Most physiology textbooks don’t spend too much space or time to discuss the physiology of blood. How strange!
The blood, streaming through our body, has many different functions, all crucially important for the functioning of our body.
In this introduction, some of these functions will be briefly described and further expanded in the next pages.
A. What is blood?
In general, blood is very similar to other tissues: it consists of cells and an extracellular environment. But in blood, this extracellular environment is fluid, called plasma. This plasma contains many compounds such as blood proteins, coagulation factors etc.
There are three types of cells in blood:
a. the red blood cells (=erythrocytes)
b. the white blood cells (=leucocytes)
c. the platelets.
The erythrocytes are involved in the transportation of oxygen and are discussed in the following pages in this chapter (D.2. Erythrocytes).
The leucocytes are involved in the continuous battle of our body against infections (D.3. Leukocytes).
The platelets are involved in stopping bleeding (=coagulation) and are discussed in D.4. Thrombocytes and Hemostasis.
The most common cells in the blood are:
a. erythrocytes (4-5 million cells/mm3),
b. followed by the platelets (250,000 – 500,000/mm3)
c. then the leucocytes (4,000-11,000/mm3).
The relation between cells and plasma is called the hematocrit; a very useful test of the blood.
It is usually determined by taking a drop of blood from a finger puncture, stored in a capillary tube and centrifuged for 5 minutes.
The centrifuging will push all the cells to one side of the tube (heavier) and the plasma to the other side (lighter).
As shown in the diagram, the red column consists of all the cells together and the transparent column is the plasma.
Since the vast majority of cells are erythrocytes (approx. 90%), the cell column is red. Sometimes, if there are a lot of leucocytes (because of an ongoing infection!), then a buffy coat (a white ring) becomes visible between the erythrocytes and the plasma.
From the diagram, you can see that a normal hematocrit is about 45%.
There is a whole system in our blood that stops the bleeding from our vessels if there is a hole in the wall of those vessels; the coagulation system (D.5. Coagulation System).
And, finally, the white blood cells (leukocytes) are there to protect us from infections (bacteria, viruses, strange bodies). This topic is the subject of Microbiology and Immunology (see D.3. Leukocytes).