REGULATION OF

BLOOD FLOW
Physiology
2024
 Learning Outcomes
ØDescribe the factors responsible maintaining the blood flow
to the body
ØDescribe the structure of microcirculation and name the
characteristics of the various vessel types
ØDescribe the roles of the smooth muscle cells in the blood
vessel wall
ØName the factors that are responsible for vasoconstriction
and vasodilation
ØRelate between changes in the microcirculation and the
regulation of systemic blood pressure
 Learning Outcomes

ØDescribe peripheral resistance in the circulation

ØExplain the relationship between pressure and peripheral
resistance
ØExplain the relationship between cardiac function, blood
vessel filling, blood pressure and blood flow
 Direction of blood flow
• Heart à aorta à arteries à arterioles à capillaries
• Capillaries à venules à venes à vena cava à heart

• The heart is the pump,

• the elastic arteries are pressure reservoirs;
• the arterioles direct blood to individual tissues by
selectively constricting and dilating (the site of
variable resistance);
• the capillaries are exchange sites;
• the veins hold more than half of the blood in the
circulatory system, they are blood reservoirs.

Guyton and Hall Textbook of Medical

Physiology, 2021 by Elsevier

4
 Pressure produced by contraction
of the left ventricle is stored in the
elastic walls of arteries and slowly
released through elastic recoil

Human Physiology: An Integrated Approach 6e Pearson

Pressure difference
Blood pressure is the force that blood exerts against the inner walls of blood
vessels.

Guyton and Hall Textbook of Medical

Physiology, 2021 by Elsevier

The pressure is highest in arteries and lowest in veins

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 Blood Vessel
Structure

The thickness of the

smooth muscle–
connective tissue
layers surrounding the
intima varies in
different vessels.
Interrelationships of Pressure, Flow, and
Resistance
• Blood flow is determined by two factors:
(1) pressure difference of the blood between the two ends of
the vessel
(2) the impediment to blood flow through the vessel, which is
called vascular resistance

Guyton and Hall Textbook of Medical

Physiology, 2021 by Elsevier
Blood flow
• the quantity of blood that passes a given point in the
circulation in a given period of time
• It can be calculated by the following formula, which is
called Ohm’s law :

• Blood flow is expressed in ml/min or L/min.

• Resistance occurs as a result of friction between the flowing
blood and the intravascular endothelium all along the inside
of the vessel.
length (l), the radius (r), dynamic viscosity (η)
viscous resistance is proportional to the viscosity of the fluid
and the length of the tube but inversely proportional to the
fourth power of the radius of the blood vessel.
Flow rate should not be confused with velocity of
flow

• Blood flow means flow rate, the volume of blood

that passes a given point in the system per unit of
time.

• Velocity is a measure of how fast blood flows past a

point.
• In contrast, flow rate measures how much (volume)
blood flows past a point in a given period of time.

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 Regulation of Tissue Blood Flow

• Local Control: most tissues have the ability to control their

own local blood flow in proportion to their specific
metabolic needs.
• Humoral Control: control by substances secreted or
absorbed into the body fluids, such as hormones (e.g.
angiotensin II, vasopressin) and locally produced factors
(e.g. kinins, histamine).
Local Control of Blood Flow in Response to
Tissue Needs
• Some of the specific needs of the tissues for blood
flow:
• Delivery of oxygen to the tissues
• Delivery of other nutrients such as glucose, amino acids,
and fatty acids
• Removal of carbon dioxide from the tissues
• Removal of hydrogen ions from the tissues
• Maintenance of proper concentrations of ions in the
tissues
• Transport of various hormones and other substances to
the different tissue.
Certain organs have special requirements

• Blood flow to the skin determines heat loss from the body,
helps control body temperature.
• Delivery of adequate quantities of blood plasma to the
kidneys allows the kidneys to filter and excrete the waste
products of the body and to regulate body fluid volumes
and electrolytes.
Local Control of Blood Flow

• Acute control: by rapid changes in local vasodilation or

vasoconstriction of the arterioles and precapillary sphincters
that occur within seconds to minutes.
• Long-term control: slow, controlled changes in flow over a
period of days, weeks, or even months. These changes come
about as a result of an increase or decrease in the physical
sizes and numbers of blood vessels supplying the tissues.
Human Physiology: An Integrated Approach 6e Pearson
Acute Control of Local Blood Flow

