Physiology 1, Fall 2008, LPC

Chapter 15 - Blood Flow & the Control of Blood Pressure

 

I – Blood Vessels

A.Structure

    Endothelium – smooth lining
    Elastic tissue - allows vessel to spring back to original size, keeps tone
    Smooth muscle – in arteries and arterioles, allows vessels to push blood forward after being stretched from bolus of blood forced into them by ventricular systole
    Fibrous tissue – connects vessel with surrounding tissue

B. Miscellaneous

Arterioles, metarterioles and capillaries have sphincters that control blood flow
Metarterioles allow WBCs to go directly from arteries to veins
Capillaries and postcapillary venules – site of exchange between the blood & interstitial fluid. Large volume allows blood to slow, allowing time for exchanges.
Many capillaries surrounded by pericytes that control exchange and can differentiate to endothelial or smooth muscle cells
Angiogenesis = grow new blood vessels

II - Blood Pressure

A. Systemic

Driving force from ventricular systole
Elastic recoil allows arteries to conserve energy of ventricular systole
Table 15-1 – good summary of pressure, flow, resistance
Pressure originates & varies in left ventricle
Pressure in arteries & arterioles highest immediately after ventricular systole, lowest during ventricular relaxation (diastole)
Pressure decreases as distance from heart increases and volume of vessels increases
Little or no variation in pressure in capillaries, venules, veins
Blood returned in veins to right atria by force from skeletal muscles squeezing veins, backflow prevented by one-way valves

B. Estimating blood pressure

Blood pressure cuff applied to upper arm, inflated until arterial blood flow stopped
Cuff pressure released until systolic arterial blood pressure forces pulsatile flow ( = systolic pressure), detected by sound or electronically
Cuff pressure released until blood flows continuously (diastolic pressure)

C. Mean arterial pressure determined by cardiac output and peripheral resistance

    Cardiac output = blood pumped by left ventricle
    Peripheral resistance determined by resistance of arterioles
    Blood volume and distribution of blood affect pressure also

D. Blood volume effect

More volume -> more pressure
Volume affected by cardiovascular & renal systems

Cardiovascular system – vasoconstriction or vasodilation & change cardiac output (heart rate X stroke volume)
Kidneys can decrease volume by excreting fluid

III - Resistance in the Arterioles

Radius of arterioles inversely proportional to resistance
Smooth muscle controls radius
Control mechanisms

  1. sympathetic reflexes respond to homeostatic needs
  2. metabolic needs determine local blood distribution via sphincters
  3. hormonal control of volume and autonomic reflexes

Smooth muscle fibers in arterioles constrict in response to stretch

Hyperemia is paracrine response to –O2, + CO2 & -pH, can be caused by increased metabolic demand or occlusion

Control of arteriolar diameter via norepinephrine

Tonic release -> normal diameter
Increased release -> vasoconstriction
Decreased release -> vasodilation

IV – Distribution of Blood to the Tissues

Distribution changes according to metabolic needs
Blood flow regulated by precapillary sphincters

V – Exchange at the Capillaries

Extensive distribution and slow flow allow exchange of materials between interstitial fluid and capillary contents
Density of capillaries determined by metabolic needs of tissue
Precapillary sphincters determine radius of vessel which determine resistance to flow

Gases & lipophilic substances can go through capillary endothelium
Continuous capillaries allow water & small dissolved solutes to pass
Fenestrated (leaky) capillaries allow high volumes of fluid to exchange, found mostly in kidney & intestine. Transcytosis brings proteins & macromolecules across endothelium

Velocity of blood flow depends on cross-sectional area
Highest cross-sectional area in capillaries, -> slowest rate, allows for exchange
Diffusion rate determined by concentration gradient

Hydrostatic pressure -> filtration at arteriole end of capillary
Colloid osmotic pressure -> absorption at venous end of capillary
Net pressure = hydrostatic pressure – colloid osmotic pressure
Usually more fluid out of capillaries than absorbed

VI – Lymphatic System

Functions

  1. return fluid & proteins filtered out of capillaries to the circulatory system
  2. fat transport
  3. filter to help capture & destroy foreign pathogens

One-way movement of interstitial fluid from tissues into circulation

    Interstitial fluid pressure and smooth muscles force interstitial fluid into lymphatic vessels
    Skeletal muscle contraction & contraction of lymph vessels, one-way valves keep lymph going towards spot under collarbones for return to venous circulation
    Lymph nodes filter out foreign particles & contain immunologic cells that respond to invasion

Edema – result of alterations in capillary exchange

  1. increase in capillary hydrostatic pressure (ex. – heart failure)
  2. decrease in plasma protein concentration (malnutrition or liver failure)
  3. increase in interstitial proteins (shock)

VII – Blood Pressure Regulation

Baroreceptor reflex

Stretch receptors in carotid arteries & aorta
Baroreceptors report to medullary cardiovascular control center, respond within two heartbeats of stimulus
Stretch -> increased sympathetic response -> vasoconstriction, decreased heart rate & decreased force of contraction in heart
Stretch -> increased parasympathetic response -> decrease in heart rate

Respiratory center coordinates with circulatory system to supply O2
Higher brain centers influence blood pressure (fright -> increased BP or, faint if see blood)

Orthostatic hypotension

Blood pressure goes down when standing after lying down
Decrease in stretch of carotid & aortic baroreceptors -> cardiovascular control center in medulla -> increased sympathetic output & decreased parasympathetic output -> increased peripheral resistance & cardiac output, -> normal blood pressure