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The human respiratory system consists of the respiratory tract (upper and lower) and the lungs. The respiratory system is responsible for gas exchange between the organism and the environment. How is the respiratory system built and how does it work?

The human respiratory systemis supposed to enable breathing - the process of gas exchange, namely oxygen and carbon dioxide, between the organism and the environment. Every cell in our body needs oxygen to function properly and generate energy. The breathing process is divided into:

  • external respiration - supplying oxygen to cells
  • internal respiration - intracellular

External breathing occurs due to the synchronization of the respiratory system with the nervous centers and is divided into a number of processes:

  • lung ventilation
  • gas diffusion between alveolar air and blood
  • transport of gases through the blood
  • gas diffusion between blood and cells

Structure of the respiratory system

The respiratory tract consists of:

  • upper respiratory tract , that is: nasal cavity ( cavum nasz ) and throat ( pharynx )
  • lower respiratory tract : larynx ( larynx ), trachea ( trachea ), bronchi ( bronchi ) - right and left, which are further divided into smaller branches, and the smallest ones turn into bronchioli ( bronchioli )

The final part of the airway leads to the alveoli ( alveoli pulmonales ). The inhaled air passing through the respiratory tract is cleaned of dust, bacteria and other small impurities, moisturized and warmed. On the other hand, the structure of the bronchi, by combining cartilage, elastic and smooth muscle elements, allows for the regulation of their diameter. The throat is where the respiratory and digestive systems intersect. For this reason, when swallowing, breathing stops and the airway closes through the epiglottis.

  • lungs- paired organs located in the chest.

In anatomical and functional terms, the lungs are divided into lobes (the left lung into two lobes, and the right one into three), the lobes are further divided into segments, segments into lobules, and lobules into clusters.

They surround each lungtwo layers of connective tissue - parietal pleura ( pleura parietalis ) and pulmonary pleura ( pleura pulmonalis ). Between them is the pleural cavity ( cavum pleurae ), and the fluid in it allows the lung covered with the pulmonary pleura to adhere to the parietal pleura fused with the inner wall of the chest. In the place where the bronchi penetrate the lungs, there are pulmonary cavities, into which, next to the bronchi, also arteries and pulmonary veins.

Lung ventilation

The essence of ventilation is to draw atmospheric air into the alveoli. Since air always flows from higher pressure to lower pressure, the right muscles are involved in every inhalation and exhalation, enabling the suction and pressure movement of the chest.

At the end of exhalation, the pressure in the alveoli is equal to the atmospheric pressure, but while drawing in air, the diaphragm ( diaphragma ) and the external intercostal muscles ( musculi intercostales) contract externi ), this increases the volume of the chest and creates a vacuum that sucks in the air.

When the demand for ventilation increases, additional inspiratory muscles are activated: the sternocleidomastoid muscles ( musculi sternocleidomastoidei ), smaller pectoral muscles ( musculi pectorales minores ), anterior toothed muscles ( musculi serrati anteriores ), trapezius muscles ( musculi trapezii ), levers of the scapula ( musculi levatores scapulae ), major and minor parallelogram muscles ( musculi rhomboidei maiores et minores ) and inclined muscles ( musculi scaleni ) .

The next step is to exhale. It begins when the inspiratory muscles relax at the peak of the inhalation. Usually this is a passive process, as the forces generated by the stretched elastic elements in the lung tissue are sufficient for the chest to decrease in volume. Alveolar pressure rises above atmospheric pressure and the resulting pressure difference removes air to the outside.

The situation is slightly different when exhaling strongly. We deal with it when the breathing rhythm is slow, when the exhalation requires overcoming increased breathing resistance, e.g. in some lung diseases, but also in phonatory activity, especially while singing or playing wind instruments. The motoneurons of the expiratory muscles are stimulated, which include: the intercostal musclesinternal muscles ( musculi intercostales interni ) and the muscles of the anterior abdominal wall, especially the rectus abdominis muscles ( musculi recti abdominis ).

Respiratory Rate

Respiratory rate is highly variable and depends on many different factors. A resting adult should breathe 7-20 times per minute. Factors leading to an increase in the rate of breathing, professionally called tachypnea, include exercise, lung conditions, and extrapulmonary respiratory distress. On the other hand, bradypnoea, i.e. a significant decrease in the number of breaths, may result from neurological diseases or central side effects of narcotic drugs. Children differ from adults in this respect: the smaller the toddler, the higher the physiological respiratory rate.

