Transport of Oxygen and Hemoglobin

Study Notes

A liter of arterial blood contains 200 mL of oxygen at atmospheric pressure.
Oxygen is found in two forms: dissolved in plasma and red blood cells (RBCs) or reversibly bound to hemoglobin in RBCs.
The amount of oxygen dissolved in blood is proportional to the partial pressure of oxygen (PO2) in blood, following Henry's Law.
Out of the 200 mL of oxygen, only 3 mL is dissolved, the remaining 197 mL is bound to hemoglobin at a normal PO2 of 100 mmHg.

Each hemoglobin molecule is composed of four subunits, each containing a heme group (with iron) and a polypeptide chain.
The four polypeptides make up the globin molecule.
Each heme group binds one oxygen molecule, allowing each hemoglobin molecule to bind four.

The curve is sigmoidal, meaning the binding of one oxygen molecule to hemoglobin facilitates the binding of more oxygen.
This is because the binding of one oxygen molecule changes the conformation of hemoglobin, making it easier for other oxygen molecules to bind.
The steep slope between 10 and 60 mmHg PO2 shows that the binding of oxygen to hemoglobin is highly efficient at lower PO2 levels.
The plateau between 70 and 90 mmHg PO2 indicates that hemoglobin is almost fully saturated with oxygen.

The affinity of hemoglobin for oxygen can be altered by PCO2, H+ concentration, temperature, and 2,3-diphosphoglycerate (DPG).
Increased PCO2, H+, temperature, and DPG shift the curve to the right, meaning at any given PO2, hemoglobin has a lower affinity for oxygen, facilitating oxygen release.
Decreased PCO2, H+, temperature, and DPG shift the curve to the left, indicating a higher affinity for oxygen.

Carbon dioxide is transported in the blood in three forms:
- Dissolved in plasma (7%)
- Bound to hemoglobin as carbaminohemoglobin (23%)
- As bicarbonate ion in plasma (70%)