12 The Transport of O2 and CO2 in the Blood Flashcards Preview

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Flashcards in 12 The Transport of O2 and CO2 in the Blood Deck (24)
1

Red blood cells

  • Function
  • Hemoglobin
  • Importance of hemoglobin

  • Function
    • Transfer of O2 from lungs to tissue and CO2 from tissue to lungs
  • Hemoglobin
    • A molecule in the RBC used to overcome the poor dissolving capability of O2  
    • Necessary to deliver O2
      • O2 dissolves poorly in the plasma
  • Importance of hemoglobin
    • When there's O2 in the lungs, it diffuses across alveolar capillary membrane into the RBC
    • O2 binds to hemoglobin molecules
    • O2 bound to the hemoglobin molecules is transported through the bloodstream to the tissues
    • In the tissues, where the O2 tension is lower, O2 is released from the hemoglobin
    • O2 diffuses out into the tissues

2

Hemoglobin structure and function

  • Structure allows...
  • 2 parts
  • Synthesis

  • Structure allows
    • Cooperative binding to bind in the lung and release it in capillaries
  • 2 parts
    • Heme moiety
      • Protoporphyrin ring with an iron atom in center
    • 4 globin chains
      • 2 alpha
      • 2 beta
  • Synthesis
    • Synthesized in the bone marrow and reticulocytes
    • Bone marrow is taken in by cells
    • Transferrin has iron bound to it
    • Some iron is transferred to ferritin
    • Iron is taken into the mitochondria where some molecules are being formed into the protoporphyrin ring
    • Iron binds to the ring and is excreted from the mitochondria in the plasma reticulum
    • Alpha and beta chains are generated in the cytoplasm
    • Chains come together to form hemoglobin

3

Cooperativity

  • First oxygen binds to Fe in heme of Hb
    • Fe is drawn into the plane of the porphyrin ring
  • Conformational changes that are transmitted to adjacent subunits
    • "Allosteric regulation"
  • Increases adjacent subunits' affinity for O2

4

Cooperativity and oxyhemoglobin curve

  • Cooperativity allows for...
  • Oxygen-hemoglobin dissociation curve
  • Shifts in the curve
    • Normal position of curve depends on...
    • Right shift
    • Left shift

  • Cooperativity allows for...
    • Rapid loading and unloading of oxygen
  • Oxygen-hemoglobin dissociation curve
    • Measures cooperativity
    • Sigmoid shape
  • Shifts in the curve
    • Normal position of curve depends on...
      • Concentration of 2,3-DPG
      • H+ ion concentration (pH)
      • CO2 in RBCs
    • Right shift
      • Decreased oxygen affinity
      • High 2,3-DPG
      • High H+ (low pH, acidic)
    • Left shift
      • Increased oxygen affinity
      • Low 2,3-DPG
      • Low H+ (high pH, basic)

5

Arterial oxygen content

  • Major function of the cardiovascular and respiratory systems
  • Oxygen is transported in the blood in two ways
  • The amount of oxygen carried in each form is dependent on...

  • Major function of the cardiovascular and respiratory systems
    • Provide an adequate amount of oxygen to the tissues
    • Failure to accomplish this goal results in tissue hypoxia
  • Oxygen is transported in the blood in two ways
    • (1) In physical solution in the plasma as dissolved oxygen
    • (2) In chemical combination with hemoglobin (HbO2)
  • The amount of oxygen carried in each form is dependent on...
    • The partial pressure of oxygen (PaO2) to which each medium is exposed

6

Arterial oxygen content:
Dissolved oxygen

  • The amount of oxygen transported as dissolved oxygen in blood (at 37OC) is defined by the equation
  • The relationship between dissolved oxygen and the partial pressure of oxygen in the blood 

  • The amount of oxygen transported as dissolved oxygen in blood (at 37OC) is defined by the equation
    • Dissolved oxygen (ml O2/100 ml blood) = 0.003 x PaO2 
  • The relationship between dissolved oxygen and the partial pressure of oxygen in the blood
    • Linear
    • Low oxygen carrying capacity
    • That is, the amount of dissolved oxygen is directly proportional to the PaO2, regardless of the value

7

Arterial oxygen content:
Combined with hemoglobin

  • Hemoglobin 
  • The relationship between the hemoglobin saturation (SaO2) and the PaO2
  • The relationship between the partial pressure of oxygen in arterial blood and the saturation of hemoglobin is influenced by...

