Gas Transport: lungs and Periphery Flashcards
PO2 differences in air versus systemic blood?
The partial pressure of O2 in inspired ambient air (PIO2) at sea level is about 159 Torr. However, the PO2 decreases to 149 Torr as water vapor (PH2O) is added to the inspired air in the upper airway. The alveolar PO2 is only about 104 Torr because inspired air is diluted and mixed with alveolar air. A PAO2 of about 104 Torr is equilibrated with the blood flowing through the lung capillaries. However, there is a further decline in the PO2 between the alveolus (PAO2) and the systemic arterial blood (PaO2) which is termed the alveolar to arterial difference. In systemic arterial blood, the PaO2 is normally between 96 to 100 Torr. The PaO2 obtained from a measurement of an arterial blood gas, together with hemoglobin concentration and cardiac output, can be used to estimate O2 delivery to the metabolizing tissues of the body.
Explain the influence of alveolar ventilation on Alveolar O2 and CO2?
Hyperventilation is when alveolar ventilation exceeds metabolic rate or exceeds the demand for either oxygen uptake or carbon dioxide removal. Hyperventilation results in an increase in the PAO2 that approaches but never reaches the PIO2 as A V x increases. Conversely, the alveolar PCO2 declines with hyperventilation so that doubling alveolar ventilation from the resting rate reduces the PACO2 by about one half. With hypoventilation, alveolar ventilation is at a rate inadequate for the body’s demand for O2 consumption or for CO2 elimination. With hypoventilation, the alveolar PO2 declines and the PAO2 increases.
Explain the solubility of respiratory gases?
Within the alveolus, O2 is present as a constituent gas of a gas mixture. Before diffusion can take place, O2 must physically dissolve in the materials and liquids of the barriers that separate alveolar gases from blood. Respiratory gases have different solubilities in body fluids. For example, CO2 is about 24 times more soluble than O2 in body fluids. When a gas dissolves in a liquid, it will exert a partial pressure so long as it does not chemically unite with anything in the liquid. After a short time, the partial pressure of the gas dissolved in a liquid will be the same as partial pressure of the gas in the air to which it was equilibrated. The partial pressure of a gas dissolved in a liquid is directly related to its concentration in the liquid, as stated by Henry’s Law. The volumes of O2, CO2, and N2 that can be physically dissolved in water or blood plasma at 37qC, when exposed to 1 atmosphere (760 Torr) of thegas, are presented in the table.
Concentration versus partial pressure? (henry’s law)
Explain how CO2 different solubility affects it in the lungs using henry’s law?
Explain the difference between concentrations and partial pressures in a soultion?
The solubility, and hence concentration, of a gas in a liquid depends upon the nature and temperature of the liquid. For example, respiratory gases are slightly more soluble in water than in plasma. In addition, the solubility of gases in liquids increases as the temperature of the liquid decreases. So, during hypothermic states, the solubility of gases increases modestly from normothermic conditions. The concentration of a gas dissolved in a liquid is directly related to its solubility in the liquid. The concentration of a highly soluble gas, like CO2, will be greater than that of a less soluble gas, like O2, at the same partial pressure. For example, compare the concentration of O2 and CO2 dissolved in 100 ml of plasma, when the plasma is equilibrated with the same partial pressure of O2 or CO2 (figure). The concentration of CO2 is about 24 times greater than O2 because of the greater solubility of CO2 even though their partial pressure are identical.
Explain the O2 that physically dissolves in the capillary and systemic blood?
When pulmonary capillary blood is equilibated with an aleolar PO2 of 100 Torr, the amount of O2 in physical solution is only 0.3 ml O2/100 ml of blood (0.3 vol%). This small concentration of physically dissolved O2 in pulmonary capillaries is related to the low solubility of O2 in plasma. If a person had a total blood volume of 6L consisting of only plasma (no erythrocytes or RBCs), then the total O2 content possible would be only 18 cc in the entire 6L blood volume. With a typical resting oxygen consumption of 250 cc/min and O2 is only in physical solution, cardiac output would have to be over 83 L/min to supply O2 to the metabolzing cells, provided all the O2 was extracted. However, if O2 content is measured in arterial blood, with RBCs present, the normal O2 content is about 19.6 vol%. The difference between whole blood and plasma O2 content is accounted for by the presence of the RBC. The RBC carries over 98% of the O2 present in blood. The bulk of O2 is transported by reversible chemical combination with hemoglobin that is located inside the RBC. Only a small portion of the O2 is transported as a physically dissolved gas in plasma.
