Understanding Blood Pressure
I. Vascular Pathway
A. Arteries
1. Elastic
2. Muscular
3. Arterioles
B. Capillaries
C. Venules
D. Veins
II. Arteries – take blood away from the heart
A. Elastic (conducting arteries)
1. high elasticity, allows for distension and recoil under pressure changes
2. help dampen pressure oscillations, smooth blood flow in arterioles
3. aorta, pulmonary (common carotids, subclavian, common iliacs)
B. Muscular (distributing arteries)
1. distribute blood to organs
C. Arterioles
1. smallest arteries
2. resistance vessels – regulate blood pressure
3. vasoconstriction and vasodilation
III. Capillaries
A. Only site of gas, nutrient, waste exchange between blood and interstitial fluids
B. Types
1. Continuous – complete endothelium, most common
a. omnipresent, except in cartilage and epithelium
b. allows diffusion of water and small solutes
2. Fenestrated – contain pores in endothelial lining
a. allows for exchange of particles as large as small peptides
b. digestive system
3. Sinusoids – gaps between endothelial cells
a. allow for exchange of plasma proteins
b. liver – production site of plasma proteins
c. bone marrow
C. Capillary Beds
1. a single arteriole feeds dozens of capillaries
2. precapillary sphincters
a. guard capillaries
b. close contact with local tissue conditions
1. constriction – decreased blood flow to tissue
2. relaxation – increases blood flow to tissue
-occurs when [O2] declines, high [CO2]
3. arteriovenous anastomoses – direct connection between arteriole and
venule —– bypasses capillary bed
IV. Veins – collect blood and return it to heart
A. Venules – smallest veins collect blood from capillaries
B. Venous system under less pressure, thinner walls
C. One-way valves
D. Distortion of vessel walls cause weak valves and pooling of blood
1. varicose veins, hemorrhoids
V. Distribution of Blood (volume at 5L)
A. Heart, arteries and capillaries — 1/3 of blood volume (~1.5L)
B. Veins contain 2/3 of blood volume (~3.5L)
1. Venous reserve (liver, bone marrow, skin —-> ~1L)
a. hemorrhaging (blood loss)
1)stimulates SNS —- venoconstriction in skin and liver can
add 1L to serve brain
VI. Cardiovascular Physiology
A. Blood pressure – force blood exerts against vessel wall
1. hydrostatic pressure in arterial system that moves blood thru
capillaries
2. blood flows from a high ——> low pressure
3. the higher the pressure gradient, the higher the flow rate
4. Pressure gradients
a. Arterial — 100 mmHg to 35 mmHg
b. Capillary — 35 to 18 mmHg along length
c. Venous — 18 to about 2 mmHg
B. Peripheral Resistance – opposes movement of blood in arterial system
1. Friction
2. Blood Flow(Q) =P/R
a. increase resistance, decrease blood flow
b. increase pressure, increase blood flow
3. Increase resistance, increases blood pressure
C. Sources of resistance
1. Vessel length
a. increase length—-> increase resistance
2. Vessel diameter
a. decrease diameter—-> increases resistance
b. vasoconstriction and vasodilation
c. arterioles
3. Viscosity – internal friction
a. RBC’s – hematocrit
b. blood is 3-4 times as thick as water
c. takes 3-4 times the pressure to push blood than water
D. Poiseuille’s Law – factors affecting rate of blood flow (ml/min) thru a vessel
Q = Pr4 / 8l
P — pressure difference at ends of vessel
r4 — radius of vessel
— viscosity of blood
l — length of vessel
1. Vessel length
a. a vessel twice as long will offer twice the resistance
2. Significance of r4 in arteriolar resistance
a. diameter = 1 == 1 ml/min
diameter = 2 == 16 ml/min
diameter = 3 == 256 ml/min
VII. Arterial Pressure
A. Systolic pressure —> peak BP during ventricular systole
B. Diastolic pressure —> minimum BP during ventricular diastole
C. 120 mmHg / 80 mmHg
D. Pulse – pressure oscillations with heartbeat
1. Elastic rebound – recoil of aorta during diastole moves blood to coronary
vessels and capillaries. (during systole coronary vessels are
squeezed and blood flow temporarily stops)
E. Pressure decreases as you move away from heart
F. Hypertension (vs. Hypotension)
1. over 150/90
2. stresses vessels
a. promotes arteriosclerosis, aneurysms, heart attack, stroke
3. left ventricular hypertrophy —–> coronary ischemia
VIII. Venous Pressure and Venous Return
A. Low Venous Pressure