  • Hemoglobin
    • Major means of transporting oxygen in the blood
    • When completely saturated with oxygen, one gram of hemoglobin is capable of carrying 1.34 ml of O2
      • 1.34 ml O2 / 100 ml blood x [Hgb] x %sat
    • ​High oxygen carrying capacity
  • The relationship between the hemoglobin saturation (SaO2) and the PaO2
    • Defined by the oxy-hemoglobin association curve shown below
      • Non-linear relationship
    • Relatively steep portion between a partial pressure of oxygen in the range of 10-50 mmHg
    • Relatively flat portion in the range > 70 mmHg
  • The relationship between the partial pressure of oxygen in arterial blood and the saturation of hemoglobin is influenced by...
    • pH
    • Temperature
    • Concentration of inorganic phosphates, such as 2,3-diphosphoglycerate (2,3-DPG)

8

Arterial oxygen content:
Combined with hemoglobin

  • Relationship between the partial pressure of oxygen and the hemoglobin saturation is conveniently defined by the P50
  • An elevation in temperature...
  • More alkaline pH...
  • A reduction in the arterial partial pressure of oxygen from a value of 100 mmHg to 70 mmHg...
  • Further reductions to the levels typical of tissue oxygen partial pressures (about 40 mmHg) will result in...
  • Major form of O2 transport in the blood

  • Relationship between the partial pressure of oxygen and the hemoglobin saturation is conveniently defined by the P50
    • The PO2 at which hemoglobin is 50% saturated
  • An elevation in temperature...
    • Shifts the oxyhemoglobin saturation curve to the right
    • Results in an elevation in the P50
  • More alkaline pH...
    • Shifts the curve to the left
    • Results in a reduction in the P50
  • A reduction in the arterial partial pressure of oxygen from a value of 100 mmHg to 70 mmHg...
    • Has a minimal impact on the amount of oxygen combined with hemoglobin
  • Further reductions to the levels typical of tissue oxygen partial pressures (about 40 mmHg) will result in...
    • Significant “unloading” of oxygen at the tissue bed
  • Major form of O2 transport in the blood
    • Hemoglobin associated O2 transport

9

Oxygen content of arterial blood (CaO2)

  • Definition
  • Equation
  • Ex. a patient with normal body temperature (37OC), a partial pressure of arterial oxygen of 80 mmHg, a corresponding saturation of hemoglobin of 96%, and a blood hemoglobin concentration of 15 g/dl, we can summarize the oxygen content of the blood as follows
    • Dissolved oxygen
    • Combined oxygen
    • Total CaO2

  • Definition
    • Sum of the oxygen carried by hemoglobin and the amount dissolved in plasma
    • Depends mostly on oxygen combined to Hgb
  • Equation
    • CaO2 (ml O2/dl blood) = [1.34 (ml O2/gram of completely saturated hemoglobin) x Hgb (grams/dl blood) x SaO2/100] + [0.003 (ml O2/mmHg) x PaO2 (mmHg)]
    • CaO2 = Hgb-O2 + dissolved O2 = (1.34 x [Hgb] x % sat) + (0.003 x PO2)
    • CaO2 ≈ 1.34 x [Hgb] x (SaO2 / 100)
  • Ex. a patient with normal body temperature (37OC), a partial pressure of arterial oxygen of 80 mmHg, a corresponding saturation of hemoglobin of 96%, and a blood hemoglobin concentration of 15 g/dl, we can summarize the oxygen content of the blood as follows
    • Dissolved oxygen
      • 80 mmHg x 0.003 ml O2 / mmHg = 0.24 ml O2 / dl blood
    • Combined oxygen
      • 1.34 ml of O2 / g Hgb x 15 g Hgb/dl blood x 0.96 = 19.3 ml O2 / dl blood
    • Total CaO2
      • 19.54 ml O2 / dl blood

10

Arterial blood oxygen delivery

  • The O2 provided to each organ
  • Total oxygen delivery (DO2) 
  • Required to convert CaO2 into ml O2 per liter of blood
  • Tissue hypoxia can result from...

