]Red Blood Cells
The function of red blood cells is to facilitate transport of respiratory gases between the lungs and cells.
It has a biconcave disk shape that curves inward on both surfaces. It looks like a jelly donut, but the
cross-section looks like a dumbbell, thinner in the middle and thicker at the edges. There are 2
advantages of having such a shape. One, the shape provides maximal surface area and minimal
diffusion distance for its volume. The narrow middle allows gases to diffuse in and out very rapidly, and
increases the efficiency of oxygen and CO2 diffusion. Two, the shape has a high degree of flexibility to it.
Red blood cells can flex itself to fit into small capillaries. The flexibility allows the cell to squeeze
through narrow capillaries. If the red blood cell was spherical, it would not be able to squeeze through.
RBCs are the most abundant blood cell. In males there are about 5 to 5.5 million RBCs per microlitre,
and in females about 4.5 million RBCs per microlitre. There are no subcellular organelles in RBCs. The
red blood cell is composed of almost 2/3 water, a little bit of proteins, lipids, and ions, and the
remaining one third is hemoglobin. RBCs have 2 important enzyme systems. First the glycolytic enzymes
generate energy. There is no mitochondria so ATP is produced anaerobically. Second are the carbonic
anhydrase, which transport CO2. These enzymes deteriorate as the cell ages. The RBC has no nucleus
and no ribosomes so it cannot regenerate itself.
Hemoglobin has 4 globular proteins. 2 pairs of identical globular proteins are known as globin. It’s
made of 2 alpha and 2 beta chains. The molecular weight of hemoglobin is 64 kilodaltons (about the
same size as albumin, which is 69). Each RBC has 200-300 million molecules. Associated with each 1 of
the 4 chains is heme (for a total of 4 hemes per hemoglobin). Each heme molecule has 1 iron atom in
the 4s form, and each iron nhas an oxygen molecule. In the lungs, hemoglobin becomes saturated with
oxygen (and appears bright red). This is why arterial blood is bright red. In the tissues, oxygen
dissociates from hemoglobin (and appears dark red). This is why venus blood is dark red.
Hemoglobin has many functions. It needs a certain amount of oxygen to carry out energy. The
solubility of oxygen in plasma is very low and not enough for humans. In blood (because of hemoglobin)
the carrying capacity is 20mL of oxygen per 100mL of blood, that’s 70x more than plasma. As a result of
iron being able to bind to oxygen forms a reversible complex called oxyhemoglobin. Another function of
hemoglobin is to transport CO2, but only to some extent. It also acts as a buffer and maintains blood pH
levels. Males have 16g hemoglobin per 100mL and females have 14. Each gram of hemoglobin holds
1.34mL O2. Factors affecting the ability of hemoglobin to bind and release to oxygen include
temperature, ionic composition, pH, CO2, and intracellular enzyme concentration.
The production of RBC is called erythropoiesis. As a fetus, sites of hematopoiesis takes place in the yolk
sac. After the end of the first trimester, it is produced in the liver and spleen. At birth, blood cell