Some diseases also display red cells of abnormal shape—e.g., oval in pernicious anemia, crescent-shaped in sickle cell anemia, and with projections giving a thorny appearance in the hereditary disorder acanthocytosis.
The number of red cells and the amount of hemoglobin vary among different individuals and under different conditions; the number is higher, for example, in persons who live at high altitudes and in the disease polycythemia.
Single celled organisms such as bacteria and fungi, and some multicellular creatures such as sponges, corals and flatworms, simply absorb the nutrients they need and get rid of their waste using a passive process known as diffusion (which is much like soaking in and draining out).
More complex animals have developed some kind of circulatory system.
The first is haem, a flat ring structure that holds an iron atom at its centre.
Haem is held closely by proteins known as globin, and this combination forms haemoglobin, which is itself packaged up in red blood cells to be transported around the body.
The cell is flexible and assumes a bell shape as it passes through extremely small blood vessels.
It is covered with a membrane composed of lipids and proteins, lacks a nucleus, and contains The function of the red cell and its hemoglobin is to carry oxygen from the lungs or gills to all the body tissues and to carry carbon dioxide, a waste product of metabolism, to the lungs, where it is excreted.
The average red cell in humans lives 100–120 days; there are some 5.2 million red cells per cubic millimetre of blood in the adult human.
Though red cells are usually round, a small proportion are oval in the normal person, and in certain hereditary states a higher proportion may be oval.