Page 141 - Textbook of Pathology, 6th Edition

3. Causes in the veins. Blockage of venous drainage may Interarterial anastomoses in the 3 main trunks of the 125
lead to engorgement and obstruction to arterial blood supply coronary arterial system.
resulting in ischaemia. The examples include the following: iii) Parallel arterial supply. Blood supply to some organs and
i) Luminal occlusion of vein: tissues is such that the vitality of the tissue is maintained by
a) Thrombosis of mesenteric veins alternative blood supply in case of occlusion of one. For
b) Cavernous sinus thrombosis example:
ii) Causes in the vessel wall of vein: Blood supply to the brain in the region of circle of Willis. CHAPTER 5
a) Varicose veins of the legs Arterial supply to forearm by radial and ulnar arteries.
iii) Outside pressure on vein: iv) Double blood supply. The effect of occlusion of one set of
a) Strangulated hernia vessels is modified if an organ has dual blood supply. For
b) Intussusception example:
c) Volvulus
Lungs are perfused by bronchial circulation as well as
4. Causes in the microcirculation. Ischaemia may result by pulmonary arterial branches.
from occlusion of arterioles, capillaries and venules. The Liver is supplied by both portal circulation and hepatic
causes are as under: arterial flow.
However, collateral circulation is of little value if the
i) Luminal occlusion in microvasculature:
a) By red cells e.g. in sickle cell anaemia, red cells parasitised vessels are severely affected with spasm, atheroma or any
by malaria, acquired haemolytic anaemia, sludging of the other such condition.
blood. 2. General and cardiovascular status. The general status
b) By white cells e.g. in chronic myeloid leukaemia of an individual as regards cardiovascular function is an
c) By fibrin e.g. defibrination syndrome important determinant to assess the effect of ischaemia. Some
d) By precipitated cryoglobulins of the factors which render the tissues more vulnerable to
e) By fat embolism the effects of ischaemia are as under: Derangements of Homeostasis and Haemodynamics
f) In decompression sickness. i) Anaemias (sickle cell anaemia, in particular)
ii) Causes in the microvasculature wall: ii) Lowered oxygenation of blood (hypoxaemia)
a) Vasculitis e.g. in polyarteritis nodosa, Henoch-Schönlein iii) Senility with marked coronary atherosclerosis
purpura, Arthus reaction, septicaemia. iv) Cardiac failure
b) Frost-bite injuring the wall of small blood vessels. v) Blood loss
vi) Shock.
iii) Outside pressure on microvasculature:
a) Bedsores. 3. Type of tissue affected. The vulnerability of tissue of the
body to the effect of ischaemia is variable. The mesenchymal
FACTORS DETERMINING SEVERITY OF ISCHAEMIC tissues are quite resistant to the effect of ischaemia as
INJURY. The extent of damage produced by ischaemia due compared to parenchymal cells of the organs. The following
to occlusion of arterial or venous blood vessels depends upon tissues are more vulnerable to ischaemia:
a number of factors. These are as under: i) Brain (cerebral cortical neurons, in particular).
ii) Heart (myocardial cells).
1. Anatomic pattern. The extent of injury by ischaemia iii) Kidney (especially epithelial cells of proximal convoluted
depends upon the anatomic pattern of arterial blood supply tubules).
of the organ or tissue affected. There are 4 different patterns
of arterial blood supply: 4. Rapidity of development. Sudden vascular obstruction
results in more severe effects of ischaemia than if it is gradual
i) Single arterial supply without anastomosis. Some organs since there is less time for collaterals to develop.
receive blood supply from arteries which do not have
significant anastomosis and are thus functional end-arteries. 5. Degree of vascular occlusion. Complete vascular
Occlusion of such vessels invariably results in ischaemic obstruction results in more severe ischaemic injury than the
necrosis. For example: partial occlusion.
Central artery of the retina
Interlobular arteries of the kidneys. EFFECTS. The effects of ischaemia are variable and range
from ‘no change’ to ‘sudden death’.
ii) Single arterial supply with rich anastomosis. Arterial supply
to some organs has rich interarterial anastomoses so that 1. No effects on the tissues, if the collateral channels
blockage of one vessel can re-establish blood supply develop adequately so that the effect of ischaemia fails to
bypassing the blocked arterial branch, and hence the occur.
infarction is less common in such circumstances. For 2. Functional disturbances. These result when collateral
example: channels are able to supply blood during normal activity but
Superior mesenteric artery supplying blood to the small the supply is not adequate to withstand the effect of exertion.
intestine. The examples are angina pectoris and intermittent
Inferior mesenteric artery supplying blood to distal colon. claudication.
Arterial supply to the stomach by 3 separate vessels 3. Cellular changes. Partial and gradual ischaemia may
derived from coeliac axis. produce cellular changes such as cloudy swelling, fatty