Hemodynamics Flashcards
Flow vs. velocity
Flow (Q) = volume per unit time (ml/min)
- o CONSTANT through system–the cardiovascular system is a closed loop, so flow through the capillaries MUST be same as flow through the aorta (on average).
- o Total flow in the cardiovascular system is the CARDIAC OUTPUT (CO)
Velocity (v) = distance per unit time (cm/sec)
- o v=Q/A
- o Velocity depends inversely on cross -sectional area (A): velocity is slowest through sections with biggest cross-sectional area (like a river). Total cross-sectional area is smallest in the aorta (fastest flow), and greatest in capillary beds and pulmonary circulation (slowest flow in these areas of exchange).
Flow equation
Poiseuille’s Equation
expanded version of flow equation
- the flow varies with the 4th power of the radius.
- understand how each variable affect flow based on the equation
- Poiseuille’s Law is only valid for single vessels, not valid for parallel vessles
Laminar Flow
- smooth, streamlined, and most efficient
- velocity slowest at edge of tube, fastest in center
Turbulent Flow
- irregular, with eddies & vortices
- requires more pressure for same average velocity
- compared to laminar flow
- factors that increase turbulent flow: large diameter, high velocity, low viscosity, abrupt changes in diameter, irregularities on tube walls.
- Turbulent flow produces shearing force – viscous drag of fluid flowing through tube, which exerts force on the walls. Shear forces can damage vascular endothelium, which promotes formation of thrombi and embolisms. Damage to the vascular endothelium is a first step in the development of atherosclerotic plaques.
Pulsatile Flow
- Heart pumps intermittently, creating pulsatile flow in the aorta — arterial pressure is not constant.
- Systolic pressure = peak aortic (~arterial) pressure;
- Diastolic pressure = minimum aortic pressure
- Systole = contraction phase of cardiac cycle;
- Diastole = relaxation phase
- normal systolic/diastolic pressure <120/80 mmHg (normal range for systolic pressure ~ 90 – 120 mmHg; diastolic ~ 60 – 80 mmHg)
- pulse pressure = systolic–diastolic = 120–80 = 40 mmHg
- in capillary beds, no pulse variation, pressure (and thus flow) is continuous. Pulse pressure, mean pressure and velocity all decrease from aorta to capillaries.
- Important because pulsatile flow requires more work – basically acceleration of mass vs. maintaining constant velocity (example: stop & go driving at rush hour uses more gas)
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Mean arterial pressure (MAP) ~ diastolic pressure + 1/3(systolic – diastolic)
- MAP is NOT the arithmetic average of systolic and diastolic pressures because diastole is longer than systole (at resting heart rates)
- MAP depends on HR, this equation is approximately correct for resting heart rate. At higher heart rates, diastole is relatively shorter, so MAP approaches the average between systolic & diastolic pressures.
Compliance
Compliance (C, in ml/mmHg) equals change in volume (∆V, in ml) that results from a change in pressure (∆P, in mmHg)
- Compliance represents the elastic properties of vessels (or chambers of the heart).
- Veins are more compliant than arteries – more ∆V per ∆P
- Degree of compliance in main arteries contributes to transformation of pulsatile flow from heart into continuous flow in microcirculation
- More compliance in aorta = lower pulse pressure.
- Compliance is determined by relative proportion of elastin fibers versus smooth muscle and collagen in vessel walls.
- Arteriosclerosis (≠ atherosclerosis) = general term for loss of compliance caused by thickening and hardening of arteries. Some arteriosclerosis is normal with age; pulse pressure 40 mmHg in young adults, ~60+ mmHg in elderly people.
- Figure shows compliance of aortas from autopsies of people of different ages – younger aortas are more compliant.
Law of Lapace
- Tension in the vessel wall increases as pressure and radius increase. Thus, hypertension increases stress on vessel (and chamber) walls.
- In an aneurysm, the weakened vessel wall bulges outward, increasing the radius, thus increasing the tension that cells in the wall have to with stand 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.
Bulk Transport and Transport Rate
Bulk transport describes the movement of substances through the CV system.
Transport rate is flow time concentration:
x=Q.[x]
where x is the amount of substance x, Q is the flow, and [x] is the concentration of x
For instance, how much O2 is carried to a muscle in 1 minute? O2/min = Q.[O2]
where O2/min = transport rate (ml O2/min), CO = cardiac output (ml blood/min), and [O2] = concentration of O2 (ml O2/ml blood)
Fick’s Principle
Fick’s Principle is an expansion of the bulk transport idea to consider how much of a substance is used by a tissue. The basic idea is that the amount used is equal to the amount that enters the tissue minus the amount that leaves, and the amount can be determined as the flow times the concentration, as above.
Myocardial oxygen consumption
Myocardial oxygen consumption is defined as the amount of oxygen consumed per minute (ml O2/min), and is often expressed as ml O2/min/100 g tissue.
o Typically the oxygen content is ~0.2 ml O2/ml blood.
mVO2 at rest is ~ 8 ml O2/min/100 g and can increase to ~70 ml O2/min/100g.
o Note that oxygen consumption for the whole body can be determined by looking at the difference between oxygen levels in the pulmonary vein minus the pulmonary artery, which is opposite from the usual expression (of arterial minus venous concentration) because blood in the pulmonary vein is oxygenated and blood in the pulmonary artery is deoxygenated.
Fractional O2 Extraction (EO2) from blood
EO2 is the amount of oxygen used by a tissue expressed as a fraction of the original (arterial) oxygen concentration.
Transcapillary Transport
movement of cargo between capillaries and tissue
With 2 mechanisms
- Solvent & Solute Movement
- Two opposing forces determine solvent movement – hydrostatic pressure and oncotic pressure
- Diffusion
Hydrostatic Pressure, P
- Hydrostatic pressure is simply fluid pressure as we have been considering so far–blood pressure in this case.
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Net hydrostatic pressure in a capillary bed is the difference between capillary pressure and interstitial pressure. Solvents move from high pressure to low pressure.
- BP in capillaries ~ 25 mm Hg
- P in interstitial space ~ 0 mm Hg (or very low anyway)
- Hydrostatic pressure promotes FILTRATION (movement of fluid out of capillaries)
Oncotic Pressure, π
- Oncotic pressure (colloid osmotic pressure) is the osmotic force created by proteins in the blood and interstitial fluid. α Globulin and albumin are major determinants of oncotic pressure.
- Solutes move from high concentration to low concentration. Solvents move toward high concentration of solutes.
- Oncotic pressure of blood in capillaries (πc) is higher than oncotic pressure of interstitial fluid (πi)
- Capillary oncotic pressure promotes REABSORPTION of fluid (movement of fluid into capillaries)
Starling Equation for transcapillary transport (AKA Starling’s law of the capillary)
- o Net movement of water in and out of a capillary is simply the outward force minus inward force, or the balance between filtration and reabsorption. Factors that increase blood pressure (hypertension) or reduce oncotic pressure (liver disease) tend to promote filtration. Excess filtration causes edema (swelling) in tissues.
- o Net flux is not constant from arterial to venous end of capillaries. Pc is higher on arterial side and lower on venous side. πc is lower on arterial side and higher on venous side. Thus, there is a tendency toward filtration on the arterial side and reabsorption on the venous side.
- o Net flux is different in different capillary beds (eg: capillaries in kidney favor filtration, capillaries in gut favor reabsorption). Net flux is regulated primarily by control of capillary hydrostatic pressure (via vasoconstriction/vasodilation of arterioles).
