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The adaptation of red blood cells

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Aerobic respiration provides the energy required by all living organisms to function normally, as muscle movement and certain cell reactions are reliant on energy the transport of oxygen to the cells of tissues and organs by blood vessels is vital for respiration to occur. Oxygen is carried in red blood cells by the pigment haemoglobin, a globular protein consisting of four polypeptide chains. Both of these components of blood are structured specifically to carry out their functions of oxygen transport, hence ensuring efficient respiration.

Erythrocytes, or red blood cells, possess a unique structure that enables them to play their role in oxygen transport. They are small, at 7μm in diameter, compared to that of average animal cells (approximately 20μm). This allows them to be transported via the thin-walled capillaries, which have very small lumen of less than 8μm in diameter, carrying oxygen as close as possible to the respiring tissues. Haemoglobin within the small red blood cell can exchange oxygen with the external environment quickly, as they are close to the plasma membrane; this again allows efficient diffusion of oxygen.

Red blood cells are described as biconcave discs, flat, with a dent in both sides. This shape defines the cell’s large surface area to volume ratio, indicating that oxygen can be transferred quickly between the cell and its surroundings, as haemoglobin molecules are close to the cell plasma membrane.

Unlike general animal cells, erythrocytes lack nuclei, mitochondrion and endoplasmic reticulum, giving space more haemoglobin can be carried by the red blood cells with such a structure, hence increasing the number of oxygen molecules being transported. The structure of red blood cells maximizes the amount of oxygen carried in the blood and the rate at which they reach and diffuse into oxygen requiring cells.

Oxygen is combined with haemoglobin in red blood cells in order to be transported. The four polypeptide chains of a haemoglobin molecule each contain a prosthetic haem group, able to bond with a single oxygen molecule (two oxygen atoms). An iron ion, Fe (II), in the haem group binds with an oxygen molecule. Haemoglobin molecules which are combined with oxygen are known as oxyhaemoglobin, and are bright red in colour. Therefore, a single haemoglobin molecule can carry a maximum of four oxygen molecules, or eight oxygen atoms.

To ensure efficient respiration, haemoglobin must combine with and release oxygen easily, providing respiring tissues with a constant supply of oxygen. When haemoglobin combines with the maximum amount of oxygen, it is termed saturated. The haemoglobin dissociation curve outlines the ability haemoglobin molecules to do so when exposed to different concentrations, or partial pressures, of oxygen. It shows that at low concentrations of oxygen, haemoglobin binds with small amounts of oxygen, giving low percentage saturation. The percentage saturation increases with the partial pressure of oxygen, this behavior indicates the high percentage saturation of haemoglobin in the presence of high partial pressures of oxygen, such as in the lungs, and contributes to the readiness of haemoglobin to release oxygen into respiring tissues with low oxygen concentrations.

The shape of the dissociation curve reflects the ability of haemoglobin to change shape following the combination of one oxygen molecule with one haem group, making it easier for successive combinations of oxygen molecules with Fe (II) ions to occur. The gradient of the curve changes, it becomes steeper as more oxygen molecules bind with haem groups, indicating that a small change in partial pressure is countered by a large change in percentage saturation. This property increases the rate at which oxygen I associated with haemoglobin, and ultimately affects the transport of oxygen and the process of respiration positively.

Saturation of haemoglobin is influenced by carbon dioxide concentration from respiring cells, as well as by the partial pressure of oxygen. A high partial pressure of carbon dioxide results in the release of oxygen by haemoglobin; the entire curve shifts to the right. This is known as the Bohr shift and is advantageous in that high carbon dioxide concentrations in respiring tissues cause haemoglobin to release the much needed oxygen more readily, ensuring efficient respiration in cells. Carbon dioxide diffused into red blood cells dissolve to give carbonic acids which break down to form hydrogen ions and hydrogen carbonate ions. Haemoglobin reacts with these hydrogen ions to haemoglobinic acid, HHB, and releases the oxygen being carried in the process. This ensures that oxygen is readily released into areas of high carbon dioxide concentration and low partial pressure of oxygen. The effect of carbon dioxide on haemoglobin and its ability to transport oxygen shows that the adaptation of the molecule ensures efficient respiration.

Although haemoglobin and red blood cells are clearly very effective in aiding respiration, their efficiency can be hindered by certain factors. Carbon monoxide found in exhaust fumes and cigarette smoke bind more easily (250 times more readily than oxygen), and almost irreversibly with haemoglobin to form carboxyhaemoglobin. Even at low concentrations, carbon monoxide can be fatal in preventing oxygen from binding with haemoglobin and ultimately decreasing the amount of oxygen being transported, causing suffocation. The structure and properties of haemoglobin that favour respiration cannot prevent the disruption of oxygen transport by carbon monoxide, therefore it can be concluded that haemoglobin are only near perfection by design.

Low air pressures at high altitudes decrease haemoglobin saturation in the lungs due to the low partial pressure of oxygen present. This in turn means that less oxygen will be supplied to tissues that require it, hindering respiration and resulting in altitude sickness. An increased red blood cell count can overcome this lack of oxygen; however there appears to be no adaptation by the individual red blood cells to prevent this hindrance by high altitudes, therefore the red blood cells and haemoglobin are only almost ideal for their role as oxygen transporters.

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