Cardiovascular System
Blood Pressure
P. 359-363
Blood Pressure
 Definition:
 The force blood exerts against the inner walls of blood vessels.
 Arterial Blood Pressure
 Rises and falls in a pattern corresponding to phases of cardiac
cycle
 Maximum pressure during ventricular contraction is called
systolic pressure
 When ventricles relax, arterial pressure drops and the lowest
pressure is present until the next contraction is called diastolic
pressure
BP Cont.
 Example:
120/80
Systolic Pressure
Diastolic Pressure
 Pulse: expanding and recoiling of an artery
Factors that influence Arterial BP
 Heart Action:
 Determines how much blood enters the arterial system with
each ventricular contraction
 Stroke Volume:
 Volume of blood discharged from left ventricle; = about 70 mL
in an avg. weight male.
 Cardiac Output:
 Stroke volume/minute
Calculate by multiplying stroke volume by heart rate
 Example:
 If S.V. is 70 mL and heart rate is 72, C.O. is 5,040 mL/min.


BP varies with C.O.; if S.V. or heart rate increase, C.O rises and
so does BP; if S.V. or heart rate decreases, C.O and BP decrease.
Blood Volume
 Varies with age and sex somewhat; directly with
body size
 Avg. blood volume is 5 L. or 8% of body weight
 B.P. is usually directly proportional to blood volume
within cardiovascular system.
 Example:

A hemorrhage would cause blood loss and cause B.P. to
decrease. Blood volume also falls if fluid volume is upset like in
dehydration. Fluid and blood replacement can restore B.P. and
volume.
Peripheral Resistance
 Friction between blood and walls of blood vessels
 Factors that change P.R. also change B.P.
 Example:
 Contraction of smooth muscles in arteriolar walls increase B.P.
 Dilation of arterioles decrease B.P.
Blood Viscosity
 Ease in which fluid molecules flow past one another.
 Greater viscosity=greater resistance to flow
 Blood cells and plasma proteins increase blood
viscosity
 B.P. increases with increased viscosity and decreases
with decreased viscosity.
Control of B.P.
 Depends mostly on cardiac output and regulation of
peripheral resistance
 Cardiac output factors:

Starlings Law of the Heart:
Relationship between myocardial fiber length and force of
contraction.
 Allows heart to maintain a balanced flow of blood.


Baroreceptors


Found in walls of aorta and carotid arteries, sense changes in B.P.
SA Node:

Increases or decreases heart blood pressure based off impulse
from Medulla Oblongata.
Cardiac Output Factors Cont.
 Cardio-inhibitor Reflex:
 Happens when heart rate decreases and so does cardiac output
and B.P.
 Returns B.P. back towards normal.
 Conversely:

Decreasing B.P. initiates cardio-accelerator reflex, SA node
received impulse and causes heart to beat faster, increasing
cardiac output and restoring pressure.
 Other Factors:
 Emotional responses such as fear and anger
 Physical exercise
 Increase in body temp.
Peripheral Resistance Factors
 Changes in arteriolar diameter
 Vasomotor center of Medulla sense changes in
normal B.P.

Can send impulses to increase or decrease P.R. by increasing
or decreasing outflow.
 Certain chemicals
 CO2, O2, H ions
 Influence P.R. by affecting capillary sphincters and smooth
muscles.
Venous Blood Flow
 B.P. is low by the time it gets to veins that heart action is
only a partial result.
 Results from skeletal muscle contraction, breathing
movements, and vasoconstriction of veins.



As skeletal muscles contract they push against nearby vessels
squeezing blood through vein valves aiding in returning blood back
to the heart.
Respiration movements move blood through veins by applying
pressure in thoracic cavity
Vasoconstriction return blood to heart by stimulating smooth
muscles to contract during low pressure
 Veins provide a blood reservoir that adapts to changes in
blood volume. If low pressure, vasoconstriction forces
extra blood out and to the heart.
BP Chart
 Hypertension:
 Disease known as high
blood pressure
 Hypotension:
 Disease known as low
blood pressure
Review
 What is the relationship between cardiac output and





blood pressure?
How does blood volume affect blood pressure?
What is the relationship between peripheral resistance
and blood pressure? Between blood viscosity and blood
pressure?
What factors affect cardiac output?
What is the function of baroreceptors in the walls of the
aorta and carotid artery?
How does the vasomotor center control diameters of
arterioles?