The primary three components of external respiration are the surface area of the alveolar membrane, the partial pressure gradients of the gasses, and the matching of perfusion and ventilation. The alveoli have a very high surface area to volume ratio that allows for efficient gas exchange. The alveoli are covered with a high density of capillaries that provide many sites for gas exchange. The walls of the alveolar membrane are thin and covered with a fluid, extra-cellular matrix that provides a surface for gas molecules in the air of the lungs to diffuse into, from which they can then diffuse into the capillaries.
Partial pressure gradients differences in partial pressure allow the loading of oxygen into the bloodstream and the unloading of carbon dioxide out of the bloodstream. These two processes occur at the same time. Gas exchange in the alveolus : External respiration is a result of partial pressure gradients, alveolar surface area, and ventilation and perfusion matching. Oxygen has a partial pressure gradient of about 60 mmHg mmHg in alveolar air and 40 mmHg in deoxygenated blood and diffuses rapidly from the alveolar air into the capillary.
Equilibrium between the alveolar air and capillaries is reached quickly, within the first third of the length of the capillary within a third of a second. The partial pressure of oxygen in the oxygenated blood of the capillary after oxygen loading is about mmHg.
The process is similar in carbon dioxide. The partial pressure gradient for carbon dioxide is much smaller compared to oxygen, being only 5 mmHg 45 mmHg in deoxygenated blood and 40 mmHg in alveolar air. Equilibrium between the alveolar air and the capillaries for carbon dioxide is reached within the first half of the length of the capillaries within half a second. The partial pressure of carbon dioxide in the blood leaving the capillaries is 40 mmHg.
The exchange of gas and blood supply to the lungs must be balanced in order to facilitate efficient external respiration. While a severe ventilation—perfusion mismatch indicates severe lung disease, minor imbalances can be corrected by maintaining air flow that is proportional to capillary blood flow, which maintains the balance of ventilation and perfusion. Perfusion in the capillaries adjusts to changes in PAO 2.
Constriction in the airways such as from the bronchospasms in an asthma attack lead to decreased PAO 2 because the flow of air into the lungs is slowed.
Alternatively, breathing in higher concentrations of oxygen from an oxygen tank will cause vasodilation and increased blood perfusion in the capillaries.
Ventilation adjusts from changes in PACO 2. When airflow becomes higher relative to perfusion, PACO 2 decreases, so the bronchioles will constrict in order to maintain to the balance between airflow ventilation and perfusion. When airflow is reduced, PACO 2 increases, so the bronchioles will dilate in order to maintain the balance.
Cellular respiration is the metabolic process by which an organism obtains energy through the reaction of oxygen with glucose. Internal respiration refers to two distinct processes. The first is the exchange of gasses between the bloodstream and the tissues. The second is the process of cellular respiration, from which cells utilize oxygen to perform basic metabolic functions. Gas exchange occurs in the alveoli so that oxygen is loaded into the bloodstream and carbon dioxide is unloaded from the bloodstream.
Afterwards, oxygen is brought to the left side of the heart via the pulmonary vein, which pumps it into systemic circulation. Red blood cells carry the oxygen into the capillaries of the tissues of the body.
Oxygen diffuses into the cells of the tissues, while carbon dioxide diffuses out of the cells of the tissues and into the bloodstream. The factors that influence tissue gas exchange are similar to the factors of alveolar gas exchange, and include partial pressure gradients between the blood and the tissues, the blood perfusion of those tissues, and the surface areas of those tissues.
Each of those factors generally increase gas exchange as those factors are increased i. Cellular respiration is the metabolic process by which an organism obtains energy through the reaction of oxygen with glucose to produce water, carbon dioxide and ATP, which is the functional source of energy for the cell. Adapted from Thomas and Lumb 6 and Leach and Treacher Of clinical relevance:. Increased 2,3-DPG production is seen in anaemia, which may minimize tissue hypoxia by right-shifting the ODC and increasing tissue oxygen release.
Inorganic phosphate is a substrate for the production of 2,3-DPG and thus capillary haemoglobin oxygen release may be impaired if hypophosphataemia is not corrected. Causes of hypophosphataemia can be divided into: decreased intestinal absorption e.
In critical care, hypophosphataemia is often seen in sepsis, after operation, in refeeding syndrome, in diabetic ketoacidosis due to increased urinary phosphate excretion , and during renal replacement therapy. Hypophosphataemia is also noted after an acute liver injury caused by, for example, paracetamol overdose and after hepatic resection.
First of all, the word delivery implies that all the oxygen so described is delivered to, and utilized by, metabolizing cells. Secondly, the word delivery implies an active external process responsible for ensuring arrival of oxygen at the cell.
Notwithstanding these comments, we will continue with oxygen delivery within the context of this article in order to remain consistent with common custom and usage. Global oxygen delivery describes the amount of oxygen delivered to the tissues in each minute and is a product of the cardiac output and arterial oxygen content. It is important to note that this is clearly an overall measure of oxygen delivery and does not describe regional differences—oxygen flux to each tissue bed is not constant throughout the body, rather the microcirculation responds to altering tissue metabolic demands by varying the regional and local blood flow.