Explain the hemoglobin moelcule and its use in transport? Gram equivalent combining weight of Hb for O2?
The hemoglobin molecule has an atomic weight of 67,000 atomic mass units (amu). Each Hb molecule is capable of combining with four molecules of O2. Thus, the gram weight of Hb combining with one mole of O2 is 67,000 grams Hb/4 O2, or 16,700 grams of Hb. This is termed the gram-equivalent combining weight of Hb for O2. One mole of O2 is equal to 32 grams and occupies 22.4 L,STPD. Hence, the number of ml of O2 that combines with 1 gram of Hb is equal to: 22,400 ml O2/16,700 gm equivalent of Hb = 1.34 ml O2/g of Hb. The gram-equivalent combining capacity of Hb of 1.34 ml O2/g Hb is important to remember because it is used to calculate the amount of O2 that attaches to Hb when the blood is completely saturated with O2. For example, if blood contains 15 grams of Hb/100 ml of blood, the O2 capacity of Hb is 20.1 ml O2/100 ml blood (20.1 vol%). This is the amount of O2 bound to Hb when all the binding sites are occupied, or Hb is 100% saturated with O2.
Hb takes the total O2 binding capacity from .3ml O2/dL to 20.0ml O2/dl
Explain the Oxyhemoglobin dissociation curve?
When oxygen combines with Hb, it is appropriately termed oxyhemoglobin (oxy-Hb). When Hb is totally devoid of O2, it has a relatively low affinity for O2. However, the polypeptide chains of Hb interact in such a manner that once the first O2 molecule attaches to Hb, it facilitates the binding of the other O2 molecules. These O2 binding characteristics of Hb account for the sigmoidal-shaped oxyhemoglobin dissociation curve (see figure). The amount of O2 that attaches to Hb is related to the PO2 of the surrounding plasma. In the normal lung capillary, the PO2 of plasma is normally the same as alveolar PO2. The extent of O2 combination with Hb is termed % saturation (SbO2). The SbO2 is the ratio of the actual amount of O2 combined with Hb to the O2 capacity of Hb at full saturation, expressed as a percentage. The oxyhemoglobin dissociation curve is formed by plotting the SbO2 as a function of the PO2. Hb becomes 100% saturated with O2 (SbO2 = 100%) when the PO2 reaches about 250 Torr. However, Hb is typically 97.5% saturated at a normal alveolar PO2 of 100 Torr because of the unique sigmoidal shape of the oxy-Hb dissociation curve. As illustrated, mixed venous blood of the pulmonary artery is denoted by “V” on the oxy-Hb dissociation curve. It has a PO2 of about 40 Torr and an O2 content of 14.8 vol%, with a SbO2 of 75%. As this mixed venous blood is equilibrated with an alveolar PO2 of 100 Torr, the blood O2 content increases to 19.6 vol%, with an Sb O2 of 97.5%. This is indicated by “A” on the curve to signify arterialized (oxygenated) blood. Thus, with oxygenation in lung capillaries, the O2 content increases from 14.8 to 19.6 vol%, which corresponds to an arterialto-venous content difference of 4.8 vol% or a net increase in blood O2 content of 4.8 vol%. At the same time, the PO2 of blood increases from 40 Torr in the pulmonary artery to about 100 Torr after equilibrium with alveolar air. This yields a pulmonary artery-to-vein PO2 difference(a-v PO2 differenceof 60 Torr).
Explain the shape of the Oxyhemoglobin dissociation curve and why its important?
