• Partial pressure of Oxygen (O2):– In the air of the alveoli:

higher– In blood capillaries: lower

• Partial pressure of Carbon dioxide (CO2):– In the air of alveoli: lower– In blood capillaries:

higher

O2 diffuses across the surface of alveolus and blood capillaries into the blood.

CO2 diffuses out of the blood capillaries into the alveoli and then expelled through the nose or mouth into atmosphere

Oxygen transport in the blood• O2 in the blood that leaves the alveoli

transported to body cells for cellular respiration.

• In the blood:– O2 combines with haemoglobin

• In the lungs:– Partial pressure of O2 higher– Oxyhaemoglobin is formed– O2 carried in the form of oxyhaemoglobin to all

parts of the body.

Haemoglobin + oxygen oxyhaemoglobinLungs

Body cells

In the tissue: Partial pressure of O2 lower Oxyhaemoglobin dissociates O2 molecules release

Carbon dioxide transport CO2 released by respiring cells (from

tissue) can be transported in three ways: 7% transported as dissolved CO2 in blood

plasma 23% of CO2 combines with haemoglobin (Hb)

and transported as carbaminohaemoglobin 70% transported in the blood in the form of

bicarbonate ions (HCO3-)

CO2 released by body cells diffuses into blood plasma and then into red blood cells (RBC).

It reacts with water to form carbonic acid (H2CO3).

In the blood, carbonic acid is converted into bicarbonate ions (HCO3-) and hydrogen ions (H+) by the enzyme carbonic anhydrase.

HCO3- diffuse from RBC into blood plasma. Most of HCO3- carried in blood plasma to the lungs.

1

HCO3- diffuse from blood plasma into RBC to form H2CO3 again.

H2CO3 breaks down into CO2 and water to diffuse out of blood capillaries into the alveoli to be expelled during exhalation.

Smaller amount of CO2 combines with Hb to form carbaminohaemoglobin in RBC.

CO2 returned to the lung in this form.

CO2 gas can dissolve directly in blood plasma to be returned to the lungs.

2

3

In tissue capillaries; Partial pressure O2 in blood higher than

cells Oxyhaemoglobin breaks down and

releases O2 which then diffuses through capillary walls into the cells.

Cellular respiration; Partial pressure of CO2 higher in cells

than tissue capillaries CO2 diffuses out of cells into tissue

capillaries to be transported back to the lungs