• Increases in tissue metabolism increase tissue

blood flow
• Reduced oxygen availability increases tissue blood
flow

• Autoregulation of Blood Flow During Changes in

Arterial Pressure—Myogenic Mechanisms
Vasodilator Theory for Acute Local Blood
Flow Regulation
• The greater the rate of metabolism or the less the
availability of oxygen (or some other nutrients) to a tissue,
the greater the rate of formation of vasodilator
substances in the tissue cells.
• The vasodilator substances diffuse through the tissues to
the precapillary sphincters, metarterioles, and arterioles to
cause dilation
• e.g. adenosine, carbon dioxide, nitric oxide, phosphate
compounds, histamine, potassium ions, and hydrogen ions.
Guyton and Hall Textbook of Medical
Physiology, 2021 by Elsevier
The flow of blood through the arteries and arterioles
causes shear stress on the endothelial cells because of viscous
drag of the blood against the vascular walls. This stress triggers
nitric oxide (NO) release

Nitric oxide synthase (eNOS)

enzyme in endothelial cells
synthesizes NO from
arginine and oxygen. NO
activates soluble guanylate
cyclases, resulting in
conversion of cyclic
guanosine triphosphate
(cGTP) to cyclic guanosine
monophosphate (cGMP),
which ultimately causes the
blood vessels to relax.
Guyton and Hall Textbook of Medical
Physiology, 2021 by Elsevier
• The precapillary sphincters and metarterioles open and
close cyclically several times per minute, with the duration
of the open phases being proportional to the metabolic
needs of the tissues for oxygen.
• The cyclical opening and closing is called vasomotion .
Myogenic theory

• Sudden stretch of small blood vessels causes the smooth

muscle of the vessel wall to contract.
• high arterial pressure stretches the vessel, reactive vascular
constriction results, which reduces blood flow nearly back to
normal.
• at low pressures, the degree of stretch of the vessel is less, so the
smooth muscle relaxes, reducing vascular resistance and helping to
return flow toward normal.
• Myogenic contraction is initiated by stretch-induced
vascular depolarization, which then rapidly increases
calcium ion entry from the extracellular fluid into the cells,
causing them to contract.
 The sympathetic nervous system innervate smooth
muscle in blood vessel walls
• Contraction of smooth muscle à reduction in the diameter of
the vessel (vasoconstriction).
• If vasomotor impulses are inhibited, the muscle fibers relax,
and the diameter of the vessel increases (vasodilation).
• Changes in the diameters of arteries and arterioles greatly
influence blood flow and blood pressure.
• Changes in the diameter of veins affect the amount of blood
returning to the heart.

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Human Physiology: An Integrated Approach 6e Pearson
b2-receptor subtype has a higher affinity for epinephrine

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Peripheral resistance

Changes in arteriole diameters regulate peripheral resistance.

Blood vessels with smaller diameters offer a greater resistance
to blood flow, factors that cause arteriole vasoconstriction
increase peripheral resistance, which raises blood pressure.

Peripheral resistance is under tonic sympathetic control.

Increased sympathetic activity à vasoconstriction

Increased sympathetic activity constricts veins à increase

venous return

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 Cardiac output and venous return each can be examined
separately as a function of right atrial pressure
Vascular function curve
• Left ventricular end-diastolic volume
depends on venous return, which also
determines right atrial pressure.
• Venous return back to the heart is driven by a
pressure gradient.
• Thus, as right atrial pressure increases, this
pressure gradient decreases, and venous
return also decreases.

Cardiac and vascular function curves

*Mean systemic pressure is the value for the right atrial pressure at which venous return
is zero and right atrial pressure is at its highest value
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 Cardiac and vascular function curves
Right atrial pressure is related to
venous return, end-diastolic volume,
and end-diastolic fiber length:

As venous return increases, right atrial

pressure increases, and end-diastolic
volume and end-diastolic fiber length
increase.

Increases in end-diastolic fiber length

produce increases in cardiac output
(Frank-Starling mechanism).

When right atrial pressure reaches a

value of approx. 4 mm Hg, cardiac
output can no longer keep up (approx.
9 L/min).

The cardiac function curve is cardiac output as a function of right atrial pressure. The
vascular function curve is venous return as a function of right atrial pressure.
The curves intersect at the point (filled circle) where cardiac output and venous
return are equal.
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Combining these curves provides a useful tool for predicting
the changes in cardiac output that will occur when various
cardiovascular parameters are altered

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The point at which the two curves
intersect is the unique operating or
equilibrium point of the system in
the steady state. 36