Blood, haemostasis and thrombosis

Blood is one of those under-appreciated things. It continuously circulates the body delivering oxygen and nutrients to where they need to be, and removing things that shouldn’t be there. The heart pumps it around and has been doing so without fail, since you were about minus 34 weeks old (ie. a very young fetus). Haemostasis is the balanced interaction of blood cells, blood vessels and plasma proteins. This post will look at how the body reacts in response to injury to the endothelium of a blood vessel and what are some potential consequences of this.

First of all, we need to look at the vessels that blood travels through. The inner most layer is know as the intima, the middle layer is the media, and the outermost layer is the adventitia. The relative thickness of these depend on the types of vessels, for example, arteries have comparatively more smooth muscle (part of the media) than veins. Blood generally has laminar flow where cellular elements of the blood move quickly through the centre of the lumen, surrounded by slower moving plasma. The endothelium of the vessel is generally non-thrombogenic (it does not enable blood to attach to it and form a thrombus) however if the endothelium is injured, there is abnormal blood flow, or hypercoagulation of the blood, the formation of a thrombus is more likely.

Let’s imagine that your endothelium was injured. Immediately after the injury occured, there would be vasoconstriction of the vessel, platelets become spherical with projections and adhere to the endothelium and each other, basically functioning to plug the hole. Minutes later, coagulation factors become activated and fibrin forms. Within hours, fibrinolysis is activated to breakdown the clot and remove it.

A thrombus is a solid mass of blood and constituents formed within an intact vascular system. It is more common in patients with artherosclerosis, where fibro-fatty lesions in the intima, protrude into the vessel lumen causing turbulent blood flow, or in veins where blood can be quite slow flowing. After formation of a thrombus on the vessel wall, there is the potential for it to fragment, and form an embolus that is free to move elsewhere. A danger associated with an embolus, is that it can be carried in the blood and can get lodged in another vessel. This might be when flowing through arteries to progressively smaller vessels causing tissue necrosis, or it could be as blood enters the pulmonary circulation.

This is a very brief overview of how the blood responds to endothelium injury, and what some of the consequences are if a thrombus forms.


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