Transport of Gases

Oxygen Transport

Each decilitre of blood carries 19.8 ml of 02 of which 5 ml diffuses into tissues.

3% is transported dissolved in plasma and 97% is carried by the RBCs. Four Fe2+ ions of each haemoglobin can bind with 4 molecules of O2 and it is carried as oxyhaemoglobin.

Oxyhaemoglobin dissociates near tissues due to increase in acidity and decrease in pH. It can also be caused due to high temperature.

In a normal person, the haemoglobin level is about 15 per 100 ml.

The capacity of 1 g of haemoglobin to combine with O2 is 1.34 ml.

Therefore, arterial blood carries about 20 ml of O2/100 ml of blood.

Under normal condition, the O2 level falls to about 14.4 ml/100 ml in the venules.

It indicates that under normal condition, approximately 5 ml oxygen is transported by blood.

Under strenous conditions or during exercise, the O2 level falls to about 4.4 ml/100 ml i.e., approximately 15 ml of O2 is transported by Hb during exercise.

Bohr's Effect:

The relationship between the pO2 and percent saturation of haemoglobin when represented on a graph is termed as oxygen-haemoglobin dissociation curve and is sigmoid in shape. A rise in pCO2 or fall in pH decreases oxygen affinity of haemoglobin, raising the P50 value. This is called Bohr's effect (P50 value is the value of pO2 at which haemoglobin is 50% saturated with oxygen to form oxyhaemoglobin). Conversely a fall in pCO2 and rise in pH increases oxygen affinity of haemoglobin and shifts the curve to left. Foetal haemoglobin has a higher affinity for O2 because it binds BPG less strongly. Therefore, oxygen haemoglobin dissociation curve for foetal haemoglobin will appear on the left side.

Oxygen-hemoglobin dissociation curve at normal body
temperature showing the relationship between haemoglobin saturation and pO2

Myoglobin present in the muscles also has more affinity for O2. But since it has only one Fe2+ group, the curve obtained will be hyperbolic, not sigmoid.

Dissociation curve for haemoglobin and myoglobin at 37°C, pCO2 40 mmHg and pH 7.

Concept Builder

1. Carbon Monoxide Poisoning:

If a person sleeps in a closed room with a lamp burning the absence of sufficient amount of oxygen causes an incomplete combustion of carbon and produces carbon monoxide in the room.

As the person inhales carbon monoxide, it diffuses from the alveolar air to the blood and binds to haemoglobin forming carboxyhaemoglobin.

The later is a relatively stable compound and cannot bind with oxygen.

So, the amount of haemoglobin available for oxygen transport is reduced.

The resulting deficiency of oxygen causes headache, dizziness, nausea and even death.

Carbon monoxide combines with haemoglobin at the same point on haemoglobin molecule as does oxygen.

It binds with haemoglobin 250 times faster than oxygen.

2. SARS (Severe Acute Respiratory Syndrome):

The first patient of SARS was reported on February 26, 2003 in China.

The causative agent is human Corona virus.

It is a new member of influenza virus family which is considered as a mutant form of influenza virus.

Carbon Dioxide Transport

CO2 is transported in three ways :

1. In dissolved State: About 7% of CO2 is transported after getting dissolved in plasma. The pCO2 in the arterial blood is 40 mm of Hg and in the venous blood, it is 45 mm of Hg. About 0.3 ml of extra CO2 is carried per 100 ml of blood in this form.

2. As Bicarbonate: Nearly 70% of CO2 is transported from tissues to lungs in this form. CO2 diffuses from tissues into the blood and enters RBCs. It combines with water to form H2CO3 which dissociates into H+ and HCO3. Being catalysed by carbonic anhydrase, it is a very fast step.

H+ combine with hemoglobin replacing its association with K+ and form hemoglobinic acid.

Due to it, the level of HCO3 increases in RBCs, which start coming out of it along the concentration gradient. To maintain ionic balance, Cl move in from plasma into RBCs.
In the plasma HCO3 combine with Na+ or K+ to form NaHCO3 or KHCO3.

3. As Carbamino-Hb : About 20-25% of CO2 is transported in this mode. CO2 combines with NH2 group of Hb and forms carbamino-Hb. This combination of CO2 and Hb is a reversible reaction.

Transport of the respiratory gases

Release of CO2 in the alveoli of lung:

In the pulmonary capillaries, CO2 starts diffusing out into alveoli.

Carbamino-Hb spits into CO2 and Hb. As Hb of RBC takes up O2, it releases H+ in RBC.

The H+ start combining with the available HCO3 in RBC to form H2CO3 which splits into H2O + CO2 and CO2 starts diffusing out (Reverse of the reactions).

As a result HCO3 from plasma starts moving in along concentration gradient and for ionic balance; Cl start moving out.

This way CO2 is released into lungs.

Hamburger's Phenomenon:

HCO3-ions diffuse out into plasma and Cl ions enter into the RBCs at the level of tissues (intemal respiration). This is known as "chloride shift" or "Hamburger phenomenon". At the level of external respiration or alveoli, Cl move out as HCO3 move in this is called reverse of chloride shift.

Haldane's Effect

It is related to the transport of CO2 in the blood. It is based on the simple fact that oxyhaemoglobin behaves as strong acid and releases an excess of H+ ions which bind with bicarbonate (HCO3) ions to form H2CO3 which dissociates into H2O and CO2.

Secondly, due to the increased acidity, CO2 loses the power to combine with haemoglobin and form carbamino-haemoglobin.

Effect of oxyhaemoglobin formation or dissociation on CO2 transport is called Haldane's effect