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LOCAL CONTROL OF BLOOD FLOW
LEARNING OBJECTIVES:
At the end of the tutorial, the student should be able to:
 Describe the intrinsic regulation of blood flow.
 Describe the vasodilator and oxygen lack theories.

Tell us the mechanisms of Autoregulation.
 Give examples of acute metabolic control of local blood flow.
LECTURE OUTLINE:
LOCAL OR INTRINSIC REGULATION:
OF BLOOD FLOW:
Tissues and organs of the body are able to intrinsically
regulate, to varying degree, their own blood supply in
order to meet their metabolic and functional needs.
This is termed local or intrinsic regulation of blood
flow.
TISSUE NEEDS FOR BLOOD FLOW INCLUDE:
1. Delivery of oxygen to the tissues
2. Delivery of nutrients, such as glucose, amino
acids, and fatty acids
3. Removal of carbon dioxide and hydrogen
ions from the tissues.
6. Transport of hormones and other
substances to the different tissues.
MECHANISMS OF BLOOD FLOW CONTROL:
Local blood flow control can be divided into two phases:
 ACUTE CONTROL AND
 LONG-TERM CONTROL.
ACUTE CONTROL:
Achieved by rapid changes in
local vasodilation/vasoconstriction of the
arterioles (seconds to minutes) to provide very
rapid maintenance of appropriate local tissue
flow.
LONG-TERM BLOOD FLOW REGULATION:
Long-term control means 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 actual blood vessels supplying the tissues.
ACUTE CONTROL OF LOCAL BLOOD FLOW:
• Increases in tissue metabolism lead to
increases in blood flow.
• Decreases in oxygen availability to tissues
increases tissue blood flow.
Theories for the regulation of local blood flow:
(1) VASODILATOR THEORY AND
(2) OXYGEN LACK THEORY.
VASODILATOR THEORY FOR ACUTE LOCAL BLOOD FLOW REGULATION:
Vasodilator substances are:
o Adenosine,
o Carbon dioxide,
o Adenosine phosphate compounds,
o Histamine,
o Potassium ions, and
o Hydrogen tons.
 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.

These then diffuse through the tissues to the precapillary
sphincters to cause dilation.

The vasodilator substance is released from the tissue mainly in
response to oxygen deficiency
OXYGEN LACK THEORY FOR LOCAL BLOOD FLOW CONTROL:
In the absence of adequate oxygen, blood
vessels simply relax and therefore naturally
dilate.
Increased utilization of oxygen in the tissues as a
result of increased metabolism could decrease
the availability of oxygen to the smooth muscle
fibers in the local blood vessels causing location
vasodilation.
EXAMPLES OF ACUTE METABOLIC CONTROL OF LOCAL BLOOD FLOW:
REACTIVE HYPEREMIA:
When the blood supply to a tissue is blocked
and then is unblocked, blood flow through the
tissue usually increases immediately.
Lack of flow sets into motion all of those
factors that cause vasodilation.
ACTIVE HYPEREMIA:
When any tissue becomes highly active, the rate of blood flow through the tissue
Increases.
AUTOREGULATION:
The capacity of the tissues to regulate their own blood is referred to as
Autoregulation.
TWO THEORIES:
1. MYOGENIC THEORY OF
AUTOREGULATION.
2 .METABOLIC THEORY OF
AUTOREGULATION.
1. MYOGENIC THEORY OF AUTOREGULATION:
 Muscle responds to tension in the
vessel wall.
 LAW OF LAPLACE:
o The wall tension is
proportionate to the distending
pressure times the radius of the
vessel.
2. METABOLIC THEORY OF AUTOREGULATION:
 Vasodilator substances tend to accumulate in active tissues; these
metabolites also contribute to Autoregulation.
 When blood flow decreases, they accumulate and the vessel dilates, when
blood flow increases, they tend to wash away.
MECHANISM FOR DILATING UPSTREAM ARTERIES WHEN MICROVASCULAR
BLOOD FLOW INCREASES—The Endothelium-Derived Relaxing Factor (Nitric
Oxide):
o
When blood flow through a
microvascular portion of the circulation
increases, this secondarily entrains another
mechanism that dilates the larger arteries as
well.
o Rapid flow of blood through the
arteries/arterioles causes shear stress on the
endothelial cells → Contorts endothelial cells
→ Increase in the release of nitric oxide →
Relaxation of blood vessels.
LONG TERM BLOD FLOW REGULATION:
• Long-term local blood flow regulation occurs by changing the degree of
vascularity of tissues (size and number of vessels).
• Oxygen is an important stimulus for regulating tissue vascularity.
The regulatory
mechanism controlling
blood vessel growth
according to a tissue's
need for oxygen.
• For example: increased vascularity in tissues of animals that live at high
altitudes, where the atmospheric oxygen is low.
Importance of Vascular Endothelial Growth Factor in Formation of New Blood
Vessels
A dozen or more factors that increase growth of new blood vessels have
been found, almost all of which are
small peptides.
Angiogenic factors promote new vessel
growth by causing new vessels to
sprout from other small vessels.
Blood vessels can also be made to
disappear when not needed.
A diagram illustrating the role of VEGF in the formation of new blood vessels that
support tumor growth.
Development of Collateral Circulation—A phenomenon of Long-term Local Blood
Flow Regulation:
• A new vascular channel usually develops around the
blocked artery or vein and allows at least partial resupply
of blood to the affected tissue.
• E.g. development of collateral blood vessels occurs after
thrombosis of one of the coronary arteries.
REFERENCES:
• TEXT BOOK OF MEDICAL PHYSIOLOGY GUYTON & HALL.
11TH EDITION
Pg # 191-199