Transport of oxygen and carbon dioxide

Overview

Blood must carry large amounts of oxygen from the lungs to the tissues and carbon dioxide back. Plasma alone could carry very little, so both gases depend on haemoglobin and on chemical reactions inside red cells.

Oxygen transport

Two forms

• Bound to haemoglobin: about 97%.
• Dissolved in plasma: about 3%; this is what the PO2 measures.

Each gram of haemoglobin can carry about 1.34 mL of O2 when fully saturated. With a haemoglobin of 15 g/dL, fully saturated arterial blood carries about 20 mL O2 per 100 mL.

At rest, tissues take about 5 mL O2 per 100 mL of blood; venous blood leaving them is still about 75% saturated. This fraction given up (about 25%) is the utilisation coefficient; it can rise to 75–85% in heavy exercise.

The oxygen-haemoglobin dissociation curve

This curve shows haemoglobin saturation against PO2. It is sigmoid (S-shaped) because binding one O2 molecule makes it easier for the next to bind (cooperative binding).

• Flat upper part: at PO2 above about 70 mmHg, saturation stays high. A fall in alveolar PO2 (mild lung disease, altitude) causes little drop in O2 loading.
• Steep middle part: in the tissues (PO2 about 20–40 mmHg), a small drop in PO2 releases a lot of O2.

The P50 is the PO2 at which haemoglobin is 50% saturated. A higher P50 means lower affinity.

Shifts of the curve

2,3-BPG (2,3-DPG)

A product of red cell glycolysis that binds deoxyhaemoglobin and lowers affinity for O2. It rises in chronic hypoxia, anaemia and high altitude, helping release O2 to tissues.

Fetal haemoglobin

HbF (α2γ2) binds 2,3-BPG poorly, so it has higher affinity for O2 (left shift). This lets fetal blood take O2 from maternal blood across the placenta.

Myoglobin

Myoglobin in muscle has a hyperbolic curve and a very high affinity. It stores O2 and releases it only when PO2 falls very low, as in heavy exercise.

Carbon monoxide poisoning

How it happens: CO binds haemoglobin about 200–250 times more strongly than O2, forming carboxyhaemoglobin. Common causes are faulty gas heaters and fires in closed rooms.

Effects:

• Less haemoglobin is available to carry O2, so O2 content falls.
• The remaining haemoglobin holds its O2 more tightly (left shift), so less O2 is released to tissues.
• PO2 is normal, because dissolved O2 is unaffected, so the carotid bodies are not strongly stimulated and breathing does not increase much.
• Pulse oximeters may read falsely normal.

What you see: headache, confusion, collapse; a cherry-red colour is classically described but often absent.

Treatment: 100% oxygen (shortens the half-life of carboxyhaemoglobin); hyperbaric oxygen in severe cases.

Carbon dioxide transport

Bicarbonate and the chloride shift

1. CO2 diffuses into red cells.
2. Carbonic anhydrase rapidly converts CO2 + H2O → H2CO3 → H+ + HCO3−.
3. H+ is buffered by haemoglobin (deoxyhaemoglobin is a good buffer).
4. HCO3− moves out into the plasma in exchange for Cl−, which moves in. This is the chloride (Hamburger) shift.

Because of the chloride shift, venous red cells contain more Cl− and are slightly larger than arterial red cells. The process reverses in the lungs.

The Haldane effect

Deoxygenated haemoglobin binds more CO2 (as carbamino) and more H+ than oxygenated haemoglobin.

• In the tissues, O2 release increases the blood's ability to pick up CO2.
• In the lungs, oxygen binding drives CO2 off the haemoglobin.

The Haldane effect roughly doubles the amount of CO2 released in the lungs.

Clinical relevance

Anaemia

How it happens: low haemoglobin.

What you see: O2 content falls although PO2 and saturation are normal; 2,3-BPG rises to help unloading.

Cyanosis

Blue colour appears when deoxygenated haemoglobin in capillary blood exceeds about 5 g/dL. It may not appear in severe anaemia, and it appears more easily in polycythaemia.

Quick revision

• O2: 97% bound to Hb, 3% dissolved; 1.34 mL per g Hb.
• Sigmoid curve; P50 about 26–27 mmHg.
• Right shift: ↑CO2, H+, temperature, 2,3-BPG (Bohr effect).
• Left shift: HbF, CO, alkalosis, cold, stored blood.
• CO: normal PO2, low content, left shift; treat with 100% O2.
• CO2: 70% bicarbonate, about 20% carbamino, 7% dissolved.
• Chloride shift: Cl− in, HCO3− out. Haldane: deoxy-Hb carries more CO2.

Written by The House Job from standard physiology references (Guyton and Hall Textbook of Medical Physiology; BRS Physiology).

FAQ