Forms of Oxygen in Blood
Oxygen is transported in the blood in two forms:
- Dissolved O2: A small fraction (around 2%) of total oxygen is dissolved directly in the blood plasma. This dissolved oxygen is crucial as it is the only form that exerts partial pressure, driving oxygen diffusion across membranes.
- O2 Bound to Hemoglobin: The vast majority (around 98%) of oxygen is transported bound to hemoglobin, a protein found inside red blood cells (erythrocytes). Hemoglobin consists of four subunits (two α and two β chains), each containing a heme group with an iron atom. Each iron atom can reversibly bind one oxygen molecule, allowing a single hemoglobin molecule to carry up to four oxygen molecules. This binding of oxygen to hemoglobin is rapid and reversible. When oxygenated, it is called oxyhemoglobin, and when deoxygenated, it's called deoxyhemoglobin.
♦ Haemoglobin: Haemoglobin is the molecule that allows O2 to bind and dissociate in the blood, One molecule of Haemoglobin (Hb) binds 4 molecules of O2.
There are 3 main types of haemoglobin.
- Haemoglobin A, is composed of 2 α-chains and 2 β-chains, 95%
- Haemoglobin A2, is composed of 2 α-chains and 2 δ-chains, 1,5-3,5%
- Haemoglobin F, is composed of 2 α-chains and 2 γ-chains, it is found in the fetus and has a higher affinity for O2. (Haemoglobin S is found in patients with sickle-cell anomia, the problem lies in a mutation of the two β-chains).