Question

Where does the blood absorb oxygen in the human body?

Answer 1

Hint: Respiration is a biochemical process occurring in living organisms. It is associated with the exchange of oxygen and carbon dioxide between the external environment and the blood followed by utilizing oxygen to produce energy.

Complete answer:
The respiratory system is responsible for the conduction of gases and the exchange of oxygen and carbon dioxide at the level of lungs and organs.
The impure and deoxygenated blood absorbs oxygen in the alveoli of the lungs.
The air we breathe in reaches the alveoli in the lungs to supply oxygen.
The partial pressure of oxygen is low and that of carbon dioxide is higher at the tissue level whereas it is inverse at the site of alveoli in the lungs.
When deoxygenated blood reaches pulmonary capillaries or alveolar capillaries, oxygen begins to form oxyhemoglobin in capillary blood. The carbon dioxide is present in the deoxygenated blood mainly in the form of $NaHCO_3$. Oxyhemoglobin being acidic facilitates the breakdown of $NaHCO_3$ to release $CO_2$ out to the alveoli. This is known as the ‘Haldane Effect'.

The exchange barrier is triple-layered and is formed by
1. Epithelium of alveoli
2. Epithelium of pulmonary capillary
3. Basement membrane between them

Note: ‘Bohr's effect' is seen at the tissue level where the partial pressure of carbon dioxide is relatively higher. Oxyhemoglobin begins to dissociate releasing $O_2$ to tissues and the free haemoglobin binds with carbon dioxide to form carboxyhemoglobin.