The sigmoidal shape of the oxy-Hb dissociation curve has physiological importance for both the loading of O2 in the lungs and for unloading O2 in the tissue capillaries. Note that the upper portion of the curve, between a PO2 of 70 to 100 Torr, is nearly flat (see figure). This portion of the curve is often referred to as the association part of the curve because it is important in the loading of O2 (association of O2 with Hb) in the lung capillary. The association part of the curve insures oxygenation of most of the Hb even when alveolar PO2 is decreased due to altitude ascension or pulmonary disease. Note that the SbO2 decreases from 97.5% at a PO2 of 100 Torr to 92% at a PO2 of 70 Torr with only a change of 1.0 vol% in blood O2 content. Thus, this flat portion of the oxy-Hb dissociation curve insures nearly normal loading of Hb with O2 even when the alveolar PO2 is reduced from normal.
On the other hand, the steep sloping part of the curve, between a PO2 of 50 and 20 Torr is termed the dissociationportion of the curve, as shown (see figure). The dissociation portion of the curve is important in the tissue capillaries where a large amount of O2 can be unloaded for a relatively small change in the PO2. For example, a decrease in the PO2 from 50 to 20 Torr reduces the blood O2 content by over 10 vol% or by nearly 50%. Thus, a sizable portion of the O2 carried by Hb is availablefor use by the tissues for a relatively small change in the PO2. In other words, Hb relinquishes a relatively large amount of O2 for a small change in the PO2. The transition from the association to dissociation portion of the curve is normally at a PO2 of around 60 Torr. The curve is very steep below, and relatively flat above this PO2.
Explain the shift in the oxyhemoblobin curve?
The oxy-Hb dissociation curve is also capable of shifting to the right or to the left. An increase in the blood PCO2 or hydrogen ion concentration [H+] (decrease pH) shifts the curve to the right, whereas a decrease in PCO2 or [H+] (increase pH) shifts the curve to the left. Shifts in the oxy-Hb dissociation curve due to changes in the blood PCO2 or pH are termed the Bohr effect. An increase in blood temperature or 2,3-biphosphoglycerate (2,3BPG) levels in the RBC also shift the oxy-Hb dissociation curve to the right, while a decrease in temperature or 2,3-BPG shifts the curve to the left. A shift in the oxy-Hb dissociation curve to the right means that more O2 is liberated for a given decrease in the PO2. Stated another way, a shift in the curve to the right indicates that the affinity of Hb for O2 is reduced, so that for a given plasma PO2, more O2 is freed from Hb. In contrast, a shift in the curve to the left means more O2 will be attached to Hb (increased affinity) for a given PO2. Thus, less O2 is available to the tissues or is freed from Hb at a given PO2.
what does an increase in temperature and increase in PCO2 lead to on the oxyhemoglobin curve?
Explain the significance of shifts in the oxyhemoglobin curve?
Shifts in the oxy-Hb dissociation curve have little effect on O2 loading or the association part of the curve. As a result, shifts in the curve have a minimal effect upon HbO2 affinity in the lung capillaries. The primary effects of shifts in the oxy-Hb dissociation curve are upon the unloading of O2 in the tissue capillaries. Shifts to the right result in increased free O2 and less O2 bound to Hb. On the contrary, a curve shift to the left means less O2 is free and more is bound to Hb at a given PO2. In an actively contracting skeletal muscle, the need for O2 uptake is nearly maximal; there is likely to be a local increase in the PCO2, and [H+] and temperature from increased metabolic activity. All these stimuli tend to shift the oxy-Hb curve to the right to free more O2 for a given PO2. Thus, more O2 is available to the muscle during periods of increased O2 demand and utilization from shifts in the oxy-Hb curve in the muscle capillary.
Effect of 2,3BPG on the oxyhemoglobin curve?
If Hb is removed from the RBC and placed in solution, the resulting oxy-Hb curve lies far to the left of the normal curve. If 2,3-diphosphoglycerate (2,3-BPG) is added to this Hb solution, the oxy-Hb curve is shifted to the right as the 2,3-BPG concentration is increased.
2,3-BPG is present in trace amounts in most mammalian cells as an intermediate product of glycolysis, but is more abundant in RBCs. Unlike most mammalian cells that convert 1,3-BPG to 3phosphoglycerate (3-PG) to generate an ATP (figure), the RBC, during periods of low oxygen, converts 1,3-BPG to 2,3-diphosphoglycerate prior to the conversion, the 2-3-BPG formed attached to the globulin chain of Hb to reduce the affinity of Hb for O2. This results in a shift in the oxy-Hb curve to the right. The synthesis of 2,3-BPG increases during anaerobic or hypoxic conditions, and 2,3-BPG formation appears to be an important mechanism for responding to acute increases in tissue O2 needs. With anemia, the oxy-Hb dissociation curve is shifted to the right as a result of increased 2,3-BPG formation. The net effect of 2,3-BPG is similar to that of an increase in [H+] in that it enhances O2 release from Hb by reducing its affinity for O2.