Lung volumes and capacities

  • TLC (total lung capacity) -total lung capacity- volume that is in the lungs after the deepest inhalation
  • IC -inspiratory capacity- pulled into the lungs during the deepest inhale after a calm exhalation
  • IRV (inspiratory reserve volume) -inspiratory reserve volume- pulled into the lungs during the maximum inspiration at the peak of free inspiration
  • TV (tidal volume) -tidal volume- inhaled and exhaled freely while inhaling and exhaling
  • FRC -residual functional capacity- remains in the lungs after calm exhalation
  • ERV (expiratory reserve volume) -expiratory reserve volume- removed from the lungs during maximum exhalation after free inhalation
  • RV (residual volume) -residual volume- always remains in the lungs during maximal exhalation
  • VC (vital capacity) -vital capacity- removed from the lungs after maximum inhalation during maximum exhalation
  • IVC (inspiratory vital capacity) -inspiratory vital capacity- drawn into the lungs after the deepest exhalation at maximum inhalation; may be slightly greater than VC because at the time of maximal exhalation followed by maximal inhalation, the alveolar conductors close before the air filling the bubbles is removed

During free inspiration, tidal volume is 500 mL. However, not all of this volume reaches the alveoli. About 150 ml fills the respiratory tract, which does not have conditions for gas exchange between air and blood, i.e. the nasal cavity, pharynx, larynx, trachea, bronchi and bronchioles. This is called anatomical respiratory dead space. The remaining 350 mL is mixed withwith air constituting the functional residual capacity, it is simultaneously heated and saturated with water vapor. In the alveoli, again, not all of the air is gaseous. In the capillaries of the walls of some of the alveoli, blood does not flow or does not flow enough to use all the air for gas exchange. This is the physiological respiratory dead space and is small in he althy people. Unfortunately, it can increase significantly in disease states.

The average respiration rate during rest is 16 per minute, and the tidal volume is 500 mL, multiplying these two values, we get pulmonary ventilation. From this it follows that about 8 liters of air are inhaled and exhaled per minute. During fast and deep breaths, the value may increase significantly, even from a dozen to twenty times.

All these complicated parameters: capacities and volumes were introduced not only to confuse us, but have an important application in the diagnosis of pulmonary diseases. There is a test - spirometry that measures: VC, FEV1, FEV1 / VC, FVC, IC, TV, ERV and IRV. It is essential for the diagnosis and monitoring of diseases such as asthma and COPD.

Gas diffusion between alveolar air and blood

The basic structure that makes up the lungs is the alveoli. There are about 300-500 million of them, each with a diameter of 0.15 to 0.6 mm, and their total area is from 50 to 90 m².

The walls of the alveoli are built by a thin, flat, single-layer epithelium. In addition to the cells that make up the epithelium, there are two other types of cells in the vesicles: macrophages (gut cells) and also type II follicular cells that produce the surfactant. It is a mixture of proteins, phospholipids and carbohydrates produced from fatty acids in the blood. The surfactant, by reducing the surface tension, prevents the alveoli from sticking together and reduces the forces needed to stretch the lungs. From the outside, the vesicles are covered with a network of capillaries. Capillaries getting into the alveoli carry blood rich in carbon dioxide, water, but with a small amount of oxygen. In contrast, in alveolar air, the partial pressure of oxygen is high and that of carbon dioxide is low. Gas diffusion follows a gradient of gas particle pressure, so capillary erythrocytes trap oxygen from the air and get rid of carbon dioxide. Gas molecules must pass through the alveolar wall and the capillary wall, and more precisely through: a layer of fluid covering the alveolar surface, alveolar epithelium, basement membrane, and endotheliumcapillaries.

Transport of gases through the blood

  • oxygen transport

Oxygen first dissolves physically in plasma, but then diffuses through the envelope into the red blood cells, where it binds to hemoglobin to form oxyhemoglobin (oxygenated hemoglobin). Hemoglobin plays a very important role in the transport of oxygen, because each of its molecules combines with 4 oxygen molecules, thus increasing the ability of blood to transport oxygen up to 70 times. The amount of oxygen transported dissolved in plasma is so small that it is irrelevant to respiration. Thanks to the circulatory system, blood saturated with oxygen reaches every cell of the body.

