#16
Erythrocytes aka red blood cells make up about 44% of the blood in the human body and is a biconcave disk and takes it colour from pigment hemoglobing. Oxygenated blood is bright red and as oxygen is released the color becomes a darker blue-red. The average male adult has 5.5 million red blood cells per millimetre in his body, while the average female has about 4.5 million red blood cells per millimetre of blood. The red blood cell is specially designed for carrying oxygen through the arteries. A minuscule 2% of dissolved oxygen that enters the blood stream is transported by plasma. A mature red blood cell has no nucleus.Instead each cell is filled with about 280 million molecules of the respiratory pigment called hemoglobin, a molecule containing iron that binds with oxygen. Without hemoglobin oxygen would not be able to travel through the blood. There are two major factors in mammals that play a role in determining when oxygen is picked up and released. The two factors are the concentration of oxygen and the acidity of the surrounding fluid. The concentration of oxygen is usually measured in partial pressure. When the partial pressure of oxygen is low, the bond linking oxygen and hemoglobin weakens. Then the hemoglobin molecule will tend to release it's oxygen molecule(s) that it is carrying. An increase in acidity will also loosen the bond between the oxygen and hemoglobin and lead to the release of oxygen. But the acidity is influenced by the presence of dissolved carbon dioxide.