Increasing 2,3 BPG leads to?
Carbon monoxide posoining versus anemia effects on the oxyhemoglobin curve?
In both cases, the available Hb sites become almost fully O2 loaded in the lungs (i.e., all available sites are filled). The anemic individual has a reduced total amount of Hb so the individual would be 100% saturated if the alveolar PO2 was 100 mm Hg. In the case of CO poisoning, 40% of the sites are occupied by CO leaving 60% of sites available for binding of O2 to Hb. Thus, the 40% CO poisoned individual can only saturate 60% of Hb with O2 at an alveolar PO2 of 100 mm Hg. The addition of the second Y-axis for saturation illustrates this key point. In terms of O2 content, both the CO poisoned individual and the anemic individual have a reduced content relative to the normal individual (~20 ml O2/100 ml blood). It is important to also note that the CO poisoned individual will have a leftward shift in the Oxy-Hb curve while the anemic individual’s curve may actually shift rightward. As a result, O2 will disassociate as the blood enters tissues with a PO2 approaching 30 mm Hg for both the normal and anemic individual. The content of O2 that disassociates from Hb in the anemic individual is sufficient for tissue at rest. However, if tissue oxygen demand significantly increases e.g. exercise, the anemic individual has little remaining O2 to meet the increased demand leading to tissue hypoxia in the metabolically active tissue. This partially explains the observation that an anemic patient will easily fatigue. In the case of the CO poisoned individual, note that there is very little O2 disassociation occuring at a normal tissue PO2 of ~30 mm Hg. It is not until the tissue PO2 drops to 10 to 15 mm Hg, that the CO poisoned Hb will unload 50% of its total content. This is due to the fact that CO not only reduces total content of O2, the affinity of Hb for O2 in the poisoned individual is increased. Thus, tissue PO2 levels decrease until a tissue PO2 is achieved that result in O2 disassociation. While the content of both the CO poisoned individual and the anemic patient are roughly half of the normal individual, the altered disassociation of O2 in the CO poisoned individual results in tissue hypoxia at rest.
Methemoglobinemia versus anemia effects on the oxyhemoglobin curve?
Methemoglobin is an abnormal hemoglobin in which the iron moiety of unoxygenated hemoglobin is the ferric (FE 3+) state rather than the ferrous (FE2+) state. When hemoglobin is in the ferric state (methemoglobin) the oxygen content of blood is reduced. Methemoglobin naturally occurs in the body at low levels (1-2%), and endogenous systems are in place to reduce the ferric state back to the ferrous state. Higher concentrations define methemoglobinemia.
The oxyhemoglobin dissociation curve of blood in an anemic individual follows a curve similar to that of an nonanemic individual. Although the oxygen content is lower, unbinding of half of the oxygen (50% oxygen saturation) occurs at the same PO2. With 50% methemoglobin, the methemoglobin is unable to bind oxygen. In addition, the leftward shift of the oxyhemoglobin dissociation curve means that hemoglobin is less willing to give up its oxygen. Consequently, tissue hypoxia is more severe than in those with a 50% anemia.
Relationship of blood PO2 and SbO2 to total O2 content?
The PO2 of blood is usually a poor indicator of the total blood O2 content because it only reflects the small amount of O2 physically dissolved in plasma and not the O2 attached to Hb. Since most of the O2 is carried by Hb, the Hb concentration is normally the most important factor in determining blood O2 content. The amount of O2 attached to Hb at a particular PO2 is positively correlated with the Hb concentration. The higher the Hb concentration, the greater the O2 content of blood at a given PO2. However, like the PO2, the SbO2 is also a poor indicator of actual O2 content or concentration because the SbO2 is calculated as the ratio of actual O2 content of Hb to the O2 capacity of Hb. Thus, when the Hb concentration is decreased, both actual content and capacity of Hb for O2 are reduced at a given PO2, so the SbO2 is largely unchanged (figure). In summary, blood O2 content varies directly with the Hb concentration. In short, neither the PO2 nor the SbO2 are reliable indicators of blood O2 content when the Hb concentration is not known.
