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The contribution of otherfactors can be gleaned by considering that average oxygenconsumption in humans at rest is 250 milliliters of oxygen perminute (mL O2/min), a rate that cannot be supported simplyby diffusion and delivery of dissolved gas to tissues. In fact,transport of gases is a complex process that affects other processes beyond respiration, including acid–base balance, whichwill also be discussed in this chapter.TRANSPORT OF OXYGENThe concentration of dissolved gas in a liquid is directly proportional to the partial pressure of the gas in the atmosphereto which the liquid is exposed (Henry’s law) and the solubilityof the gas in the solvent.
For each mm Hg PO2, only 0.003 mLO2 will dissolve in 100 mL of blood at body temperature.Because PaO2 (PO2 in arterial blood) is normally approximately100 mm Hg (equilibrated with PAO2, PO2 in alveolar air), theamount of dissolved oxygen in arterial blood is normally only0.3 mL O2/100 mL blood (Fig. 15.1). The actual measuredconcentration of oxygen in normal arterial blood is approximately 20.4 mL O2/100 mL blood. What accounts for this largedifference? The answer is binding of O2 to the hemoglobin inred blood cells.
The average concentration of hemoglobin(Hb) is 15 g/100 mL blood, and each gram (g) of hemoglobinbinds 1.34 mL O2 when fully saturated. Therefore, by far, thebulk of oxygen transport is accomplished by hemoglobin.Oxygen Binding Capacity and Oxygen Contentof BloodThe oxygen binding capacity of blood is given by the followingformula:O2 binding capacity = (1.34 mL O2/g Hb) ×(g Hb/100 mL blood)Thus, for the normal hemoglobin concentration of 15 g/100 mLblood,O2 binding capacity = (1.34 mL O2/g Hb) ×(15 g Hb/100 mL blood) = 20.1 mL O2/100 mL bloodOxygen content of blood can be calculated by the followingformula:O2 content = % saturation × O2 binding capacity +dissolved oxygenGiven normal values of 15 g hemoglobin/100 mL blood andapproximately 100% oxygen saturation of arterial blood atPO2 of 100 mm Hg, oxygen content in arterial blood can becalculated as shown below:Arterial O2 content = 100% × (1.34 mL O2/g Hb) ×(15 g Hb/100 mL blood) + (0.003 mL O2/100 mL blood/mm Hg) × (100 mm Hg) = 20.4 mL O2/100 mL bloodArteriovenous Oxygen Gradient andOxygen ConsumptionCompared with this arterial O2 content (20.4 mL O2/100 mLblood), in normal venous blood, in which PO2 is approximately 40 mm Hg and hemoglobin is 75% saturated,Venous O2 content = 75% × (1.34 mL O2/g Hb) ×(15 g Hb/100 mL blood) + (0.003 mL O2/100 mL blood/mm Hg) × (40 mm Hg) = 15.2 mL O2/100 mL bloodEach deciliter of blood delivers approximately 5 mL of O2 tothe tissues as it passes through the systemic circulation,because the arteriovenous difference in oxygen content ofblood is 5 mL.
Based on this arteriovenous difference inOxygen binding capacity is the maximum amount ofoxygen that can be bound to hemoglobin in blood (at100% saturation). The actual oxygen content of blood dependson the degree of saturation of hemoglobin. Dissolved oxygen isa minor portion of the oxygen content of blood; in a personbreathing normal air, it is only 0.3 mL/100 mL blood, compared to the total content of oxygen of 20.4 mL/100 mL bloodat 100% saturation.180Respiratory PhysiologyOxygen TransportO2 in solution in plasma0.003 mL O2/100 mL plasma/mm Hg PO2O2 combined with Hb1.34 mL O2/g HbO2HbO2HbO2HbO2HbBloodstreamAlveoliof lungRed bloodcellPlasmaBodytissuesFigure 15.1 Oxygen Transport Oxygen diffuses into the blood flowing through alveolar capillariesand is transported to tissues, where it diffuses out of the blood along its concentration gradient.
Transportof oxygen in the blood is mainly in the form of oxygen combined with hemoglobin, with only a minor portioncarried in the form of dissolved oxygen.oxygen content of blood and cardiac output, oxygen consumption can be estimated. Thus, at rest,O2 consumption = [a − v]O2 × cardiac output =(5 mL O2/100 mL) × 5000 mL/min = 250 mL O2/minAt the usual respiratory quotient (CO2 production/O2 consumption) of 0.8, CO2 production is 200 mL/min.of oxygen from a molecule of hemoglobin makes dissociationof the other bound oxygen molecules from that moleculemore likely.To summarize:■■THE OXYHEMOGLOBIN DISSOCIATION CURVEAs discussed, at PO2 of 100 mm Hg, hemoglobin is nearly100% saturated with oxygen (actually, 97.5%), and at40 mm Hg, hemoglobin is 75% saturated.
