External respiration. The mechanism of air renewal in the lungs.
Respiration refers to the processes that ensure gas exchange between the body and the external environment, and within the body.
Gas exchange between the external environment and the pulmonary alveoli, i.e. external respiration. External respiration is the process in which pulmonary ventilation renews part of the gas mixture in the alveoli. Blood in the gas-exchange zone of the pulmonary capillaries is enriched with oxygen and gives off carbon dioxide. External respiration occurs because of changes in thoracic volume.
Pleural cavity, inspiration and expiration. *Pleural cavity: the chest cavity and the surface of the lungs are covered with a serous membrane, i.e. the pleura. The lungs are covered by the visceral pleura, while the parietal pleura attaches to the wall of the chest cavity. Between the two layers an airtight, slit-like space forms, i.e. the pleural cavity, which is filled with serous fluid. The pressure in the pleural cavity is lower than atmospheric pressure, which keeps the lungs stretched and makes them follow changes in thoracic volume. * Inspiration, i.e. inhalation: the contracting inspiratory muscles raise the ribs and the dome of the diaphragm flattens. As a result, thoracic volume increases and intrapulmonary pressure falls below atmospheric pressure. The resulting negative pressure sucks air into the lungs. * Expiration, i.e. exhalation: the ribs move down, the diaphragm domes upward, thoracic volume decreases, and the rise in intrapulmonary pressure forces part of the air in the lungs out into the external air.
Tidal volume, pulmonary ventilation. * During quiet breathing, 0.4–0.6 liters of air enters the lungs and is removed with each breath. *Pulmonary ventilation takes place during inhalation and exhalation. As a result, the air in the alveoli is continuously renewed. At rest, the breathing rate can slow to as low as six breaths per minute, while during physical work it can rise to more than 60 per minute. At rest, the number of breathing cycles per minute is normally 12–16, i.e. the minute volume of breathing.
Vital capacity i.e. the vital volume of the lungs, or lung volume (VC). This is the volume of air exhaled after an inhalation of maximal depth. On average it is 3.5–5 l. The volume of air exhaled to maximal depth after a maximal inhalation is the expiratory vital capacity (EVC), and the volume of air inhaled to maximal depth after a maximal exhalation is the inspiratory vital capacity.
Gas exchange between alveolar air and pulmonary capillary blood, i.e. diffusion in the lungs. This gas exchange takes place by diffusion, from the higher partial pressure to the lower one. Partial pressure (p) is the part of the total pressure that corresponds to the amount of each gas in a gas mixture. Since the alveolar pO2 is 102 mmHg while in the blood surrounding the alveoli it is only 40 mmHg, it is natural for O2 to diffuse from the alveoli into the surrounding blood, where it is arterialized (arterial blood forms). The diffusion of CO2 from the blood into the alveoli is driven by the higher pCO2 in the blood (47 mmHg) compared with the alveoli.
Partial pressure (p) is the part of the total pressure that corresponds to the amount of each gas in a gas mixture. pO2 = 159 mmHg; p CO2= 0.2 mmHg; p N2= 600.8 mmHg.
Partial pressures of gases in atmospheric air. pO2 = 159.0 mmHg; p CO2= 0.29 mmHg; p N2= 600.8 mmHg.
*alveolar gases pO2 = 103mmHg; p CO2= 40 mmHg; p N2= 570 mmHg.
*Blood carries oxygen: 1) in physically dissolved form (a small amount in the blood); 2) bound to hemoglobin (Hb), which carries most of the O2 the body needs. Hb consists of four polypeptide chains, each of which contains a prosthetic group, heme. Each heme contains one divalent iron atom; O2 is bound, without changing the valence of the iron atom, into an easily reversible (unstable) compound with heme. Hb becomes oxyhemoglobin, and this reaction is called oxygenation. *Blood carries carbon dioxide: 1) dissolved in blood plasma and electrolytes (plasma 5%; erythrocytes ~7%); 2) bound to proteins in erythrocytes (Hb) and, to a small extent, to plasma proteins (11%); 3) as bicarbonate (mainly sodium and potassium salts) in plasma and erythrocytes (plasma 94%, erythrocytes 82%); 4) a very small amount also as undissociated carbonic acid.
Oxyhemoglobin is an unstable compound that forms when O2 combines with hemoglobin. One hemoglobin molecule can bind 4 O2 molecules, because one Hb contains 4 hemes, each of which has one iron atom that binds O2.
Blood O2 capacity, CO2 volume %.
Gas exchange between capillary blood and tissues. The transfer of oxygen from the blood to the tissues and the removal of carbon dioxide from the tissues occur through the diffusion of these gases across the capillary walls. The diffusion of gases is driven by the difference in partial pressures. In the tissues, the partial pressure (O2) is considerably lower than in the blood flowing into the capillaries. O2 dissolved in the blood plasma diffuses into the tissue fluid and from there into the cells. This is caused by the difference in pO2 between the blood and the cells. As a result of diffusion, the O2 partial pressure of the blood begins to fall. As pO2 decreases, the release of O2 by hemoglobin increases. The released O2 molecules pass from the erythrocytes into the plasma and from there into the tissues. Consequently, the pO2 of the blood falls even further. Gas exchange across the capillary walls is also helped by the filtration of the liquid part of the blood under the influence of blood pressure. Together with the filtration fluid, oxygen dissolved in the blood also leaves the capillaries. The capillary blood of the systemic circulation does not give up all of its O2 to the tissues. If the O2 volume percent in arterial blood is 19, in the venous blood draining from the tissues it falls to 11%. This means oxygen was given to the tissues to the extent of 8 vol%. The difference in O2 vol% between the arterial blood flowing to the tissues and the venous blood flowing away is called the arteriovenous difference. This indicator shows how much oxygen every 100 ml of blood gives to the tissues. To find out what share of the O2 transported by the blood goes to the tissues, the O2 utilization coefficient is calculated.