Explain CO2 transport in the blood?
Tissue-produced CO2 is transported in the systemic venous blood to the lung where it is excreted by exhalation. In the tissue, CO2 diffuses as a physically dissolved gas into capillary blood by simple diffusion. Each minute, the metabolizing cells of the body produce 200 to 300 ml of CO2, in a resting adult. CO2 must continuously be excreted to avoid acidbase problems. However, systemic arterial blood entering the tissue capillary already contains a considerable amount of CO2. A certain amount of CO2 is retained and maintained in the blood for the purpose of acid-base balance. To keep blood PCO2 constant and preserve acid-base balance, the amount of CO2 exhaled by the lung needs to equal the amount of CO2 produced by the tissues of the body.
CO2 in systemic Arterial blood?
Systemic arterial blood entering the tissue capillary contains an appreciable amount of CO2. However, most of the CO2 present is notin the form of a physically dissolved gas. As shown in the CO2 hydration reactionabove, CO2 reacts with water (hydration) to produce carbonic acid (H2CO3). H2CO3 is unstable and readily dissociates into bicarbonate (HCO3-) and hydrogen (H+) ions. The relative concentrations of the different chemical transport forms of CO2 in arterial blood are shown below the CO2 hydration reaction (figure). In arterial blood entering the tissue capillary at a pH = 7.4, most of the CO2 is present as bicarbonate ion. Smaller amounts of CO2 are transported as the physically dissolved gas (CO2) and carbonic acid (H2CO3). The H + formed from the dissociation of carbonic acid is buffered so the concentration of free H+ ions is negligible compared to HCO3-. Some CO2 is also present as carbamino groups of both Hb and plasma proteins, formed by the chemical reaction shown in equation 2 (figure). Thus, CO2 is present in systemic blood as several chemicals entities that include physically dissolved CO2, bicarbonate ion, carbamino groups of proteins, and carbonic acid. In the tissue capillary, tissue-produced CO2 is added to the CO2 already present in arterial blood.
Explain Loading of the CO2 in the tissue Capillary?
Physically dissolved CO2 enters the tissue capillary from the cells by simple diffusion but a small portion (5%) remains in this form. However, it accounts for the increase in the PCO2 from arterial to venous blood. Most of the CO2 added in the tissue capillary is converted to other transport forms of CO2. About 90% of the total CO2 added in the capillary depends on the RBC for transport, while the remaining 10% is independent of the RBC. About 63% of the CO2 added in the tissue capillary enters the RBC and undergoes hydration to form carbonic acid. H2CO3 is unstable and quickly dissociates to HCO3- and H+. This hydration reaction is facilitated in the RBC because of the enzyme carbonic anhydrase. However, carbonic anhydrase is absent from the plasma so that CO2 hydration with HCO3formation is very small in plasma (5%). However, as HCO3- is formed in the RBC during CO2 loading and hydration, its concentration increases. This establishes a diffusion gradient and HCO3- exits the RBC to the plasma and in doing so, leaves behind a net positive charge on the inside of the RBC membrane. To preserve electrical neutrality of the RBC membrane, chloride ions (Cl-) move into the RBC from plasma. This exchange of Cl- for HCO3- is called the chloride shift, or Hamburger effect. Another portion of the CO2 (21%) entering the RBC in the tissue capillary forms carbamino groups with the peptides chains of Hb. A small amount of CO2 (5%) remains as a physically dissolved gas inside the RBC to match that of plasma. The H+ produced as a result of CO2 hydration and carbamino group formation in the RBC are buffered to a large extent by Hb. As hydrogen ions attached to Hb, they displace potassium ions (K+). This results in a net gain in the number of osmotically active particles inside the RBC and prompts a net inward diffusion of H2O from the plasma to maintain osmotic balance. This inward flux of water causes the volume and size of the RBC to increase slightly as it traverses the tissue capillary. About 10% of the total CO2 added in the tissue capillary is notdependent on the RBC for transport. It remains as physically dissolved CO2 (5%) and accounts for the increase in the PCO2 from arterial to venous blood. Another 5% of CO2 undergoes hydration to form HCO3-. However, this reaction is slow in plasma because carbonic anhydrase is not present. An even smaller amount ofCO2 (<1%) forms carbamino groups with plasma proteins.