The oxyhemoglobin dissociation curve (Fig. 15.2) describes the relationshipbetween oxygen saturation of blood (SO2) and PO2. The sigmoidal shape of the curve is due to cooperative binding ofhemoglobin. Each molecule of hemoglobin is capable ofbinding four oxygen molecules; when one molecule binds toan oxygen binding site, the other three sites bind oxygen morereadily, causing the middle portion of the curve to be steep.Thus, exposure of blood to the high PO2 in the respiratoryzone of the lung results in binding of a substantial amount ofoxygen. Note also that due to the sigmoidal shape, PAO2 canfall substantially without greatly affecting the degree of saturation of hemoglobin (at PO2 of 80 mm Hg, SO2 is still above95%).
On the other hand, as PO2 falls in blood coursingthrough systemic capillaries, typically to 40 mm Hg, bloodPO2 is on the steep portion of the curve, which facilitatesdelivery of oxygen to the tissues. Dissociation of one molecule■Cooperative binding of oxygen to four binding sites perhemoglobin molecule is responsible for the sigmoidalshape of the oxyhemoglobin dissociation curve.In the lungs, blood will become fully saturated withoxygen over a wide range of PO2, due to the flat, upperportion of the oxyhemoglobin dissociation curve.At the lower PO2 levels in tissue capillaries, small changesin PO2 result in dissociation of relatively large amountsof oxygen, due to the steep middle portion of the curve,facilitating oxygen delivery to the tissues.Factors Affecting the OxyhemoglobinDissociation CurveIn addition to these qualities, another important characteristicof the oxyhemoglobin dissociation curve is that it is shifted tothe right under conditions of increased PCO2, low pH, andhigh temperature (see Fig.
15.2). These conditions occurlocally during tissue hypoxia and increased metabolism (forexample, during exercise), and the rightward shift of the curveresults in decreased hemoglobin affinity for oxygen, and thusenhances delivery of oxygen to tissues.
The metabolite of redblood cell glycolysis, 2,3-diphosphoglycerate (2,3-DPG; alsoknown as 2,3-bisphophoglycerate), also shifts the curve to theright and is elevated during hypoxia.Oxygen and Carbon Dioxide Transport and Control of RespirationCLINICAL CORRELATEAnemiaAnemia is the state of low hemoglobin or red blood cell count inblood, resulting in reduced oxygen binding capacity. It may be theresult of blood loss, insufficient erythropoiesis (reduced red bloodcell production), or hemolysis (red blood cell destruction).
Irondeficiency may also cause anemia in menstruating women.Hypoxia associated with anemia cannot be corrected by adminis-181tration of oxygen, because only the minor proportion of oxygenthat is dissolved in arterial blood will be raised by elevating alveolar oxygen concentration, and oxyhemoglobin (oxygen bound tohemoglobin) will remain low due to the hemoglobin deficiency.Although mild anemia can be treated by a variety of interventionsthat address the primary cause of the anemia, severe anemiarequiring immediate medical attention must be treated by bloodtransfusion.Signs and symptomsAbnormalitiesof productionStomatitis and glossitisLow dietary ironThyroid diseaseLiver diseaseMalabsorptionChemotherapy/ RadiationShortness of breath (late)Abnormalitiesof destructionExcessive blood loss (menorrhagia)HemolysisSickle cell diseaseLaboratory studiesMean corpuscular volume(MCV), reticulocyte count,blood smear, hemoglobinelectrophoresis. Otherstudies as indicated on a percase basis:serum iron, totaliron binding capacity,serum ferritin, B12, folateAnemia Destruction of red blood cells, bleeding, or abnormalities of red blood cell production are causesof anemia.
Laboratory studies are useful in differential diagnosis of the cause.Oxygen delivery to the fetus involves transfer of oxygenfrom maternal blood to fetal blood across the placenta.Fetal blood contains a form of hemoglobin known as fetalhemoglobin (hemoglobin F) that has higher affinity foroxygen than adult hemoglobin (hemoglobin A), facilitatingtransfer of oxygen from maternal to fetal blood. Thus, theoxyhemoglobin dissociation curve for hemoglobin F is shiftedto the left, compared with the dissociation curve for hemoglobin A.
At a given PO2, oxygen saturation of hemoglobinF will be higher than that of hemoglobin A. HemoglobinF is replaced by hemoglobin A within the first 3 monthsof life.TRANSPORT OF CARBON DIOXIDEThe concentration of carbon dioxide is highest in the mitochondria, where it is produced during cellular respiration.From there, it diffuses to the interstitium and eventually intothe blood, which transports it to the alveoli. Within the blood,CO2 is transported in three forms (Fig.
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