00:01
So now let's take a look
at how we control blood flow
through our vessels.
00:06
Tissue perfusion
refers to our blood flow
through the body's tissues,
and involves the delivery of
oxygen and nutrients to,
and the removal of waste
from our tissue cells.
00:18
We also have gas exchange
at the lungs,
absorption of nutrients
in our GI tract,
and urine formation
in the kidneys.
00:28
The rate of flow is precisely
just the right amount
to provide proper function
to a tissue or organ.
00:37
The rate of blood flow
is going to be controlled
by both extrinsic
and intrinsic factors.
00:43
Extrinsic controls include the
sympathetic nervous system,
and hormones that control the flow
of blood through the whole body.
00:52
They also act on
arteriolar smooth muscles
to reduce the flow to regions
that need it the least.
00:59
Our intrinsic controls
also referred to as autoregulation
is more of a local control
of blood flow.
01:07
Where blood flow is adjusted to meet
the specific tissues requirements.
01:12
Local arterioles
that feed capillaries
can undergo modification
of their diameters
and organs are able to regulate
their own blood flow
by varying the resistance
of their own arterioles.
01:27
Autoregulation or local control
is going to regulate blood flow
to that local area.
01:35
An example of autoregulation
is reactive hyperemia,
which increases blood flow to an
area due to intrinsic factors.
01:45
There are two main types
of intrinsic mechanisms
that determine the final
autoregulatory response at a tissue.
01:53
We have metabolic controls
and myogenic controls.
01:58
With metabolic controls
increase in tissue
metabolic activities
are going to result in
a declining level of oxygen,
as well as increasing levels
of our metabolic products
like H+, potassium, adenosine,
or prostaglandins.
02:16
The effects of changes
on the local level
cause a direct relaxation
of arterioles
and relaxation of our
precapillary sphincters.
02:26
This causes the release
of nitric oxide
as well as
powerful vasodilators,
our endothelial cells.
02:36
Endothelins also released from
the endothelium of our capillaries
are going to be potent
vasoconstrictors.
02:44
Nitric oxide and endothelins
are usually balanced
unless blood flow is inadequate.
02:51
In which case,
nitric oxide is going to
win control
and cause vasodilation.
02:57
Also inflammatory chemicals
can cause vasodilation.
03:05
Another type of local control
are myogenic responses.
03:09
Local vascular smooth muscles
are going to respond to changes
in the main arterial pressure
in order to keep
perfusion constant
while avoiding
damage to our tissues.
03:22
In myogenic control,
passive stretch is going to cause
our smooth muscles
to respond by constricting.
03:30
Here, increase
mean arterial pressure
stretches the vessel wall
more than normal.
03:37
And turn, the smooth muscles respond
by constricting and causing
a decrease
in the blood flow to that tissue.
03:46
Opposite of passive stretch
is reduced stretch.
03:50
Here the decreased
mean arterial pressure
causes less stretch
than normal.
03:56
In response,
our smooth muscles would dilate
causing an increase
in the blood flow
to that tissue.
04:04
We also have
long-term autoregulation
as part of our intrinsic controls.
04:10
This occurs when
short-term autoregulation
cannot meet the tissues nutrient
and requirements.
04:17
Long-term autoregulation
may take weeks or months
to supply an adequate
blood supply.
04:25
The number of vessels
to a region may increase
by way of angiogenesis,
and existing vessels
may become larger.
04:34
This is common in the heart
when our coronary
vessels become occluded
and also in the body of people
who move to high altitude areas.