Explain the Haldane effect?
One consequence of CO2 loading in the tissue capillary is the formation of hydrogen ions from CO2 hydration and carbamino group formation. Hemoglobin, and to a lesser extent plasma proteins, are capable of buffering many of the H+ produced as shown above. This buffering of H+ minimizes the blood pH change as it traverses the tissue capillaries. The peptide chains of Hb contain numerous amino and carboxyl groups. The carboxyl groups of Hb are capable of binding, and hence buffering, many of the H+ formed as a result of CO2 transport. Over the normal physiological pH range, much of the buffering of H+ by Hb is performed by the imidazole group of the amino acid histidine, present in the globulin chain. The imidazole group of histidine, like some other buffering groups, is closely associated with the iron atom of the heme portion of Hb. As O2 is released from the iron atom of Hb during transit through the tissue capillary, the electron structure of the Hb peptide chain is altered and groups like imidazole become more capable of binding H+. In other words, when Hb loses O2, it becomes a stronger base or weaker acid, making more sites available to buffer H+. The complete deoxygenation of 1 mM HbO2 to yield 1 mM O2 can result in the neutralization or buffering of 0.7 mM of H+. This ability of Hb to neutralize (buffer) more H+, as it releases oxygen is termed the Haldane effect. The Haldane effect is an important mechanism for facilitating the transport of CO2 by minimizing changes in free H+ or blood pH.
The CO2 dissociation curve?
Total CO2 content is plotted as a function of the blood PCO2 in the figures. The carbon dioxide curve on the left shows the amount of CO2 carried as physically dissolved CO2, HCO3-, and carbamino group according to the blood PCO2 (i.e., physically dissolved CO2). Whereas the physically dissolved CO2 increases linearly, the bulk of CO2 present in blood is in the form of bicarbonate ion, with smaller amounts forming carbamino groups. As noted from the figure on the right, more CO2 can be transported at a given PCO2 if hemoglobin is devoid of O2 because of the Haldaneeffect.
Unloading of the CO2 in the lungs?
The unloading of CO2 in the lung is the reverse of the CO2 loading in tissue capillaries. In the lung, CO2 diffuses from the capillary to the alveoli. The hydration and carbamino reactions in the RBC are driven in the reverse direction from the tissue capillary, towards CO2 formation. Thus, HCO3- moves into the RBC from plasma in exchange for Cl-. HCO3- combines with the H+ liberated from Hb when O2 attaches and forms carbonic acid, which dissociates into CO2 and H2O. Likewise, carbamino group reaction is reversed to produce CO2. However, not all the CO2 present in blood is removed during its transit through the lung capillary as some is retained for the purpose of acid-base regulation. The amount of CO2 removed (excreted) by the lung each minute is closely related to its tissue production rate (V CO2). The actual amount of CO2 removed by ventilation is determined by the respiratory control system, which, in the absence of acid-base disturbances, is set to maintain a systemic arterial PCO2 around 40 Torr.
Summarize blood gas transport?
Molecular oxygen is delivered to the alveoli by ventilation. In the alveoli, O2 moves by simple diffusion into the pulmonary capillary blood. Most O2 binds chemically, but reversibly, with hemoglobin present inside the RBC. O2 is transported by blood flow to the tissue capillaries, where it leaves the Hb to enter tissue cells by simple diffusion. In the tissue capillaries, CO2 formed as an end product of cellular metabolism, is added to the blood. The RBC is important in CO2 transport because carbonic anhydrase, present only in the RBC, facilitates the hydration of CO2 to HCO3- and H+. As a consequence, most of the CO2 carried to the lung for excretion is present as bicarbonate ions. Hb also buffers many of the hydrogen ions formed during CO2 hydration by the RBC. The ability and capacity of Hb to buffer H+ is inversely related to the amount of O2 that remains attached to Hb. When the blood enters the lung capillaries, all the reactions associated with O2 or CO2 transport are in the opposite direction from that in the tissue capillary; CO2 is exhaled (excreted) and O2 is loaded.
