• During inhalation, air is drawn into the nose; nasal air is cleaned and moistened.
  
• Further it goes down through the larynx into the trachea.
  
• The trachea divides into two tubes - the bronchi. Through them the air gets into the right and left lungs.
• The bronchi branch into many tiny bronchioles that end in alveoli. Each bubble is enmeshed with a network of blood vessels.
• Through the thin walls of the alveoli oxygen enters the blood vessels. Here begins the pulmonary circulation. The oxygen is picked up by hemoglobin, which is contained in erythrocytes, and the oxygenated blood from the lungs is sent to the left side of the heart.
• The heart pumps blood into the blood vessels (arteries and arterioles), the systemic circulation begins, where oxygen is distributed via the arteries throughout the body.
• As soon as the oxygen from the blood is consumed, the blood flows through the veins into the right side of the heart, ending the systemic circulation, and then - back to the lungs, ending the pulmonary circulation. During expiration carbon dioxide is removed from the body. Exhaled air contains about 16% oxygen (instead of 21% in the inhaled air) and approximately 5% carbon dioxide is added.

With each breath not only oxygen enters the lungs, but also dust, germs and other foreign matter. On the walls of the bronchi there are tiny villi that trap dust and germs. Located in the walls of the airways, special cells produce mucus, which helps to clean and lubricate these villi. Mucus is also located in the nasal cavity and throat. Contaminated mucus is removed from the bronchi to outside and expectorated (coughed up), or swallowed into the stomach where bacteria can be destroyed by stomach acid.

In humans, the lungs occupy about 6% of the body volume regardless of their weight. Thorax changes in volume, due to its structure. The largest amplitude of ribs is possible if during inspiration the spine extends, and during exhalation – spine flexes. Movement of the upper ribs changes thorax in the anteroposterior direction, and the lower ones – in the frontal plane.

Lung volume changes during inspiration but nor regularly in all its parts. For this there are three main reasons: firstly, the thoracic cavity increases irregularly in all directions, and secondly, not all parts of the lung are equally extensible. Thirdly, we assume the existence of the gravitational effect which promotes downward displacement of lung.

At rest an adult breathes 16-20 times per minute. In children same breath is more frequent – up to 60 breaths per minute.

During tidal respiration a human inhales and exhales about 500 ml of air. This amount is called the tidal volume.

With a deep breath, you can inhale additional 1,500 ml of air. This volume of air is called the reserve inspiratory volume.

After the tidal exhalation a human can further exhale another 1,500 ml of air. This volume of air is called reserve expiratory volume.

The sum of these three volumes constitutes vital lungs capacity (VC). In adult this capac- ity is approximately 3500 ml. VC depends on age, sex, physical fitness of a human.

Moreover, even after the deepest expiration approximately 1,000 ml of air remain in human lungs. This volume of air is called residual volume, and even more air remains in the airways (about 150 ml) which do not participate in gas exchange.

The sum of Reserve Inspiratory Volume, Reserve Expiratory Volume, and Residual Vol- ume (RV) is the Total Lung Capacity (TLC). In these contexts, a capacity can be broken down into two or more volumes.

Physical exercises have more than a beneficial effect on the entire respiratory system and redundant capabilities of the organism, i.e., the reserve of health, directly depend on the reserve possibilities of the respiratory system.