Chapter 10 Blood Vessels and Blood Pressure

Patterns and Physics of Blood flow

• blood

  • is transported to all parts of the body through the blood vessels

  • Brings fresh supplies to cells

  • removes wastes

  • transports chemical messengers

• Blood is constantly “reconditioned”

  • reconditioning organs include

    • receives more blood to maintain homeostasis

      • Kidneys (eliminate wastes)

      • Digestive tract (to pick up nutrients)

      • Skin (eliminate heat)

• Blood flow rate through the vessel is

  • proportional to pressure gradient (ΔP)

  • inversely proportional to the vascular resistance (R)

• F (flow rate of blood through a vessel) equation

  • F = ΔP/R

Factors in Blood Flow

• ΔP- Pressure gradient

  • difference in pressure between the beginning and end of a vessel

  • contraction of the heart imparts pressure to the blood—> drives the blood flow

• Vascular resistance (R)

  • Opposition to blood flow through a vessel, due to friction between the fluid and the vessel wall

  • Rαη L/r^4

    • R is proportional to viscosity ηand depends on L and r

      • η-viscosity. Directly proportional

      • L-Vessel length. Directly proportional

      • r= vessel radius Inversely Proportional (major determinant)

    • Poiseuille’s Law

      • Flow rate = (pi*ΔP*r^4)/(8*η*L)

Vascular Tree

• systemic and pulmonary circulation-each consist of a closed system of vessels

Arteries- transport blood from the heart to the organ

Arterioles-control the amount of blood that flows through each organ

Capillaries-vessels where materials are exchanged

Venules- capillaries join to form venules

Veins-venules merge to form veins that return the blood to the heart

Arteries

• rapid transit passageways to organs

  • the heart contracts to pump blood into arteries and relaxes to refill

• Arterial pressure fluctuates

  • Systolic pressure averages 120 mm Hg

  • Diastolic pressure averages 80 mm Hg

  • Changes in pressure will affect vessels (normal)

• mean arterial pressure is driving force for blood flow

  • Avg pressure driving blood forward into the tissues throughout the cardiac cycle

    

• Arterial blood pressure is the force exerted by blood against the vessel wall

• depends on the blood volume and on the vessel compliance

  • ΔP=ΔV/C

  • c- compliance

• measuring blood pressure

  • Pressure cuff (sphygmomanometer)

    • Cuff is inflated→applies pressure on brachial artery

    • If cuff pressure >systolic pressure → artery is pinched; no blood flow; no sound .

    • If cuff pressure falls below the systolic pressure →the artery partially opens; blood flow is turbulent; makes sound

    • If cuff pressure < diastolic pressure →laminar flow; no sound

Blood Pressure

• MAP- mean arterial pressure

  • MAP = diastolic pressure + (1/3) pulse pressure

  • pulse pressure = systolic pressure — diastolic pressure

  • MAP is closer to the diastolic pressure

  • 2/3 of the cardiac cycle is spent in diastole

Arterioles

• the major resistance vessels

  • radius is small→considerable resistance to flow → pulsatile pressure converts to the non-fluctuating pressure entering the capillaries

  • the pressure drop from 93 mm Hg to 37 mm Hg

    • it declines in pressure encourages blood flow to the organs downstream

Vasoconstriction - narrowing of a vessel (due to smooth muscle contraction)

Vasodilation - enlargement in circumference and radius of a vessel (due to smooth muscle relaxation)

Vascular tone - state of partial constriction of arteriolar smooth muscle, establishes a baseline of arteriolar resistance

• intrinsic myogenic activity of the smooth muscle

Local Control of Arteriole Radius

• fraction of cardiac output (CO) delivered to an organ depends on its demand for blood at each particular time

  • differences in blood flow various organs are determined by

    • differences in vascularization

    • differences in resistance offered by arterioles supplying each organ

• important in determining the blood flow and hence the distribution of CO to that organ

Local Influences on Arteriolar Radius