The Transport Of Oxygen And Carbon Dioxide In The Blood

What is the most specialised transport role of the blood?

The transport of the oxygen from the lungs to the cells of the body by the erythrocytes.

What are the erythrocytes also involved in?

The removal of carbon dioxide from the cells and its transport to the lungs for gaseous exchange.

What is the main function of erythrocytes?

Transporting oxygen.

How are erythrocytes specialised?

• Biconcave shape - larger SA than a simple disc or sphere shape, increasing SA of diffusing gases and helping them pass through narrow capillaries.

• Contain haemoglobin - red pigment that carries oxygen and gives the red colour.

In adults, where are erythrocytes continuously formed?

In the red bone marrow.

By the time mature erythrocytes have lost their nuclei (maximising the amount of haemoglobin that can fit into the cells) what happpens?

Their life is limited, so they last for about 120 days in the bloodstream.

What is haemoglobin?

A very large globular conjugated protein made up of four peptide chains, each with an iron containing prosthetic group.

How many oxygen molecules can one haemoglobin molecule bind to?

Four oxygen molecules.

What forms when oxygen binds to haemoglobin?

Oxyhaemoglobin.

What is the equation for the formation of oxyhaemoglobin?

Haemoglobin + oxygen ⇌ oxyhaemoglobin

Hb + 4O₂ ⇌ Hb (0₂)₄

What happens when erythrocytes enter the capillaries in the lungs?

The oxygen levels in the cells are relatively low. This makes a steep concentration gradient between the inside of the erythrocytes and the air in the alveoli.

What happens when oxygen moves into the erythrocytes?

It binds with the haemoglobin.

What is positive cooperativity?

The arrangement of the haemoglobin means that as one oxygen binds to a haem group the molecule changes shape, making it easier for the next oxygen molecules to bind.

Because the oxygen is bound to the haemoglobin, what is the concentration of the free oxygen in the erythrocyte?

It stays low, so a steep diffusion gradient is maintained until all of the haemoglobin is saturated with oxygen.

What happens when blood reaches the body tissues?

The situation becomes reversed: the concentration of oxygen in the cytoplasm of the body cells is lower than in the erythrocytes.

What happens when the concentration of oxygen in the cytoplasm of the body cells is lower than in the erythrocytes?

Oxygen moves out (is unloaded) of the erythrocytes down a concentration gradient.

What happens once the first oxygen molecule is released by the haemoglobin?

The molecule again changes shape and it becomes easier to remove the remaining oxygen molecules.

What is an oxygen dissociation curve an important tool in understanding?

How the blood caries and releases oxygen.

What is the percentage saturation of haemoglobin with oxygen plotted against?

The partial pressure of oxygen.

What do oxygen dissociation curves show?

The affinity of haemoglobin for oxygen.

What does a very small change in the partial pressure of oxygen in the surroundings make?

A significant difference to the saturation of the haemoglobin with oxygen, because once the first molecule becomes attached, the change in shape of the haemoglobin molecule means that other oxygen molecules are added rapidly.

Where does the curve level out on an oxygen dissociation curve?

At the highest partial pressures of oxygen because all the haem groups are bound to oxygen and so the haemoglobin is saturated and cannot take up any more.

What does it mean when the haemoglobin is completely saturated?

At the high partial pressure of oxygen in the lungs, the haemoglobin in the red blood cells is rapidly loaded with oxygen.

What does a relatively small drop in oxygen levels in the respiring tissues mean?

Oxygen is released rapidly from the haemoglobin to diffuse into the cells. This effect is enhanced by the relatively low pH in the tissues compared with the lungs.

When we are not being active, how much of the oxygen carried in the erythrocytes is released to the body cells?

Only about 25%. The rest act as a reservoir for when the demands of the body increase suddenly.

What happens as the partial pressure of carbon dioxide rises?

Haemoglobin gives up oxygen more easily (affinity for oxygen decreases), this change is known as the Bohr effect.

Why is the Bohr effect important in the body and what happpens because of it?

• In active tissues with a high partial pressure of carbon dioxide haemoglobin gives up its oxygen more easily.

• In the lungs where the proportion of carbon dioxide in the air is relatively low, oxygen binds to the haemoglobin molecules easily.

When a fetus in developing in the uterus, what is it completely dependent on?

Its mother to suply it with oxygen. Oxygenated blood from the mother runs cose to the deoxygenated fetal blood in the placenta.

What would happen if fetal and normal haemoglobin had the same affinity for oxygen?

Little or no oxygen would be transferred to the blood of the fetus.

Which haemoglobin has a higher affinity for oxygen?

Fetal haemoglobin, so it removes oxygen from the maternal blood as they move past each other.

Why is it that fetal haemoglobin only has a slightly higher affinity for oxygen?

If it was very high, it could prevent it from unloading oxygen to the fetal (respiring) tissues.

What are the three different ways in ehich carbon dioxide is transported from the tissues to the lungs?

• About 5% is carried dissolved in the plasma.

• 10-20% is combined with the amino groups in the polypeptide chains of haemoglobin to form a compound called carbaminohaemoglobin.

• 75-85% is converted to hyfrogen carbonate ions in the cytoplasm of red blood cells.

Where is most of the carbon dioxide that diffuses into the blood from the cells transported to?

The lungs in the form of hydrogen carbonate ions.

What does carbon dioxide react slowly with water to form?

Carbonic acid. It then dissociates to form hydrogen ions and hydrogen carbonate ions

In the blood plasma, what is the speed of the reaction between carbon dioxide and water?

It happens slowly.

In the cytoplasm of the red blood cells where there are high levels of carbonic anahydrase, what is the speed of the reaction between carbon dioxide and water?

This enzyme catalyses the reverable reaction. The carbonic acid then dissociates to form hydrogen carbonate ions and hydrogen ions. The negatively charged hydrogen carbonate ions move out of the erythrocytes by diffusion down a concentration gradient and negatively charged chloride ions move into the erythrocytes, which maintains the electrical balance of the cell - the chloride shift.

By removing the arbon dioxide and converting it to hydrigen carbnate ions, what do the erythrocytes maintain?

A steep concentration gradient for carbon dioxide to diffuse from the respiring tissues into the erythrocytes.

What happens when the blood reaches the lung tissue, where there is a relatively low concentration of carbon dioxide?

Carbonic anhydrase catalyses the reverse reaction, breaking down carbonic acid into carbon dioxide and water. Hydrogen carbonate ions diffuse back into the erythrocytes and react with hydrogen ions to form more carbonic acid. When this is broken down by carbonic anhydrase, it releases free carbon dioxide, which diffuses out of the blood into the lungs. Chloride ions diffuse out of the red blood cells back into the plasma down an electrochemical gradient.

Haemoglobin in the erythrocytes also plays a role in this process. It acts as a buffer and prevents changes in the pH by accepting free hydrogen ions in a reversible reaction to form haemoglobinic acid.