Medical physiology

1. LOCAL CONTROL OF BLOOD
FLOW
DR TSOHO

2. LOCAL CONTROL OF BLOOD FLOW IN
RESPONSE TO
TISSUE NEEDS
• One of the most fundamental principles of circulatory
function is the ability of each tissue to control its own
local blood flow in proportion to its metabolic needs.
• The amount of blood that goes to a particular tissue is
proportional to its level of activity.

3. 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 other ions in the tissues
• Transport of various hormones and other substances to the
different tissues

4. The amount of blood to different
tissues differ
• Liver -----------------------1500 ml/min
• Kidneys--------------------1260 ml/min
• Brain-----------------------750 ml/min
• Skin-------------------------462 ml/min
• Heart muscle-------------250 ml/min
• Skeletal muscle----------840 ml/min
• Rest of the body---------336ml/min

5. THE HEART
• Heart muscle is supplied by two coronary arteries,
• Right and left coronary arteries
• they are first branches of aorta.
• Arteries encircle the heart in the manner of a crown,
hence the name coronary arteries
• The heart also undergoes autoregulation

6. • Autoregulation is the ability of an organ to
regulate its own blood supply. In this process,
blood flow is maintained at relatively constant
levels despite variations in perfusion pressure.

7. • The two coronary arteries that supply the myocardium
arise from the sinuses behind two of the cusps of the
aortic valve at the root of the aorta
• Eddy currents keep the valves away from the orifices of
the arteries
• they are patent throughout the cardiac cycle.
• Most of the venous blood returns to the heart through the
coronary sinus and anterior cardiac veins which drain
into the right atrium.

8. • In addition, there are other vessels that empty directly into
the heart chambers. These include arteriosinusoidal
vessels, thebesian veins
• A few anastomoses occur between the coronary arterioles
and extracardiac arterioles, especially around the mouths
of the great veins
• Anastomoses between coronary arterioles in humans only
pass particles less than 40μm in diameter
• but evidence indicates that these channels enlarge and

9. Factors that regulate coronary blood flow
 Autoregulation
 Reactive Hyperemia
 Neural factors-Adrenaline and nor-adrenaline
 Metabolic Products like:
i. Potassium
ii. Hydrogen
iii. Carbon dioxide
iv. Adenosine phosphate compounds.:

10. Coronary artery disease (CAD)
• Coronary artery disease (CAD) or coronary heart disease
• caused by inadequate blood supply to cardiac muscle
• Results from occlusion of coronary artery
• When flow through a coronary artery is reduced to the point
that the myocardium it supplies becomes hypoxic
• angina pectoris develops.
• If the myocardial ischemia is severe and prolonged,
irreversible changes occur in the muscle, and the result is

11. • Many individuals have angina only on exertion, and
blood flow is normal at rest.
• Others have more severe restriction of blood flow and
have anginal pain at rest as well.
• Partially occluded coronary arteries can be constricted
further by vasospasm, producing myocardial infarction.
• However, it is now clear that the most common cause of
myocardial infarction is rupture of an atherosclerotic
plaque, or hemorrhage into it, which triggers the

12. • Measuring the rises in serum enzymes and isoenzymes
produced by infarcted myocardial cells plays an important
role in the diagnosis of myocardial infarction.
• The enzymes most commonly measured today are the MB
isomer of creatine kinase (CK-MB), troponin T, and
troponin I.
• Myocardial infarction is a very common cause of death in
developed countries because of the widespread occurrence
of atherosclerosis.
• there is a relation between atherosclerosis and circulating

13. • Lp(a) has an outer coat of apo(a)
• It interferes with fibrinolysis by down-regulating plasmin
generation
• There is also a strong positive correlation between
atherosclerosis and circulating levels of homocysteine.
• This substance damages endothelial cells.
• It is converted to nontoxic methionine in the presence of
folate and vitamin B12,
• Clinical trials are under way to determine whether
supplements of folate and B12 lower the incidence of

14. • It now appears that atherosclerosis has an important
inflammatory component as well.
• The lesions of the disease contain inflammatory cells
• There is a positive correlation between increased levels of
C-reactive protein and other inflammatory markers in the
circulation and subsequent myocardial infarction.
• Treatment of myocardial infarction aims to restore flow to
the affected area as rapidly as possible while minimizing
reperfusion injury.
• It should be started as promptly as possible to avoid

15. Blood flow to the brain
• The brain receives 15% of the cardiac output
• Uses roughly 20% of the oxygen consumed by the body
• compared to the rest of the body has a rich blood supply
and a high oxygen demand.
• Hence, it has developed mechanisms to maintain blood
flow over a fairly wide range of perfusion pressures
• This process is termed auto regulation.

