Module 8 Study Guide - Physiology I

Everything above, compressed to what you should be able to reproduce without the slides in front of you.

Core ideas¶

Within the systemic circulation and within the pulmonary circulation, beds are plumbed in parallel, so every capillary bed sees the same driving pressure. The two circulations are in series, so the entire cardiac output passes through both.
The arterial side diverges and the venous side converges. Large arteries distribute, arterioles provide resistance, capillaries exchange, veins return blood and act as capacitance.
Structure dictates function. Elastic tissue dominates the aorta, smooth muscle the small arteries and arterioles, collagen the largest vessels. Every vessel has an endothelium.
Gut and kidneys are grossly overperfused at rest because they clean, filter, and maintain the blood. Flow shifts to muscle in exercise and to skin in heat.
Four governing equations: Q = ΔP/R, R = 8ηl/πr⁴, Q = v̄ · A, and ΔP = ΔV/C.
The systemic driving pressure is MAP minus CVP. CVP is near zero, so MAP = CO × SVR.
Pulsatility disappears and the steepest pressure drop occurs across the arterioles.
MAP is the area under the pressure curve, approximated as ⅓ SBP + ⅔ DBP, and always weighted toward diastolic pressure.
Pressure depends on volume and compliance. Arteries hold little volume with low-but-sufficient compliance at high pressure. Veins hold most of the blood with high compliance and capacitance at low pressure.
Aortic elasticity lets the aorta absorb systolic volume and recoil during diastole, so arterial flow continues even when the heart is not pumping.
Flow is conserved, so mean velocity varies inversely with aggregate cross-sectional area. Velocity is lowest in the capillaries.
All flow equations assume laminar flow. Re below 2,000 is laminar, above 3,000 turbulent. Turbulence raises resistance, makes noise, and damages cells.
Radius dominates resistance both mathematically (fourth power) and physiologically (the only regulated variable). Arterioles are the resistance vessels.
Vasodilation is mostly intrinsic and local, for organ perfusion. Vasoconstriction is mostly extrinsic and systemic, through the sympathetic nervous system and hormones, for SVR and MAP.
Water crosses the capillary wall by the Starling forces. Filtration wins at the arterial end, reabsorption at the venous end, and lymphatics return the 3.6 L/day difference. When they can’t, edema forms.

Key terms¶

Arborization: The branching pattern of the circulation. Divergence on the arterial side, convergence on the venous side.
Tunica intima / media / externa: Innermost layer (endothelium, basement membrane, internal elastic membrane) / middle layer (smooth muscle, external elastic membrane) / outer layer (mostly collagen).
Endothelium: Single layer of thin flat cells lining every vessel. Its damage is almost universally the first step in cardiovascular disease.
Perfusion: Blood flow into a tissue. How well supplied with flow an organ or tissue is.
Hagen-Poiseuille equation: Q = ΔPπr⁴/8ηl. Rearranged, R = 8ηl/πr⁴.
Mean arterial pressure (MAP): Integrated area under one cycle’s arterial pressure curve. Approximated as ⅓ SBP + ⅔ DBP.
Central venous pressure (CVP): Pressure in the vena cava where it joins the right atrium. Virtually identical to right atrial pressure, about 2 to 3 mmHg.
Pulse pressure: SBP minus DBP. Proportional to arterial volume and to arterial stiffness.
Wall tension: Force needed to keep a vessel from rupturing. Proportional to P · r (Law of Laplace).
Wall stress: Wall tension divided by wall thickness h. Used for thick-walled vessels.
Compliance: ΔV/ΔP. The slope of the volume-pressure curve. Reciprocal of stiffness.
Capacitance: Volume contained at a given pressure. A single point on the compliance curve.
Distensibility: The aorta’s ability to expand with incoming volume with only a moderate pressure rise, enabled by its elasticity.
Venous return: The volume of blood returning to the heart each minute, driven by P_venules − CVP.
Laminar flow: Flow in concentric layers with a parabolic profile. Fastest at the center, zero at the wall.
Reynolds number: Re = v̄ · 2r · ρ/η. Unitless predictor of laminar or turbulent flow.
Turbulent flow: Layers break down into eddies. Blunt profile, high resistance, noisy, damaging.
Korotkoff sounds: Sounds of arterial turbulence heard when taking blood pressure with a cuff.
Murmur: Turbulent flow in the cardiac cycle around a diseased valve, heard with a stethoscope.
Viscosity: A fluid’s stickiness, its cohesive tendency to stick to itself. In blood, mostly from red blood cells.
Hematocrit: Percent of blood volume occupied by red blood cells. About 45%.
Shear rate: Difference in velocity between adjacent layers of flow. Higher shear rate, lower viscosity.
Resistance vessels: The arterioles. Their radius changes exert a disproportionately large effect on resistance.
Resting vasomotor tone: Baseline sympathetically induced vasoconstriction in most vessels.
Myogenic: Of muscle origin. Smooth muscle constricting to more stretch and pressure, dilating to less.
Precapillary sphincter: Smooth muscle at the entrance to a capillary bed that controls flow through it.
Continuous / fenestrated / sinusoidal: Junctions only, most of the body / plus cellular perforations, kidneys, intestines, choroid plexuses / plus large pores and gaps, liver, marrow, spleen.
Fick’s law: J_x = P_x · ([X_c] − [X_if]). Diffusion of small water-soluble solutes.
Filtration / reabsorption: Water moving out of the capillary into the interstitium / water moving from the interstitium into the capillary.
Starling forces: P_c, P_if, π_c, and π_if, together setting water movement across the capillary wall.
Reflection coefficient (σ): Weights net osmotic pressure. 1 if no proteins cross the wall, 0 if all do.
Hydraulic conductivity (L_p): Leakiness of the capillary wall, its ability to let water across.
Net filtration pressure: (P_c − P_if) − (π_c − π_if). Positive filters, negative reabsorbs.
Lymph: The same watery fluid once it enters a lymphatic capillary. Plasma in vessels, interstitial fluid in tissues.
Edema: Accumulation of fluid in the interstitial space. Peripheral is uncomfortable, cerebral and pulmonary can be life-threatening.

