Human Anatomy and Physiology (with age-specific features of the child's body) - Sapin M.R., Sivoglazov V.I. 2002

Splanchnology (the study of internal organs)
Respiratory system
Respiration

Respiration is the process of gas exchange between the body and the external environment. Oxygen enters the body from the environment, while Carbon dioxide is released into it. Oxygen is essential for Cells, Tissues, and Organs for oxidation processes, which release energy. Carbon dioxide (along with Water) is the end product of METABOLISM and oxidation. Respiratory arrest leads to an immediate cessation of metabolism.

Human gas exchange consists of three components: external respiration, transport of gases by the Blood, and internal (cellular or tissue) respiration.

External respiration is performed by the Respiratory System, including the Lungs, where oxygen (O₂) passes through the walls of the pulmonary alveoli and blood capillaries into the blood, while carbon dioxide (CO₂) is released from the blood into the alveoli and then expelled from the body through the airways. Inhaled and exhaled air naturally differ in their composition (Table 9).

Class="center">Table 9 Gas content (O₂ and CO₂) in inhaled and exhaled air

The transport of gases (oxygen and carbon dioxide) is carried out by the blood through Blood Vessels. Blood rich in carbon dioxide flows from The Heart to the lungs via the pulmonary Arteries. In the lungs, the blood releases carbon dioxide and is saturated with oxygen. Oxygenated blood from the lungs travels to the heart via the Pulmonary Veins. From the heart, blood is transported through the aorta and then through the arteries to the organs, where it supplies oxygen (and nutrients) to their Cells and Tissues. In the opposite direction—from cells and tissues—blood carries carbon dioxide through the veins back to the heart, and from the heart, this carbon dioxide-rich blood is directed to the lungs.

Internal respiration is the gas exchange between the blood and tissues. Oxygen from the blood passes through the walls of blood capillaries to cells and other tissue structures, where it is incorporated into metabolism. Carbon dioxide is also released from cells and tissues into the blood through the capillary walls.

Thus, blood constantly circulating between the lungs and tissues ensures a continuous process of supplying cells and tissues with oxygen and removing carbon dioxide. In the tissues, blood oxygen penetrates cells and other tissue elements, while carbon dioxide is transported in the opposite direction. This process of internal (tissue) respiration occurs with the participation of specific respiratory Enzymes.

The MECHANISM OF INHALATION and exhalation

Due to the rhythmic contraction of the Diaphragm (16— 18 times per minute) and other Respiratory Muscles (external intercostal muscles, Muscles of the shoulder girdle, and neck), the volume of the chest cavity alternately increases (during inhalation) and decreases (during exhalation). As the chest cavity expands, the lungs passively stretch and expand. Consequently, the pressure inside the lungs drops below atmospheric pressure (by 3—4 mmHg). Therefore, air from the outside rushes into the lungs through the airways. This is how inhalation occurs. During deep inhalation or forced breathing, not only the primary respiratory muscles but also the accessory muscles contract. Exhalation occurs when the inspiratory muscles relax and the expiratory muscles (internal intercostal muscles, muscles of the anterior abdominal wall) contract. The rib cage, raised and expanded during inhalation, lowers due to its own weight and the action of several muscles. Thanks to their elasticity, the stretched lungs decrease in volume. At this point, the pressure inside the lungs rises sharply, and air leaves the lungs. This is how exhalation occurs. During coughing, sneezing, or rapid exhalation, the Abdominal muscles contract, the Ribs (chest cavity) lower, and the diaphragm rises sharply.

During quiet breathing, a person inhales and exhales 500 ml of air. This volume of air (500 ml) is called the tidal volume. With a deep (additional) inhalation, another 1500 ml of air enters the lungs. This is the inspiratory reserve volume. With steady breathing, after a quiet exhalation, a person can exhale another 1500 ml of air by contracting the expiratory muscles. This is the expiratory reserve volume. The total volume of air (3500 ml), consisting of the tidal volume (500 ml), inspiratory reserve volume (1500 ml), and expiratory reserve volume (1500 ml), is called the vital capacity of the lungs. In trained, physically fit individuals, vital capacity can reach 7000—7500 ml. Due to lower body mass, women generally have a smaller vital capacity than men.

After a person exhales 500 ml of air (tidal volume) and then makes a deep exhalation (1500 ml), approximately 1200 ml of residual volume remains in the lungs, which is practically impossible to expel. A lung that has breathed always contains air. Therefore, lung tissue does not sink in water.

Within 1 minute, a person inhales and exhales 5—8 l of air. This is the minute volume of respiration, which can reach 80—120 l per minute during intense physical activity.

Of the 500 ml of inhaled air (tidal volume), only 360 ml reaches the alveoli and delivers oxygen to the blood. The remaining 140 ml stays in the airways and does not participate in gas exchange. Therefore, the airways are referred to as 'dead space'.

