces
disruptive forces=2rvp (radius,velocity,density)
cohesive forces=n (viscosity)
What are auscultatory sounds?
The surfaces of constant velocity (spirals, swirls, and eddies) heard in turbulent flow with a stethoscope
Define murmurs (bruits)
Audible sounds due to vibrations in heart or vessel walls; not usually heard under resting conditions
Define "innocent" systolic murmurs
Occur when cardiac output increases during exercise, causing turbulent aortic flow during systole
What are Korotkoff sounds of sphygmomanometry?
Also examples of the sound of turbulent flow (blood pressure measurement with inflatable cuff)
What is another cause of murmurs?
Aortic or mitral stenosis (constriction of vessels)
What is aortic regurgitation and does it generate?
Back flow into the heart caused by aortic valve defect that generates turbulence and characteristic sounds
What does turbulence do to the arterial wall and what are the complications of this?
- arterial wall may be damaged by turbulence
- development of thrombi (blood clots) is more likely in turbulent flow
- resistance to flow is increased with a consequent increased work of the heart
Define mean circulatory pressure
Equilibrium pressure that would result throughout the cardiovascular system if the heart stopped beating and is equal to 7 mm Hg
What establishes an arterial pressure gradient in a supine subject?
What does gravity affect with regards to pressure?
Lateral (or transmural) pressure through a gravitational pressure term, which either adds to or subtracts from the pressure generated by the heart
Why should blood pressure be taken at the level of the heart?
Because gravitational pressure will exist whether or not the heart is beating, thus gravity affects the measurement of BP
Why doesn't gravity affect the flow of blood in a circuit of distensible vessels?
Because gravitational pressure in the arteries is exactly counter-balanced by the same gravitational pressure at the same level in the corresponding veins
What is the transmural pressure?
PTM = (PL + ρgh) - Pext
*gravity does not affect the driving pressure on the blood, but it affects the distribution of blood throughout the system of distensible vessels and, therefore, it affects the transmural pressure
Fluids flow from ___ to ___energy
What is the pressure at any point in the circulatory system equal to?
The sum of static and dynamic pressures
Define static pressure
- present whether the blood is moving or not
- composed of the applied or lateral pressure (∆P or PL) and force of gravity (ρgh)
Define dynamic pressure
pressure that is due to kinetic energy (1⁄2ρv2)
As velocity ___, dynamic pressure becomes a ___ fraction of the total pressure
As vessel radius ___, the dynamic component ___ significantly
Compare the dynamic pressure in:
*smaller arteries, capillaries and small veins
*atria & pulmonary arteries
Aorta --> small in resting state but may be important when cardiac output increases
smaller arteries, capillaries and small veins --> negligible
vena cava --> very important at high cardiac output
atria and pulmonary arteries --> significant and of great importance at elevated cardiac output
How is direct measure of blood pressure done?
Cannulation or catheterization into a large vessel *invasive
What are some less invasive but indirect methods of measuring blood pressure?
Electromagnetic or ultrasound flow meters
How do clearance methods measure blood pressure?
They measure regional blood flow as dilution of metabolite or injected dye
What is the most common method of measuring BP?
How is sheer stress created?
By flowing blood on the endothelial wall directed along the long axis of the vessel
Define shear rate
Rate at which the axial velocity changes from the wall to the lumen
Sheer stress on the vessel wall is ___ to the viscosity and the shear rate
For Poiseuille flow, sheer stress is ___ to viscosity and flow rate, and ___ to the cube of the vessel radius
directly proportional; inversely proportional
What are the units of sheer stress?
The velocity profile in a rigid cylindrical tube is ___
Why does the outermost layer have zero velocity?
Because friction is greatest at the wall
The axial layer in the center is farthest from the wall, has minimal frictional retardation and...
...therefore, maximal velocity, vmax
The axial velocity is ___ the mean velocity, v, as averaged over the cross-section of the tube
How are flow and velocity different?
On the thruway the cars may travel with a velocity of 50 mph; the traffic flow is how many cars pass the tollbooth per hour