Internal Respiration: Gas Exchange at the Tissue Level

Internal respiration is the gas exchange process between the blood and the target tissues of the body.
- It involves the movement of oxygen out of oxygenated blood and into target tissues.
- It also involves the movement of carbon dioxide from target tissues into the blood.

Oxygen Movement:
- Oxygen detaches from hemoglobin in the oxygenated blood.
- A small amount of oxygen dissolves in the plasma.
- Oxygen moves out of the blood and into the target tissues.
- This movement is driven by the lower partial pressure (or concentration) of oxygen in the target tissues compared to the oxygenated blood.
Carbon Dioxide Movement:
- Carbon dioxide has a higher partial pressure inside the metabolizing target tissues compared to the oxygenated blood.
- Carbon dioxide moves across cell membranes and into the oxygenated blood.
- Once in the blood, carbon dioxide is converted to bicarbonate.
- It is also carried on hemoglobin and dissolves in plasma.

Internal respiration is specifically the gas exchange at the level of the target tissues.
External respiration involves:
- Moving air in and out of the lungs.
- Gas exchange at the respiratory membrane inside the lung.

In metabolizing tissues:
- The partial pressure of oxygen is much lower than in oxygenated blood.
- The partial pressure of carbon dioxide is higher than in oxygenated blood.
Due to these pressure gradients:
- Oxygen moves out of the blood and into the tissues.
- Carbon dioxide moves out of the cells and into the bloodstream.

Oxygen detaches from hemoglobin and moves out of solution.
Carbon dioxide is converted into bicarbonate, binds to hemoglobin, and dissolves in plasma.
The blood, now deoxygenated, returns to the heart and lungs.
The entire process then repeats.