1. Arterial – 65 mmHg (100 mmHg to 35 mmHg)
2. Venous – 16 mmHg (18 to about 2 mmHg)
B. Venous return – am’t of blood returning to right atrium / min
1. low venous pressure can’t overcome force of gravity
C. Mechanisms to help venous return
1. Muscular pump
a. contractions of skeletal muscle
b. one-way valves
c. knees locked at attention —> decrease venous return and CO —–> fainting
d. Swollen ankles / feet from standing –increased venous pressure
increases capillary pressure and pushes fluid out (edema)
2. Respiratory pump
a. inhale –> pulls blood up inferior vena cava
b. exhale —> pushes blood into right atrium
D. Frank-Starling Principle
1. the greater the venous return the greater the SV
-more in = more out
2. increased stretching causes more forceful contraction
3. Exercise – increase in muscular pump will increase venous
return —-> increase SV —–> increase CO
IX. Cardiovascular Regulation
A. Tissue perfusion – blood flow to tissue; supply oxygen and nutrients
1. Cardiac output
2. Peripheral resistance
3. Blood pressure
CO = HR x SV
BP = CO x PR
Increase Blood volume —–> increase BP
B. Regulation
1. Autoregulation
2. Neural
3. Endocrine
C. Autoregulation – adjustments based on local tissue conditions
1. Local vasodilators – relax smooth muscle in precapillary sphincters
(in high concentrations – arterioles)
a. Low O2 or high CO2
b. lactic acid
c. nitric oxide from endothelial cells
d. elevated H+
e. inflammatory chemicals – histamine
2. Local vasoconstrictors
a. prostaglandins / thromboxanes – from damaged tissue
-blood clotting
D. Neural
1. Cardiovascular centers (CV) in medulla oblongata
a. Cardiac center
1) cardioacceleratory center
-SNS
-increases HR
2) cardioinhibitory center (PNS)
b. Vasomotor center (vasomotor tone) (SNS)
1) vasoconstriction – large group of adrenergic (NE) neurons
-stimulate smooth muscle in arterioles
-venoconstriction – with extreme stimulation
(mobilization of venous reserve)
2) vasodilation – skeletal muscle
-nitric oxide – relaxes smooth muscle in arterioles
-cholinergic synapse – ACh triggers release of NO
from endothelial cells
2. Baroreceptor reflexes
a. Baroreceptors
1) stretch receptors; aortic and carotid sinuses
2) monitor blood pressure
Ex) decrease in BP inhibits baroreceptors —- stimulates SNS
1) stimulates cardioacceleratory and inhibits cardioinhibitory
centers —– increases HR (NE release on SA node)
and SV (NE release on myocardium)
2) stimulates vasomotor center — vasoconstriction
3) increase CO and PR — increase BP
E. Endocrine
1. Antidiuretic Hormone (ADH)
a. release – decrease Blood volume, hyperosmotic plasma,
and angiotensin II
b. action – vasoconstriction and conserves water at kidney
c. result – increases blood pressure and maintain blood volume
2. Renin – Angiotensin – Aldosterone Mechanism
a. leads to angiotensin II (large effect on BP)
1) release of aldosterone – conserves Na+, release K+
2) release of ADH
3) stimulates thirst
4) vasoconstriction of arterioles
3. Erythropoietin (EPO)
a. released by kidneys when low O2 in blood (also low BP)
b. stimulates RBC production – increases blood volume
4. Atrial Natriuretic Peptide (ANP)
a. produced by right atrium — excessive stretching (high BP)
b. action –
1) increases Na+ excretion at kidneys
2) blocks release of ADH, NE, Epi, aldosterone
X. Cardiovascular Response
A Light exercise
1. Vasodilation in skeletal muscles (relaxation of arterioles and
precapillary sphincters)
2. Venous return increases with contraction of muscle
3. CO increases — Frank-Starling Principle
4. increase blood flow to muscles and skin
B. Heavy exercise
1. Massive SNS stimulation
2. CO increases to maximum levels (20-25 L/min)
2. vasomotor centers restrict blood flow to nonessential organs
a. digestive system
3. blood races between muscle, lungs, and heart
C. Hemorrhaging
Short term responses: (temporary compensation for low blood volume)
1. Neural response (baroreceptor reflexes) and SNS
a. vasoconstriction, mobilize venous reserve, increase HR/SV
2. Release of hormones
a. Epi and NE from adrenal medulla
b. ADH and angiotensin II
Long term responses: (restore normal blood volume)
1. decline in capillary pressure triggers recall of interstitial fluids
2. ADH and aldosterone – promote fluid retention
3. increase thirst
4. EPO
