Abnormalities of Blood Flow

Hello :) The next few Pathology posts will discuss the effects of vascular disturbances in the body. This post will address abnormalities in blood flow which may occur, particularly hyperaemia, blood volume expansion and ischemia. 

Hyperaemia

Simple put, hyperaemia occurs when there is too much blood in a part of the vascular system. Hyperaemia can be active of passive and both types can be acute (rapidly developing) or chronic (slowly developing). 


Active Hyperaemia

Active Hyperaemia occurs when the blood vessels leading to an area of the body actively dilate and cause an increased flow of blood to that area. Active hyperaemia appears as a red/pink region on the surface of an organ (this is due to the increased supply of oxygenated blood). It may be pathological (occurs as a part of disease) or physiological (occurs normally). Active hyperaemia is pathological when it occurs as part of inflammation. If the active hyperaemia is pathological, inflammatory cells and engorged blood vessels will be seen histologically. Examples of when physiological hyperaemia may occur include:

• Blushing
• During Digestion
• Muscles during exercise

 Passive Hyperaemia

Passive hyperaemia is always pathological and is also known as congestion. It occurs because the venous drainage leading from a part of the body is reduced. Passive hyperaemia is usually quite a problem because the affected area does not receive a good oxygen supply. In addition, because poor drainage is present metabolic wastes build up and add further insult to injury. Passive hyperaemia appears as a dull red colour on the surface of an organ. An example of passive hyperaemia is when a tourniquet has been applied to a limb and the outflow of blood is obstructed.

Blood Volume Expansion

When there is too much blood flowing in the circulatory system (this may be due to polydipsia, high salt intake or done intentionally [eg. blood doping]) the body uses mechanisms to try and reduce the amount of blood in the system. The increase in blood volume causes an increase in atrial stretch and pressure. This stimulus triggers the cardiomyocytes to release ANP (atrial natriuretic peptide) and BNP (brain natriuretic peptide) which causes a reduction in the extracellular fluid volume and blood pressure by causing:

• Vasodilation: this causes more blood to enter tissues and so less is able to return back to the heart, reducing atrial stretch and pressure. 
• Increased glomerular filtration rate: this increases the amount of blood that is filtered by the kidneys and also allows more Na to be excreted in the urine (increasing urine production). 
• Increased sodium excretion: Since water tends to follow sodium in the body, this reduces the amount of water drawn into the blood by Na.
• Inhibition of the Renin-Angiotensin-Aldosterone System (RAAS): this prevents the hypertension that is caused by the RAAS (see this post).

• Increased vascular permeability: this increases the amount of blood that enters the tissues. 
• Inhibited endothelin release (endothelins normally constrict blood vessels and increase blood pressure)
• Inhibited vascular smooth muscle, endothelial cell and cardiac myocyte proliferation: this prevents the blood vessels from narrowing.

Ischemia

Ischemia occurs when there is not enough blood in a region of the circulatory system. It may be caused by anaemia (generalised ischemia), an intramluminal blockage of the vessel (eg a blood clot) or an extraluminal blockage of the vessel (eg by a tumor) - the latter two would be considered localised ischemia. Ischemic regions of an organ appear pale and will feel cool in a live animal. If the ischemia occurs rapidly, the cells in the affected area are likely to die to due a poor oxygen supply. However, if ischemia progresses slowly, tissues may be able to adapt.  



That's all for now, see you next time :)