As can be seen from the above equation, alterations in cardiac output, arterial oxygen saturation, and haemoglobin concentration will affect oxygen delivery. Under these circumstances, cells have a relative or absolute failure of the capacity to utilize oxygen and increasing D O 2 will have little effect in correcting the hypoxia. Any cause of microcirculatory dysfunction will affect oxygen delivery, 16 for example, sepsis where nitric oxide production is increased leading to disorders of autoregulation matching of supply with demand within the tissues along with the decreased vascular tone that manifests clinically as hypotension.
Manipulation of global oxygen delivery to improve patient outcome has been the focus of goal-directed haemodynamic therapy GDT since its inception in the s. Given that continuing evidence supports equivalent outcome with low blood transfusion triggers in many clinical contexts haemoglobin concentrations 7. The rate of oxygen consumption depends on cellular metabolic demand and can be manipulated. For example, the use of therapeutic hypothermia to reduce cerebral metabolic demand post-cardiac arrest in order to improve neurological outcome is well documented.
Factors that affect oxygen consumption. Adapted from McLellan and Walsh If D O 2 continues to decrease further below the D O 2 crit, or if V O 2 increases for a given D O 2 crit, tissue hypoxia ensues with resultant anaerobic respiration and lactate production secondary to an imbalance between ATP supply and demand producing a type A hyperlactataemia.
It is also important to highlight that even if global oxygen consumption appears to be supply independent, it does not rule out pathological oxygen supply dependency at a regional or local level, which may only manifest clinically at a later stage.
Figure 2 illustrates the theoretical biphasic relationship between oxygen consumption and oxygen delivery. Points B and E depict D O 2 crit in health and critical illness, respectively. O 2 ER is known to increase during exercise, peaking at maximal exercise at 0. This is because although D O 2 increases, it does not match the increase in V O 2 required by exercise. In critical illness, however, especially sepsis, V O 2 may continue to increase, even with increasing D O 2 demonstrated by the line EF , and D O 2 crit may be greater than in health.
The gradient of slope DE is reduced in critical illness as the tissues are less able to extract oxygen. A graph depicting the relationship between V O 2 and D O 2. Within the lung, oxygen diffuses from the alveoli into the pulmonary capillaries, driven by the gradient between the partial pressure of oxygen in the alveolar space and that in the deoxygenated pulmonary capillary blood. In the tissues, oxygen diffuses down a gradient between oxygenated blood in the systemic capillaries and the oxygen-consuming cells.
Diffusion can be described by either a phenomenological approach using Fick's laws or an atomistic approach applying the principle known as the random walk of the diffusing particles another example of which is Brownian motion.
Thus, although the global oxygen delivery oxygen flux may be manipulated through changes in cardiac output and oxygen content, at a tissue level diffusion distance and partial pressure gradients will have the greatest effect in altering the diffusive oxygen flux. This is shown in Figure 3. A diagram illustrating the importance of diffusion distance from capillary to cell and local oxygen tension in determining diffusive oxygen flow rate.
Whole-body oxygen transport and utilization can be estimated using two principle approaches: It is worth noting that expired gas analysis, although less invasive, is more direct in its measurement of cellular oxygen consumption. Estimation of oxygen mass transport, through separate measurement of cardiac output and the elements of oxygen content.
In combination with the latter approach, additional measurement of mixed venous oxygen content allows calculation of oxygen extraction and therefore oxygen consumption. Evaluation of oxygen consumption through measurement of steady state, or dynamically changing, oxygen uptake using expired gas analysis to measure gas flows and concentrations [cardiopulmonary exercise testing CPET , metabolic cart].
In addition to its use in the physiological assessment of elite athletes, CPET has been developed as a tool to assess a patient's preoperative functional capacity, that is, their ability to do external physical work, before major surgery. Also determining V O 2peak , a subject's ventilatory anaerobic threshold AT may be calculated.
While this is often presented as being evidence of the demand for oxygen outstripping supply, it may in fact be more closely related to the recruitment of muscle fibres with different patterns of metabolism. A high level of functional capacity physical fitness is an index of a substantial physiological reserve over and above resting values. This in turn is inferred to provide benefit in withstanding the physiological challenge of major surgery.
In patients undergoing major surgery, postoperative morbidity and mortality are consistently increased in individuals with lower values of AT and V O 2peak. An example of a CPET nine-panel plot data from authors' laboratory. Panels 1—3 are in the first row, 4—6 in the second row, and 5—9 in the third row.
The AT can also be ascertained by evaluating: the ventilatory equivalents for oxygen and carbon dioxide in panel 4; end-tidal oxygen tension in panel 7; and ventilatory equivalents against workload in panel 9.
The vertical red line denotes the AT. Originally, measurement of these variables required thermodilution techniques and a pulmonary artery right heart catheter; 27 however, this modality has subsequently gone out of favour following concerns about its safety.