Numbers worth knowing¶

Resting cardiac output	5.0 L/min
Cerebral share	14%, 700 mL/min
Coronary share	4%, 200 mL/min
Splanchnic share	27%, 1,350 mL/min
Renal share	20%, 1,000 mL/min
Skeletal muscle share	21%, 1,050 mL/min
Cutaneous share	5%, 250 mL/min
All other tissues	9%, 450 mL/min
Gut / kidney flow per 100 g	100 / 400 mL/100 g/min
Aorta internal radius / wall	12 mm / 2 mm
Capillary diameter	~5 µm
Continuous capillary junction	~10 nm
Capillaries downstream of the aorta	10 billion
Mean pressure: aorta / entering arterioles / entering capillaries	~100 / ~80 / ~30 mmHg
Right ventricular systolic pressure	~25 mmHg
Venule pressure / CVP	~18 / 2–3 mmHg
Blood volume	70 mL/kg, ~5 L at 70 kg
Arterial / venous share of blood volume	~15% (750 mL) / ~65% (>3 L)
Venous to arterial compliance	20:1 to 30:1
Reynolds number, laminar / turbulent	< 2,000 / > 3,000
Plasma / red cells by volume	~55% / ~45%
Viscosity of water / plasma	1 / ~1.3 centipoise
Doubling radius changes resistance by	÷ 16
Capillary P_c, arterial / venous end (slide 41)	35 / 18 mmHg
Capillary π_c	~25 mmHg
NFP, arterial / venous end	+10 / −7 mmHg
Filtered / reabsorbed / to lymph per day	24 / 20.4 / 3.6 L

Distinctions that get mixed up¶

Pair	How to keep them apart
Parallel vs series	Parallel is within a circulation: every bed gets the same blood at the same pressure. Series is between circulations: pulmonary then systemic, so all of cardiac output goes through both.
Compliance vs capacitance	Compliance is the slope of the volume-pressure curve. Capacitance is one point on it, the volume held at a given pressure.
Compliance vs stiffness	Reciprocals. Pulse pressure rises with stiffness, so it falls with compliance.
Elastic fibers vs collagen fibers	Elastic fibers stretch and recoil like rubber bands. Collagen fibers are strong and barely stretch, like rope.
MAP vs the average of SBP and DBP	MAP is the area under the curve. Diastole lasts about twice as long as systole, so MAP = ⅓ SBP + ⅔ DBP, not their simple average.
Wall tension vs wall stress	Tension ∝ P · r, for thin-walled cylinders. Stress ∝ P · r / h, for thick-walled vessels, so a thicker wall lowers it.
ΔP across a capillary bed vs P_c in it	The difference across the bed drives flow through the capillaries. The pressure inside at a point pushes fluid out through the wall.
Individual vs aggregate cross-sectional area	Each capillary is tiny, but there are so many that their combined area is the largest in the circulation. Velocity follows the aggregate.
Laminar vs turbulent	Laminar: layers, parabolic profile, quiet, Re < 2,000, equations apply. Turbulent: eddies, blunt profile, noisy, Re > 3,000, equations fail.
Intrinsic vs extrinsic	Intrinsic signals start in the local tissue or the vessel itself. Extrinsic signals come from nerves or hormones outside it and act body-wide.
Purpose of vasodilation vs vasoconstriction	Dilation is mostly intrinsic and adjusts local organ perfusion. Constriction is mostly extrinsic and maintains SVR and MAP.
Hydrostatic vs osmotic pressure	Hydrostatic pressure pushes water away from its source. Osmotic pressure pulls water toward its source.
Filtration vs reabsorption	Filtration is out of the capillary, driven by P_c and π_if. Reabsorption is into the capillary, driven by π_c and P_if.
Paracellular vs transcellular	Paracellular is between cells by bulk flow: water, ions, small polar molecules. Transcellular is through the cell: nonpolar solutes directly, proteins by vesicles.
Plasma vs interstitial fluid vs lymph	The same watery fluid, named by where it is: in vessels, in the tissues, or in a lymphatic capillary.