Regulation of breathing.
The respiratory center and the impulses passing through it. Breathing is regulated by neurohumoral means. It is controlled by the respiratory center in the medulla oblongata, which consists of an inspiratory center and an expiratory center, from which impulses go to the spinal cord nerve cells that innervate the respiratory muscles. Pulmonary ventilation can be reduced depending on the chemical composition of the blood flowing through the center (humoral regulation) and on afferent signals arriving at the respiratory center from receptors (unconditioned reflex). Under natural conditions, humoral and neural mechanisms act in interaction.
Because of the general excitability of the respiratory center, breathing rate and depth depend on influences from the higher parts (pons, cortex) and from the periphery (lung stretch receptors and skeletal muscle receptors). The excitatory state of the respiratory center is altered by the partial pressures of CO2 and O2 in arterial blood (a rise in pCO2 and a fall in pO2 in the blood increase the excitatory state of the respiratory center, which increases pulmonary ventilation). The respiratory center is very sensitive to changes in blood CO2 and less sensitive to changes in O2. The respiratory center is also affected by lactic acid and by a shift of the blood reaction toward acidity. In addition to direct contact, CO2 also affects the respiratory center reflexively, through chemoreceptors located in the blood vessels. Their sensitivity to excess CO2 and to O2 deficiency is considerably greater than that of the respiratory center’s nerve cells. The respiratory center also receives influences from receptors in the lungs (afferent nerve impulses from the lungs and impulses generated by stimulation of the proprioceptors of the respiratory muscles are conducted to the respiratory center via the vagus nerve).
Apnea. Within certain limits, a person can voluntarily alter their breathing, hold their breath (apnea) or hyperventilate (linked to the function of the cerebral cortex). Pre-start changes in breathing activity (pre-start fever) are also linked to the function of the cerebral cortex.
Breathing at lower and higher atmospheric pressure compared with air pressure at sea level. We encounter lower atmospheric pressure in high mountains and at high altitudes when not using breathing apparatus. Above 4-5 km above sea level, where pO2 drops to 96-80 mmHg, signs of O2 deficiency appear even at rest, let alone during physical work. We encounter higher atmospheric pressure in underwater work, i.e. caisson work. A water column 10 m high causes a pressure rise of ~1 atm (at sea level, 760 mmHg, it has no effect).
* Hypoxia is a shortage of O2 in the air, which occurs at a lower atmospheric pressure than at sea level. *Hypoxemia is a pathological condition brought on by hypoxia, i.e. a shortage of O2 in the air leads to a drop in, or lack of, O2 in the blood. Hypoxemia occurs when pO2 is not sufficient to saturate Hb with O2. Hb saturation with O2 begins to fall when the pO2 of atmospheric air drops below 100 mmHg (normal 159 mmHg). *Caisson disease i.e. decompression sickness occurs on rapid transition from an environment of higher air pressure to one of lower air pressure. It is an occupational disease that affects caisson workers and divers. Caisson disease is caused by gas (N) bubbles that form in the blood and tissues. To prevent caisson disease, a gas mixture is used in caissons in which part of the air nitrogen has been replaced with helium or oxygen.
Breathing during physical work.
Oxygen demand is the amount of O2 consumed by oxidation processes when performing a given task.
Oxygen ceiling, i.e. maximal O2 consumption, i.e. maximal aerobic power, is a fixed maximum value of O2 that the muscles take up per minute, regardless of how energetically the lungs and heart work. The muscles can receive less O2 than this, but not more. The oxygen ceiling is an individual indicator that shows a person’s training status. It is tested on a treadmill or ergometer by increasing the training load.
*True steady state at a work intensity where the O2 demand per minute does not exceed the O2 ceiling, a fairly rapid balance arises between the tissues’ oxygen demand and O2 delivery. Outwardly, this shows up in a true steady state, in which O2 uptake in the lungs is at a certain constant level, the height of which is proportional to the work intensity and the O2 demand. * Apparent steady state – when doing work of a certain intensity at which the O2 demand is higher than the oxygen ceiling, O2 uptake through the lungs can balance it out. In this case the steady state is only apparent, because the amount of O2 used does not cover the body’s oxygen demand. A constant level of O2 consumption arises in this situation when the oxygen ceiling is reached, i.e. when a further increase in oxygen consumption is no longer possible. The level of the apparent steady state does not depend on work intensity but on the maximal capacity of respiration and circulation.
Oxygen debt, what it depends on. If breathing cannot satisfy the tissues’ O2 supply, i.e. the O2 demand per minute is greater than the oxygen ceiling, the muscles perform work under conditions of O2 deficiency. The extent of the oxygen debt depends on the intensity and duration of the work. Oxygen debt also arises under steady-state conditions. At the start of work, a temporary shortfall in oxygen supply arises because of a certain delay in mobilizing the circulatory and respiratory systems.
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