Breathing in and out is accomplished by respiratory muscles Control of Breathing Breathing is usually automatic, controlled subconsciously by the respiratory center at the base of the brain. Breathing continues during sleep and usually even when a person is unconscious The function of the respiratory system is to move two gases: oxygen and carbon dioxide. Gas exchange takes place in the millions of alveoli in the lungs and the capillaries that envelop them.
As shown below, inhaled oxygen moves from the alveoli to the blood in the capillaries, and carbon dioxide moves from the blood in the capillaries to the air in the alveoli. Three processes are essential for the transfer of oxygen from the outside air to the blood flowing through the lungs: ventilation, diffusion, and perfusion.
Diffusion is the spontaneous movement of gases, without the use of any energy or effort by the body, between the alveoli and the capillaries in the lungs. The body's circulation is an essential link between the atmosphere, which contains oxygen, and the cells of the body, which consume oxygen. For example, the delivery of oxygen to the muscle cells throughout the body depends not only on the lungs but also on the ability of the blood to carry oxygen and on the ability of the circulation to transport blood to muscle.
In addition, a small fraction of the blood pumped from the heart Function of the Heart The heart and blood vessels constitute the cardiovascular circulatory system. Merck and Co. The cilia propel foreign particles trapped in the mucus toward the pharynx. The cartilage provides strength and support to the trachea to keep the passage open. The forced exhalation helps expel mucus when we cough.
The trachea and bronchi are made of incomplete rings of cartilage. Lungs: Bronchi and Alveoli The end of the trachea bifurcates divides to the right and left lungs. The lungs are not identical. The right lung is larger and contains three lobes, whereas the smaller left lung contains two lobes. The muscular diaphragm , which facilitates breathing, is inferior to below the lungs and marks the end of the thoracic cavity. The trachea bifurcates into the right and left bronchi in the lungs.
The right lung is made of three lobes and is larger. To accommodate the heart, the left lung is smaller and has only two lobes. In the lungs, air is diverted into smaller and smaller passages, or bronchi.
Air enters the lungs through the two primary main bronchi singular: bronchus. Each bronchus divides into secondary bronchi, then into tertiary bronchi, which in turn divide, creating smaller and smaller diameter bronchioles as they split and spread through the lung. Like the trachea, the bronchi are made of cartilage and smooth muscle. At the bronchioles, the cartilage is replaced with elastic fibers. In humans, bronchioles with a diameter smaller than 0. They lack cartilage and therefore rely on inhaled air to support their shape.
As the passageways decrease in diameter, the relative amount of smooth muscle increases. The terminal bronchioles subdivide into microscopic branches called respiratory bronchioles.
The respiratory bronchioles subdivide into several alveolar ducts. Numerous alveoli and alveolar sacs surround the alveolar ducts. The alveolar sacs resemble bunches of grapes tethered to the end of the bronchioles. In the acinar region, the alveolar ducts are attached to the end of each bronchiole.
At the end of each duct are approximately alveolar sacs , each containing 20 to 30 alveoli that are to microns in diameter. Gas exchange occurs only in alveoli. Alveoli are made of thin-walled parenchymal cells, typically one-cell thick, that look like tiny bubbles within the sacs.
Alveoli are in direct contact with capillaries one-cell thick of the circulatory system. Such intimate contact ensures that oxygen will diffuse from alveoli into the blood and be distributed to the cells of the body.
In addition, the carbon dioxide that was produced by cells as a waste product will diffuse from the blood into alveoli to be exhaled. The anatomical arrangement of capillaries and alveoli emphasizes the structural and functional relationship of the respiratory and circulatory systems. This organization produces a very large surface area that is available for gas exchange.
The surface area of alveoli in the lungs is approximately 75 m 2. This large surface area, combined with the thin-walled nature of the alveolar parenchymal cells, allows gases to easily diffuse across the cells. Terminal bronchioles are connected by respiratory bronchioles to alveolar ducts and alveolar sacs.
Each alveolar sac contains 20 to 30 spherical alveoli and has the appearance of a bunch of grapes. Air flows into the atrium of the alveolar sac, then circulates into alveoli where gas exchange occurs with the capillaries. Mucous glands secrete mucous into the airways, keeping them moist and flexible. Birds face a unique challenge with respect to breathing: They fly. Flying consumes a great amount of energy; therefore, birds require a lot of oxygen to aid their metabolic processes.
Birds have evolved a respiratory system that supplies them with the oxygen needed to enable flying. Similar to mammals, birds have lungs, which are organs specialized for gas exchange. Oxygenated air, taken in during inhalation, diffuses across the surface of the lungs into the bloodstream, and carbon dioxide diffuses from the blood into the lungs and expelled during exhalation.
The details of breathing between birds and mammals differ substantially. In addition to lungs, birds have air sacs inside their body. Air flows in one direction from the posterior air sacs to the lungs and out of the anterior air sacs.
The flow of air is in the opposite direction from blood flow, and gas exchange takes place much more efficiently. This type of breathing enables birds to obtain the requisite oxygen, even at higher altitudes where the oxygen concentration is low.
This directionality of airflow requires two cycles of air intake and exhalation to completely get the air out of the lungs. Decades of research by paleontologists have shown that birds evolved from therapods, meat-eating dinosaurs.
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