16. • Interruption of normal blood flow to the brain and spinal
cord may produce irreversible parenchymal injury within a
few minutes.
• Such vascular insults to the brain results in stroke

18. • two paired arteries which are responsible for the
blood supply to the brain
• vertebral arteries
• internal carotid arteries.
• These arteries arise in the neck, and ascend to the
cranium
Circle of Willis
• separate arteries branch from the internal carotids
and vertebral arteries
• arterial anastomosis interconnects them to form a
circle of connecting arteries at base of brain
• more than one route for blood to get to brain

21. • There are three main (paired) constituents of the
Circle of Willis:
• Anterior cerebral arteries: These are terminal
branches of the internal carotids.
• Internal carotid arteries: Present immediately
proximal to the origin of the middle cerebral
arteries.
• Posterior cerebral arteries: These are terminal
branches of the vertebral arteries.

22. • To complete the circle, two ‘connecting vessels’ are also
present:
• Anterior communicating artery: This artery connects the
two
• Posterior communicating artery: A branch of the internal
carotid, this artery connects the ICA to the posterior cerebral
artery

23. Regional blood flow to the cerebrum

24. Cushing reflex
• Cushing reflex is a protective reflex that helps save the brain
tissues from ischemic effects during the periods of reduced
cerebral blood flow
• Increase in intracranial pressure or increase in CSF pressure
compresses the cerebral blood vessels and decreases the blood
flow.
• However, blood flow is decreased only for a short period.
• It is restored immediately by means of Cushing reflex.
• When cerebral blood flow decreases by the compression of
cerebral arteries, the cerebral ischemia develops.

25. • Compression of blood vessels decreases the blood flow to
vasomotor center also.
• Local hypoxia and hypercapnea activate vasomotor center,
resulting in peripheral vasoconstriction and rise in the
arterial pressure.
• The increased arterial pressure helps to restore the cerebral
blood flow

26. Cushing reflex

27. • Cushing reflex operates only when the rise in arterial blood
pressure is proportional to increase in intracranial pressure.
• When the increase in intracranial pressure is very high and
it exceeds the arterial blood pressure, this protective
mechanism fails.
• Cerebral ischemia becomes severe, leading to irreversible
damage of the brain tissues
• Hence, it is not advisable to lower blood pressure
immediately after a stroke, to prevent further ischaemia

28. STROKE
• When the blood supply to a part of the brain is interrupted,
ischemia damages or kills the cells in the area, producing
signs and symptoms of a stroke.
• There are two general types of strokes:
hemorrhagic
 ischemic.
• Hemorrhagic stroke occurs when a cerebral artery or
arteriole ruptures, sometimes but not always at the site of a
small aneurysm.
• Ischemic stroke occurs when flow in a vessel is
compromised by atherosclerotic plaques on which thrombi
form.
.

29. • Thrombi may also be produced elsewhere (eg, in the atria in
patients with atrial fibrillation) and pass to the brain as
emboli where they then lodge and interrupt flow.
• In the past, little could be done to modify the course of a
stroke and its consequences
• it has now become clear that in the penumbra, the area
surrounding the most severe brain damage, ischemia
reduces glutamate uptake by astrocytes, and the increase in
local glutamate causes excitotoxic damage and death to
neurons

30. • In experimental animals, and perhaps in humans, drugs that
prevent this excitotoxic damage significantly reduce the effects
of strokes.
• In addition, clot-lysing drugs such as tissue type plasminogen
activator (t-PA) are of benefit in ischemic strokes.
• Both anti excitotoxic treatment and t-PA must be given early in
the course of a stroke to be of maximum benefit
• This is why stroke has become a condition in which rapid
diagnosis and treatment have become important.
• It is important to determine if a stroke is thrombotic or
hemorrhagic, since clot lysis is contraindicated in the latter.

31. Hepatic Portal System
• Another special circulation
• Found in the liver
• veins from spleen, stomach, pancreas,
• Gall bladder, intestines, superior and inferior mesenteric
merge to form hepatic portal vein
• do not take blood directly to vena cava instead take it to liver
for “inspection”
• phagocytic cells remove toxins , vitamins and minerals are
stored