Oedema

Hi :) In this post we'll learn about oedema. I'll explain what oedema is and how it occurs and finish off with some good examples of oedema. Enjoy :)

What is Oedema?

Oedema is the accumulation of excess extravascular fluids in tissue spaces or body cavities. Oedema can occur locally or there may be a general tendency for oedema to occur. Oedema may be a transudate or an exudate. Transudates are lower in proteins and cells and are non-inflammatory. An oedema may be termed an exudate if the spaces between the endothelial cells of the blood vessels have increased. This is usually a result of inflammation and results in fluid that is high in proteins and cells. Other differences include:

• Exudates will contain proteins that are normally found in blood plasma while transudates contain only albumin. 
• Exudates contain fibrin which causes clots, transudates have no fibrin.
• Exudates have a high specific gravity while transudates have a low specific gravity (this is because there are less proteins).

How Does Oedema Occur?

Oedema occurs because of changes in Starling Forces (see this post for more info). An oedema may occur when:

• The hydrostatic pressure of the blood increases. This may occur with active or passive hyperaemia.
• The oncotic pressure of the blood decreases. The most important protein in this situation is albumin. Low levels of albumin may be a result of malabsorption of amino acids in the gut, decreases synthesis of plasma proteins by the liver, or excessive loss of albumin through the kidney or gut.
• The hydrostatic pressure of the lymph fluid increases. This may occur due to a blockage of the lymph vessels (eg. a tumour).
• The oncotic pressure of the interstitial fluid increases. This occurs most commonly when there is an increased ion intake as well as renal insufficiency. 
• Increased endothelial permeability due to the neurogenic action of chemical mediators. 

Oedema usually forms in places of low tissue tension in different parts of the body. Grossly, it appears as swollen areas of tissue. Oedema may sometimes be described as 'pitting'. This is when a depression is formed in the tissue after it has been touched. Histologically, oedema can be difficult to spot but some signs to look out for include: a clear space around collagen bundles and enlarged lymphatics.  

There is a naming system used to describe where oedema is in the body and this is best explained with some examples. A transudate that forms in the pleural cavity is called 'hydrothorax', in the pericardium is called 'hydropericardium'. 'Ascites' is fluid accumulation in the peritoneal cavity while 'anasarca' refers to generalised subcutaneous oedema. 

Examples of Oedema

I'll include some examples of oedema and how they form here because it seems like this would be a good question to ask in an exam!

Ascites

Ascites (the accumulation of fluid in the abdomen) may occur as a result of right-sided heart failure. The process is described below:

• Right-Sided Heart Failure causes passive hyperaemia in the right side of the circulation (the systemic circulation).
• This causes congestion of the liver, especially in the periacinar regions.
• This causes the hydrostatic pressure in the sinusoids to increase. 
• Fluid is forced into the Space of Disse lining the sinusoids. 
• Fluid drains into the lymphatics around the portal triads and under the capsule
• Fluid leaks out through the capsule
• Ascites forms. 

Pulmonary Oedema 

Another important example is pulmonary oedema see in left-sided heart failure. 

1.   Left-Sided Heart Failure causes passive hyperaemia in the left side of circulation (the pulmonary circulation).
2. This causes congestion of the lungs
3. Increased hydrostatic pressure in the alveolar walls
4. Fluid is forced into alveoli
5. Pulmonary oedema forms
6. Over time, red blood cells and proteins leak via diapedis
7. Increased numbers of macrophages are attracted to the area to phagocytose the debris
8. Macrophages phagocytose haemosiderin. 

That's all for this post :) Please feel free to leave any suggestions, questions or comments in the space below.