Gas Exchange in the lungs

Gas exchange in the lungs occurs between the air entering the alveoli and the blood flowing through the capillaries (Fig. 55). Intensive gas exchange between alveolar air and blood is facilitated by the thinness of the so-called blood-air barrier. This barrier between air and blood is formed by the alveolar wall and the Capillary Wall. The thickness of the barrier is about 2.5 μm. The alveolar walls are composed of a single-layer squamous epithelium (alveolocytes), lined on the inside (from the alveolar lumen side) with a thin film of phospholipid called surfactant. Surfactant prevents the alveoli from collapsing during exhalation and reduces surface tension. The alveoli are surrounded by a dense network of blood capillaries, which greatly increases the surface area where gas exchange between air and blood takes place.

In the inhaled air—within the alveoli—the concentration (partial pressure) of oxygen is much higher (100 mmHg) than in the venous blood (40 mmHg) flowing through the pulmonary capillaries.

Fig. 55. Diagram of gas exchange between blood and alveolar air:

1 — alveolar lumen, 2 — alveolar wall, 3 — blood capillary wall, 4 — capillary lumen, 5 — erythrocyte in the capillary lumen.

The arrows show the path of oxygen (O₂) and carbon dioxide (CO₂) through the blood-air barrier between the blood and air is high (47 mmHg), diffusing into the alveoli, where the partial pressure of CO₂ is significantly lower (40 mmHg). Carbon dioxide is removed from the lung alveoli with the exhaled air.

Thus, the difference in pressure (tension) of oxygen and carbon dioxide in the alveolar air, as well as in arterial and venous blood, enables oxygen to diffuse from the alveoli into the blood, and carbon dioxide from the blood into the alveoli.

Transport of gases by the blood

Due to the unique property of Hemoglobin to bind with oxygen and carbon dioxide, blood is capable of absorbing these gases in significant quantities. 100 ml of arterial blood contains up to 20 ml of oxygen and up to 52 ml of carbon dioxide. One hemoglobin molecule can bind four oxygen molecules, forming an unstable compound called oxyhemoglobin. It is known that 1 ml of hemoglobin binds 1.34 ml of oxygen. 100 ml of blood contains 15 g of hemoglobin.

In body tissues, oxygen is consumed and carbon dioxide is produced As a result of continuous metabolism and intensive oxidation processes. When blood enters body tissues, hemoglobin releases oxygen to the cells and tissues. Carbon dioxide produced during metabolism diffuses from the tissues into the blood and binds to hemoglobin. This forms an unstable compound, carbaminohemoglobin. The rapid binding of hemoglobin with carbon dioxide is facilitated by the enzyme Carbonic anhydrase, which is present in erythrocytes.

Hemoglobin in red Blood Cells is also capable of binding with other gases. For instance, hemoglobin binds with carbon monoxide, which is produced during the incomplete combustion of coal or other fuels, 150–300 times faster than with oxygen. This forms a highly stable compound, carboxyhemoglobin. Consequently, even at low concentrations of carbon monoxide (CO) in the air, hemoglobin binds with carbon monoxide rather than oxygen. As a result, the body's oxygen supply and its transport to cells and tissues are disrupted and halted. Under these conditions, a person suffocates and may die due to the lack of oxygen delivery to the body's tissues.

An insufficient supply of oxygen to tissues {Hypoxia) can occur when There is a lack of oxygen in the inhaled air, such as in the mountains. A decrease in blood hemoglobin levels—anemia—occurs when the blood cannot transport oxygen (as in Carbon monoxide poisoning).

If breathing stops or ceases, suffocation (asphyxia) develops. This condition can occur during drowning or other unexpected circumstances, when a foreign body enters the airways (such as talking while eating), or due to vocal cord edema caused by illness. Food particles can be expelled from the airways by a reflex cough (cough thrust) triggered by irritation of the airway mucosa, primarily the Larynx.

When breathing stops (due to drowning, electric Shock, or gas poisoning) but the heart is still beating, artificial respiration is performed using special devices, or, in their absence, by the "Mouth-to-mouth" or "mouth-to-Nose" method, or by compressing and expanding the chest (Fig. 56).

Review and Self-Assessment Questions:

1. Name the components (phases) of gas exchange between the external environment and The Human Body that you know, and describe these phases.

2. Explain how inhalation and exhalation occur. How do you envision The Mechanism of this process?

3. State The amount of inhaled and exhaled air, providing the figures (if you remember them). What are the lung volumes mentioned in the book called?

4. Describe the gas exchange in the lungs between the inhaled air and the blood.

5. How are gases transported by the blood? What are the names of the compounds that hemoglobin forms with oxygen, carbon dioxide, and carbon monoxide?

Fig. 56. Artificial respiration techniques