  • carbon dioxide transport

Tissue carbon dioxide enters the capillaries and is transported to the lungs:

  • ok. 6% physically dissolved in plasma and in the cytoplasm of erythrocytes
  • ok. 6% bound to free amino groups of plasma and hemoglobin proteins (as carbamates)
  • majority, i.e. approx. 88% as HCO3- ions bound by the bicarbonate buffer system of plasma and erythrocytes

Gas diffusion between blood and cells

In the tissues, gas molecules once again penetrate along the gradient of elasticity: the oxygen released from hemoglobin diffuses into the tissues, while carbon dioxide diffuses in the opposite direction - from the cells to the plasma. Due to the differences in the oxygen demand of different tissues, there are also differences in oxygen tension. In tissues with intensive metabolism, the oxygen tension is low, so they consume more oxygen, while the draining venous blood contains less oxygen and more carbon dioxide. The arteriovenous difference in oxygen content is a parameter that determines the degree of oxygen consumption by tissues. Each tissue is supplied with arterial blood with the same oxygen content, while venous blood may contain more or less of it.

Internal breathing

Breathing at the cellular level is a multi-stage biochemical process that involves the oxidation of organic compounds that produce biologically useful energy. It is a fundamental process that continues even when other metabolic processes are stopped (anaerobic alternative processes are inefficient and of limited importance).

The key role is played by mitochondria - cellular organelles, which receive oxygen molecules diffusing inside the cell. On the outer membrane of the mitochondria there are all the enzymes of the Krebs Cycle (or the cycle of tricarboxylic acids), while on the inner membrane there are enzymes of the chain

In the Krebs cycle, sugar, protein and fat metabolites are oxidized to carbon dioxide and water with the release of free hydrogen atoms or free electrons. Further in the respiratory chain - the last stage of intracellular respiration - by transferring electrons and protons to successive conveyors, high-energy phosphorus compounds are synthesized. The most important of them is ATP, i.e. adenosine-5′-triphosphate, a universal carrier of chemical energy used in cell metabolism. It is consumed by numerous enzymes in processes such as biosynthesis, movement and cell division. Processing of ATP in living organisms is continuous and it is estimated that every day man converts the amount of ATP comparable to his body weight.

Breathing regulation

In the medulla is the breathing center which regulates the frequency and depth of breathing. It consists of two centers with opposite functions, built by two types of neurons. Both are located within the reticular formation. In the solitary nucleus and in the anterior part of the posterior-ambiguous vagus nerve is the inspiratory center, which sends nerve impulses to the spinal cord, to the motor neurons of the inspiratory muscles. On the other hand, in the ambiguous nucleus of the vagus nerve and in the posterior part of the posterior-ambiguous nucleus of the vagus nerve, there is the exhalation center, which stimulates the motor neurons of the expiratory muscles.

The neurons of the inspiration center send a burst of nerve impulses several times a minute, which follow the branch descending to the motor neurons in the spinal cord and at the same time the axon branch ascending to the neurons of the reticular formation of the bridge. There is a pneumotaxic center that inhibits the inspiration center for 1-2 seconds and then the inspiratory center stimulates again. Due to successive periods of stimulation and inhibition of the inspiratory center, rhythmicity of breaths is ensured. The inspiratory center is regulated by nerve impulses arising in:

  • chemoreceptors of the cervical and aortic lobes, which react to an increase in carbon dioxide concentration, concentration of hydrogen ions or a significant decrease in oxygen concentration in arterial blood; impulses from the aortic clots travel through the glossopharyngeal and vagus nerves. and the effect is the acceleration and deepening of inhalations
  • lung tissue interoreceptors and thoracic proprioreceptors;
  • Inflation mechanoreceptors are located between the bronchial smooth muscles, they are stimulated by stretching of the lung tissue, which triggers exhalation; then reducing the stretch of lung tissue on exhalation, activates other mechanoreceptors this timedeflationary ones that trigger the inhalation; This phenomenon is called the Hering-Breuer reflexes;
  • The inspiratory or expiratory setting of the chest irritates the respective proprioreceptors and modifies the frequency and depth of the breath: the deeper you inhale, the deeper you exhale;
  • centers of the upper levels of the brain: cortex, limbic system, thermoregulation center in the hypothalamus

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Respiratory system