Gas exchange in lungs and tissues
The exchange of gases in the lungs and tissues occurs as a result of diffusion.
In the alveolar air, the oxygen content is always higher than in the venous blood moving through the pulmonary capillaries. Therefore, oxygen moves from the alveoli into the blood, turning it into arterial. In the blood, oxygen combines with hemoglobin and is transported to the tissues.
In tissues, oxygen from the capillaries of the systemic circulation diffuses into the tissue fluid, and from it into cells, where it is used to oxidize organic substances.
Carbon dioxide goes the other way: it enters the blood from the tissues, dissolves in it, or binds to hemoglobin. Carbon dioxide is transported through the blood vessels to the lungs, where it passes into the alveoli and is removed with the exhaled air to the outside.
As a result, the amount of oxygen in the exhaled air decreases from 21% to 16%, and the carbon dioxide content increases from 0.03% to 4%.
Breathing is regulated by the nervous and humoral pathways.
Nervous regulation of respiratory movements is carried out by the respiratory center, which is located in the medulla oblongata. The impulses arising in the respiratory center ensure the alternation of inhalation and exhalation. Depending on the state of the body, the respiratory center can change the depth and frequency of breathing.
The medulla oblongata also provides protective respiratory reflexes – sneezing and coughing – which are sharp exhalations through the mouth or nose when irritating particles enter them.
The respiratory center is controlled by the cerebral cortex, which allows you to arbitrarily slow down or speed up your breathing.
Humoral regulation is associated with changes in the content of carbon dioxide in the blood. With its increased concentration, ventilation of the lungs increases, with a lower concentration, it slows down.
The humoral and nervous modes of regulation are interconnected, since the respiratory center reacts to the content of carbon dioxide in the blood that washes it.