Transport Of Oxygen

 TRANSPORT OF OXYGEN

Hemoglobin

• The protein hemoglobin is a molecule that is responsible for carrying almost all of the oxygen in the blood. It is composed of four subunits, each with a heme group plus a globin chain. The heme group is composed of a porphyrin ring which contains an iron (Fe) atom in its center. Normally, the Fe is in the ferrous form and can reversibly bind oxygen. 

• When the iron atom in the heme group becomes oxidized, it changes from ferrous to ferric form. The hemoglobin is then called methemoglobin or ferrihemoglobin. 

• The Ferric form of iron will not bind oxygen.

• Hemoglobin molecules are highly toxic substances, if it is leaked in nephrons then it will damage the nephron that's why hemoglobin is packed in the RBCs.

• In the resting person, the Partial pressure of oxygen in blood coming back to the lung is 40mmHg & the partial pressure of oxygen in alveoli is 100mmHg.

• When hemoglobin is gone back the lung for oxygenation is already 75% oxygenated, & when blood reaches to pulmonary capillary then oxygen moves higher pressure from alveoli to low pressure in plasma blood by simple diffusion until the partial pressure becomes equal.

OXYGEN IN BLOOD

Oxygen is carried in the blood in two forms: 

1. Dissolved in plasma and RBC water (about 2% of the total) 

•  Henry's law states that the amount of oxygen dissolved in the plasma is directly proportional to PO2.

• Its mean partial pressure of oxygen in the blood is determined by only dissolved oxygen in plasma & fluid of RBCs.

2. Reversibly bound to hemoglobin (about 98% of the total).

• Hemoglobin changes shape based on How many oxygen molecules are bound to it. The change in the shape of hemoglobin also causes a change in the affinity to oxygen which means the more oxygen is bound, the higher the hemoglobin affinity for oxygen, this is called positive cooperative binding.

TOTAL AMOUNT OF OXYGEN DISSOLVE IN 100mL OF BLOOD AT PO2 = 100mmHg & 100% hemoglobin saturation

Amount of hemoglobin bound oxygen in 100mL blood at PO2 = 100mmHg

 

1 gram Hemoglobin bind at 100% Hb saturation is 1.34mL oxygen

Hemoglobin in a healthy person is 15g Hb/100mL of blood

15g hemoglobin bind with 1.34X15= 20.1mL oxygen

Amount of Dissolved oxygen in 100mL blood at PO2 = 100mmHg

 0.003mL oxygen dissolve in 100mL of blood at Po2 = 1mmHg

Amount of dissolved oxygen at Po2= 100mmHg is 0.0030X100 = 0.3mL oxygen

TOTAL AMOUNT OF OXYGEN DISSOLVE IN 100mL OF BLOOD AT PO2 = 100mmHg is 20.1+0.3 = 20.4mL oxygen

Due to the mixing of deoxygenated blood in oxygenated blood in the pulmonary vein because bronchial vein drained in pulmonary vein then :

• The partial pressure of oxygen is dropped from 100mmHg to 95mmHg 

• % hemoglobin saturation becomes 95%

• Content of oxygen in 100mL of blood become 20mL 

Due to the continuous use of oxygen by aerobic respiration by the tissues, the partial pressure of oxygen in tissues is 15-40mmHg. So, oxygen moves from high-pressure systemic capillaries to low-pressure interstitial fluid until the pressure becomes equal. Then oxygen shifts to cells for aerobic respiration.

Dissolved oxygen first diffuses to interstitium then the partial pressure of oxygen in plasma is dramatically reduced, so hemoglobin bound oxygen comes to plasma to maintain the partial pressure of oxygen in the blood.

100mL of oxygenated blood supplies about 5mL oxygen to the tissue at Rest.

Characteristic of 100mL of venous blood:

• The partial pressure of oxygen is 40mmHg

• Dissolve oxygen in plasma is 0.12mL

• % hemoglobin saturation is 75%

• The content of oxygen is 15mL

Now again blood is ready to fully saturated in the lung.