Blood

Blood is a complex fluid  that transports a variety of molecules around the body. It carries diatomic Oxygen and the nutrients essential for the organism to function, but it also carries the wastes  produced by these same organs (Carbon Dioxide, nitrogenous wastes).

It also carries the many molecules associated with the immune system, and diffuses hormones throughout the organism.

• To define the principal constituents of blood.
• To illustrate that blood is set into motion by the heart, and that it circulates in only one direction.
• To illustrate that blood is propelled by the heart, at the rhythm of the cardiac pulses. This is the circulatory system.

This animation illustrates only a part of the composition of blood. In effect, blood is 55% plasma, the yellowish liquid that gives it fluidity.

Plasma is 90% water. It is for this reason that the density of blood is close to that of water.

The remaining 45% is made up of hematocytes. These are primarily red blood cells (99%), platelets (<1%) and white blood cells (<1%).

All three of these blood cell types  are produced in the marrow of certain bones. The same embryonic stem cell can develop into a red blood cell, a white blood cell or a platelet via a complex physiological process called hematopoiesis.

• The red blood cells (also called erythrocytes) contain hemoglobin, a protein whose function is to transport diatomic Oxygen and Carbon Dioxide. Rich in Iron, it gives blood its red color.
• The white blood cells (also called leucocytes) take on different forms, all having the function of defending the organism against foreign invaders.  They are the basis of our immune system, and their numbers can increase rapidly in case of infection.
• The platelets (also called thrombocytes)  react in the event of a lesion in the vascular wall, in order to minimize blood loss. It is this function of producing coagulation that, among other things, that causes the appearance of a scab on a scar. These same platelets are at the origin of blood clots.   

Did you know?

• Red blood cells have a life expectancy of 3 to 4 months (Platelets only live for 10 days),
• The adult human body contains about 5 liters of blood (3 liters in a child),
• Blood always circulates in the same direction inside a network of more than 10,000 km of blood vessels (arteries, veins, capillaries).