Chapter 14.1-14.5

Flow equation

Flow = ΔP / R; flow of blood depends on pressure difference and resistance, inversely related to resistance

Vasoconstriction

Decreases blood vessel radius, increases resistance, decreases flow

Vasodilation

Increases blood vessel radius, decreases resistance, increases flow

Delta P (ΔP)

Pressure difference between two points; drives blood flow from high to low pressure

Systemic circuit

Start: left ventricle; End: right atrium; function: deliver oxygen to tissues, remove CO2

Pulmonary circuit

Start: right ventricle; End: left atrium; function: oxygenate blood

Mean arterial pressure (MAP)

Average pressure in arteries during one cardiac cycle; MAP = (Systolic + 2 × Diastolic)/3

Total peripheral resistance (TPR)

Combined resistance of all systemic blood vessels

Cardiac output (CO)

Flow of blood from heart per minute; CO = MAP / TPR

Arteries

Blood vessels that carry blood away from the heart

Elastic arteries

Large diameter, less smooth muscle, act as pressure reservoirs

Muscular arteries

Smaller diameter, more smooth muscle, can vasoconstrict/dilate to regulate flow

Compliance

Ability of vessel to expand; high compliance = expands easily, low compliance = stiff

Aortic pressure variation

Pressure in aorta rises during systole, falls during diastole due to recoil

Blood pressure measurement

Figmo manometer and stethoscope; brachial artery; first sound = systolic, last sound = diastolic

Pulse pressure

Systolic – Diastolic

Arterioles

Smallest arteries, highest resistance; major site of blood flow regulation

Capillaries

Thin-walled vessels where exchange occurs; slow velocity, large cross-sectional area

Capillary types

Continuous: small gaps, most common; Fenestrated: larger pores, kidney/intestine/endocrine; Discontinuous/sinusoids: large gaps, liver/spleen/bone marrow

Precapillary sphincters

Rings of smooth muscle between arteriole and capillaries; control blood flow into capillary beds

Metarterioles

Direct connections between arterioles and venules; shunt blood when capillary bed is bypassed

Local control of microcirculation

Sphincters relax in response to metabolites like low oxygen to increase blood flow; contract when flow not needed

Starling forces

Forces governing fluid movement across capillary walls: hydrostatic and oncotic pressures

Capillary hydrostatic pressure (Pc)

Pushes fluid out of capillary → filtration

Interstitial hydrostatic pressure (Pif)

Pushes fluid into capillary → reabsorption

Capillary oncotic pressure (πc)

Pull fluid into capillary → reabsorption; created by plasma proteins

Interstitial oncotic pressure (πif)

Pull fluid out of capillary → filtration; created by proteins in interstitial fluid

Edema

Collection of fluid in interstitial space

Causes of edema

Liver disease → low plasma protein → decreased πc; heart failure → high Pc → lower limb edema; injury → high πif → local swelling

Extracellular fluid (ECF)

Plasma (20%) + Interstitial fluid (80%)

Filtration

Fluid movement out of capillary

Reabsorption

Fluid movement into capillary

Lymphatic system

Picks up excess fluid filtered out (~3 L/day) to prevent swelling

Flow velocity

Slow in capillaries to allow exchange

Cross-sectional area

Largest in capillaries to facilitate exchange

Blood viscosity

Depends on RBC and protein content; affects resistance

Length of blood vessels

Systemic vessels longer → higher resistance; pulmonary shorter → lower resistance

Arterial wall components

Endothelium, smooth muscle, elastin, collagen; varies by vessel type

Pressure reservoir

Elastic arteries store pressure during systole, recoil pushes blood during diastole

Systolic pressure

Pressure in arteries during ventricular contraction

Diastolic pressure

Pressure in arteries during ventricular relaxation

Hydrostatic pressure

Pushing force by fluid

Oncotic pressure

Pulling force by proteins

Polycythemia

High RBC count → increases blood viscosity → increases resistance

Laminar flow

Smooth, silent blood flow

Turbulent flow

Disrupted flow creating sound; used to measure blood pressure

Blood pressure cuff placement

~2 inches above elbow, stethoscope over artery

MAP calculation

1 systolic + 2 diastolic / 3 (resting state uses more time in diastole)

CO vs Flow

In systemic circuit, CO = flow; same flow in systemic and pulmonary despite ΔP difference due to resistance differences

Precapillary sphincter control

Affected by metabolites, oxygen, CO2, local needs

Filtration vs Reabsorption

Pc and πif → filtration; πc and Pif → reabsorption

Capillary exchange mechanisms

Diffusion (lipophilic), through pores (hydrophilic), transcytosis, mediated transport, bulk flow

Capillary bed

Mesh of capillaries between arterioles and venules

Vessel types function

Arteries: away from heart; Arterioles: resistance and flow control; Capillaries: exchange; Venules: return to veins

Velocity in capillaries

Slowest to allow exchange

Largest cross-sectional area

Capillaries

Sphincters contract

Increase resistance, shunt blood

Sphincters relax

Decrease resistance, allow exchange

Systemic ΔP

~85 mmHg

Pulmonary ΔP

~15 mmHg

Flow equality between circuits

Same flow due to different resistances

Resistance factors

Radius, viscosity, length of vessel

Resistance vs Flow

Inversely related

CO = MAP / TPR

Equation connecting flow, pressure, and resistance in systemic circuit

Pressure gradient

ΔP drives blood from high to low pressure

Arterioles resistance

Highest among vessels

Capillary wall thickness

~1 cell layer, 0.5 μm

Fenestrated capillaries

Large pores for proteins, kidney/intestine/endocrine glands

Sinusoidal capillaries

Large gaps, allow cells/proteins, liver/spleen/bone marrow

Hydrostatic capillary

Filters fluid out of capillaries

Hydrostatic interstitial

Reabsorbs fluid into capillaries

Oncotic capillary

Reabsorbs fluid into capillaries

Oncotic interstitial

Filters fluid out of capillaries

MAP formula

(MAP = (Systolic + 2 × Diastolic)/3)

Pulse pressure formula

PP = Systolic – Diastolic

MAP significance

Represents average driving pressure of systemic circulation

Venules

Tiny vessels connecting capillaries to veins; small exchange of material

Elastic arteries function

Pressure reservoir

Muscular arteries function

Regulate flow via smooth muscle

Precapillary sphincters location

Between arteriole and capillaries

Metarterioles location

Directly connect arteriole to venule to shunt blood

Edema localization

Depends on affected organ: liver → belly; heart → ankles; kidney → eyelids