Explain in general acidosis. Metabolic/Respiratory
Explain the basis of Respiratory Acidosis?
Hypoventilation leads to an elevated CO2, producing more hydrogen and a reduced pH
Hypoventilation leads to an elevated CO2
Basis of respiratory alkalosis?
What are the normal values for arterial blood gases?
Explain what CO2 and HCO3 that happens with Respiratory and Metabolic acidosis?
Explain the metabolic acidosis expected compensation?
Explain metabolic alkalosis expected compensation?
Respiratory Acidosis expected compensation acute and chronic?
Respiratory alkalosis acute vs chronic expected compensation?
Explain anatomic shunts?
The anatomical shunt is principally accounted for by the bronchial veins that anastomose with pulmonary veins, also known as the bronchopulmonary circulation, and the thebesian veins of the left heart that drain into the left ventricle. These vessels add some poorly oxygenated blood to the oxygenated blood before it reaches the aorta. This results in a lower systemic arterial PO2 than alveolar PO2, and accounts for the normal A - a PO2 difference of about 6 to 9 Torr. The anatomical shunt is an obligatory or normal (true) shunt inherent in the design of the cardiovascular system.
Explain an Alveolar or physiological shunt?
The physiological shunt is mainly due to the mismatching of alveolar ventilation to pulmonary blood flow, or V/Q mismatch, as illustrated in the figure. Mixed venous blood in the pulmonary artery normally enters the lung capillaries with a total O2 content of about 14.6 vol% with a PO2 of about 40 Torr. In the lung capillaries, this blood is equilibrated with alveoli that may have different PO2s depending on their V/Q . In the figure, Alveolus A has normal perfusion and ventilation ( x V/Q =1.0), so blood exposed to O2 in Alveolus A exits with a PO2 of 100 and O2 content of 19.5 vol%. While pulmonary capillary blood flow to Alveolus B is normal, ventilation is impaired due to a partially obstructed airway. As a result, Alveolus B has a low V/Q ratio of 0.1, resulting in a lower-than-normal PO2 of 50 Torr. Thus, the blood exiting Alveolus B adds only enough O2 to increase blood O2 content to 16.0 vol% and the PO2 to 50 Torr. Alveolus C has normal ventilation, but blood flow is partially obstructed. This results in a V/Q of 10 and a greater-than-normal PO2 of 130 Torr. While blood flow from Alveolus C is lower than normal, the blood exhibits a higher-than-normal PO2 and O2 content (20.0 vol%). However, in the pulmonary vein, blood perfusing alveoli of different V/Q mixes to yield a PO2 of about 64 Torr and an O2 content of 17.9 vol%. Hence, a V/Q mismatch results in an Alveolar – arterial PO2 gradient that depends mostly upon the portion of total pulmonary blood flow that perfuses alveoli with a V/Q that is below normal (Alveolus B). Thus, V/Q mismatch contributes to the Alveolar-arterial PO2 gradient. This is referred to as an alveolar or physiological shunt. Physiological shunts normally occur, but usually to a lesser degree than in the example presented. Nevertheless, the physiological shunt can add to the anatomical shunt to further increase the A-aPO2gradient.
Explain a pathological shunt
A pathological shunt results from impaired gas diffusion between alveoli and blood and is associated with some lung pathology, such as pulmonary edema or thickening of alveolar-capillary membrane, as with pulmonary fibrosis. Such a diffusion impairment typically results in a PO2 gradient between the alveoli and pulmonary capillary blood because blood passes through the lung capillary before it can completely equilibrate with the alveolar gases (figure). As a result, the A-aPO2 gradient is increased. With a pathological shunt, diffusion of O2 is often more affected than that of CO2 because of the greater diffusibility of CO2. The difference between alveolar and pulmonary capillary PO2 frequently is not present at low pulmonary blood flow, but increases as blood flow increases, such as during exercise and with hypoxia. At high blood flow in lungs with abnormal diffusion, blood traverses the lung capillary before complete equilibration can occur. Thus, the A-a PO2 difference is increased from the normal gradient caused by the anatomic and physiologic shunt.