Check yourself¶

Explain why the systemic circulation is plumbed in parallel while the whole circulation is in series, and what each arrangement accomplishes.
Trace the branching from aorta to arterioles and from venules to vena cava, and name the job of each level.
Name the three tunicas and the sublayers of each in an artery. Which sublayers do veins lack, and which do capillaries have?
Give two structural differences between an artery and a vein at the same level of branching, and the purpose of venous valves.
Which vessels need elasticity most, which need smooth muscle most, and which need collagen most? Why?
Calculate renal blood flow at rest from its share of a 5.0 L/min cardiac output. Why do the gut and kidneys get so much flow at rest?
Write the four governing equations of hemodynamics and state what each relates.
Using R = 8ηl/πr⁴, by what factor does resistance change if radius is halved? If length doubles?
Define ΔP for the systemic and pulmonary circulations. Why can CVP and RAP be used interchangeably?
Where does the steepest pressure drop occur in the systemic circulation, and what else changes there?
State the Law of Laplace and the wall stress equation. Name the two ways arteries limit wall tension.
A blood pressure reads 120 over 80. Calculate MAP and pulse pressure, showing the equation first.
Why is MAP weighted toward diastolic pressure? State both ways to calculate it.
Calculate total blood volume and arterial volume for a 70 kg person.
Describe zero, infinite, and finite compliance, and explain why the aorta needs finite compliance.
How does aortic recoil keep blood flowing during diastole?
Explain why venous compliance is high at low pressure and then nearly flat at higher pressure.
Distinguish compliance from capacitance on a volume-pressure graph, and explain why veins act as a blood reservoir.
List the three main influences on MAP, and the primary determinant of flow into and of flow out of the arteries.
Using Q = v̄ · A, explain why blood velocity is lowest in the capillaries even though pressure keeps falling.
Describe laminar flow and its profile. Which changes in velocity, vessel size, and viscosity favor it?
Give three consequences of turbulent flow and two clinical situations where we listen for it.
Explain the downward spiral between atherosclerotic plaque and endothelial damage.
How do hematocrit and shear rate each affect blood viscosity, and why?
Why are viscosity and length not used to regulate resistance? How does vessel length explain blood pressure differences between children and adults?
Explain, in terms of friction and laminae, why a larger vessel has lower resistance.
Four arterioles share 4 L/min. One constricts to 0.25 L/min. Calculate the flow through each of the others.
Explain how arteriolar constriction affects upstream pressure and downstream flow, and how flow is rerouted during a run.
Name the four sources of intrinsic vasodilation with one example each, and the two extrinsic mechanisms.
Name the main extrinsic vasoconstrictors and their receptors. Why is vasoconstriction mostly extrinsic?
Compare continuous, fenestrated, and sinusoidal capillaries by location, pathways, and what crosses.
Explain why most capillaries are functionally impermeable to proteins.
Name the four Starling forces, the direction each moves water, and its relative strength.
Write the Starling-Landis equation and define each term. What happens to the osmotic term when σ = 0?
Calculate NFP at both ends of a capillary with P_c of 35 and 18 mmHg, π_c of 25 mmHg, and interstitial pressures of zero. Interpret each sign.
Where does the 3.6 L/day of unreabsorbed fluid go, and what happens if that route is overwhelmed? Why is cerebral or pulmonary edema more dangerous than peripheral edema?