Hemodynamics and Vasculature

Question 1

Explain the relationship between pressure, flow, and resistance in the circulatory system (Flow Equation), and describe how changes in vascular resistance determine the distribution of cardiac output among tissues

Answer 1

• Q = DP/R
o Q—flow, DP—pressure difference, R—resistance.
o Analogous to Ohm’s law (V=IR).
o Flow requires a pressure difference.
o Flow is inversely proportional to resistance.
o Changes in vascular resistance can increase flow (↓in resistance) or decrease (↑in resistance) flow through a particular vessel. Orchestrated changes in vascular resistance can change the distribution of cardiac output among different tissues.

Question 2

Poiseuille’s Law:

Answer 2

o Expanded version of the flow equation: F—flow, DP—pressure difference, r—radius, h–viscosity of blood, l—length.
o The extra term is a more detailed description of resistance.
 An increase in the radius decreases resistance and increases flow.
 The radius of a vessel has a huge effect on flow (r4), so doubling the radius increases flow by 16-fold.
 Vessel diameter is the major mechanism by which flow is controlled (vasoconstriction and vasodilation).
 An increase in the length of the vessel results in increased resistance and decreased flow.
 An increase in viscosity results in increased resistance and decreased flow.

Question 3

Pulsatile flow:

Answer 3

the heart pumps intermittently, creating a pulsatile flow in the aorta. Pulsatile flow requires more work (analogy: stop and go driving requires more gas)

Question 4

Steady flow:

Answer 4

once blood reaches the capillary beds, there is no pulse variation, pressure (and thus flow) is constant and continuous

Question 5

the conversion of pulsatile flow to steady flow is achieved via _______ in the main arteries

Answer 5

compliance

Question 6

Vascular compliance:

Answer 6

the elastic properties of vessels (or chambers of the heart) by describing the change in volume (DV) that results from a change in pressure (DP)
o C=DV/DP
o The degree of compliance in main arteries contributes to transformation of pulsatile flow in microcirculation

Question 7

Arteriosclerosis:

Answer 7

loss of compliance caused by thickening and hardening of arteries. Some arteriosclerosis is normal with age

Question 8

LaPlace’s Law:

Answer 8

o T—tensions/wall stress, DP—transmural pressure (pressure across the wall), r—radius, m–wall thickness.
o Tension in the vessel wall increases as pressure and radius increase. Thus, hypertension increases stress on vessel walls.

Question 9

Aneurysm:

Answer 9

weakened vessel wall bulges outward, increasing the radius, thus increasing the tension that cells in the wall have to withstand to prevent the vessel from splitting open. Over time, cells become weaker, allowing the wall to bulge more so that tension increases further, until the aneurysm ruptures

Question 10

Fick’s Principle:

Answer 10

Xtc=[Xi] – [Xo]
• However much of substance X was used in the capillary (Xtc)—the transcapillary efflux—is the difference between the amount of substance X that went into the capillary and the amount of substance X that came out of the capillary.
• X=Q[conc], where X is the transport rate (mass/time), Q is the flow (volume/time

Question 11

Hydrostatic pressure, P:

Answer 11

fluid pressure.
o The net hydrostatic pressure in the capillary bed is the difference between capillary pressure and interstitial pressure. Solvents move from high pressure to low pressure.
o Hydrostatic pressure promotes filtration (movement of fluid out of capillaries)

Question 12

Oncotic pressure, p:

Answer 12

osmotic force created by proteins in the blood and interstitial fluid.
o Alpha-globulin and albumin are the major determinants of oncotic pressure.
o Solutes move from high concentration to low concentration. Solvents→high concentrations of solutes.
o Oncotic pressure promotes reabsorption (movement of fluid into capillaries).
o pcapillaries > pinterstitial fluid.

Question 13

Starling’s Equation for transcapillary transport:

Answer 13

o Flux = k [ ( Pc – Pi ) – ( pc – pI ) ]
 Flux—net movement across capillary wall; k—constant; Pc—capillary hydrostatic pressure; Pi—interstitial hydrostatic pressure; pc—capillary oncotic pressure; pI—interstitial oncotic pressure.
 Pc – Pi: net hydrostatic pressure; tends to be outwards→filtration.
 pc – pI: net oncotic pressure; tends to be inwards→reabsorption.

Question 14

Factors that increase _______ or reduce _______ tend to promote filtration. Excess filtration causes _______

Answer 14

• blood pressure (hypertension)
• oncotic pressure (liver disease)
• edema (swelling) in tissues, such as pulmonary and peripheral edema in heart failure

Question 15

capillary hydrostatic pressure is higher on the ______ side

Answer 15

arterial side

Question 16

capillary oncotic pressure is higher on the ______ side

Answer 16

venous side

Question 17

there is a tendency toward filtration on the ______ side and reabsorption on the ______ side

Answer 17

• arterial

- venous

Question 18

net flux is regulated primarily by

Answer 18

control of capillary hydrostatic pressure (vasodilation and vasoconstriction)