  • The O2 provided to each organ is the product of...
    • The arterial oxygen content
    • The blood flow to that individual organ
  • Total oxygen delivery (DO2)
    • The product of arterial oxygen content and cardiac output.
    • DO2 (ml O2/min) = CaO2 (ml O2/dl blood) x CO (L/min) x 10 (dl/L)
      • ​DO2 = O2 delivery
      • CaO2 = O2 content of arterial blood
      • CO = cardiac output
    • DO2 ≈ CaO2 x CO ≈ 1.34 x [Hgb] x (SaO2 / 100) x CO
  • Required to convert CaO2 into ml O2 per liter of blood
    • Multiplying by 10
  • Tissue hypoxia can result from...
    • A reduction in the arterial oxygen content
    • A decrease in tissue blood flow

11

The effects of a 50% reduction in the individual variables associated with tissue oxygen delivery

  • Normal
    • Tissue oxygen delivery = 1020
    • PaO2 = 100
    • SaO2 = 100
    • Hgb = 15
    • CO = 50
  • PaO2 --> 50
  • Hgb --> 7.5
  • CO --> 25

  • Tissue oxygen delivery = 1020
    • PaO2 = 100
    • SaO2 = 100
    • Hgb = 15
    • CO = 50
  • PaO2 --> 50
    • SaO2 --> 87
    • Tissue oxygen delivery --> 882
  • Hgb --> 7.5
    • Tissue oxygen delivery --> 518
  • CO --> 25
    • Tissue oxygen delivery --> 510

12

Transport of CO2 in the blood:
The transport of carbon dioxide from actively metabolizing cells to the lungs for excretion involves a number of processes

  • In the tissues, CO2 diffuses...
  • Once in the blood...
    • The majority of the CO2...
    • The remainder...
  • CO2 transport in arterial vs. venous blood

  • In the tissues, CO2 diffuses along its partial pressure gradient into the plasma
    • Diffusion of CO2 from tissue cells
    • 20x more soluble than O2
  • Once in the blood...
    • The majority of the CO2 enters the red blood cell
    • The remainder stays in the plasma
    • Chemical reactions in the plasma and in RBCs
  • CO2 transport in arterial vs. venous blood
    • ​Arterial
      • Most CO2 is transported as HCO3-
      • ​Very little CO2 is transported as carbamino or dissolved CO2
    • ​Venous
      • ​Still a lot of CO2 is transported as HCO3
      • But a lot more is transported as carbamino or dissolved CO2

13

Transport of CO2 in the blood:
In the plasma, CO2 can be transported in 1 of 3 ways

  • Like oxygen, the amount of CO2 dissolved in plasma is relatively small
    • Each dl of plasma will carry about 0.067 ml of CO2 for each mmHg PCO2
    • Dissolved carbon dioxide (ml CO2/dl blood) = 0.067 (ml CO2/ mmHg) x PaCO2 (mmHg)
  • CO2 can be bound by reactions with plasma proteins to form carbamino compounds
  • CO2 can be hydrated in the following reaction:
    • CO2 + H2O ⇔ H2CO3 ⇔ H+ + HCO3-
    • This reaction in plasma is relatively slow, since the critical enzyme, carbonic
      anhydrase, is not present

14

RBC CO2 transport

  • RBC
  • Hydration
  • NH2 groups

  • CO2 is dissolved in RBC
  • Hydration
    • ​H2O + CO2 H2CO3 H+ + HCO3-
    • H+ buffered by Hgb / O2 unloaded
    • Bohr effect
    • HCO3- may be carried in the plasma
  • NH2 groups of Hg form carbamino groups
    • R-NH2 + CO2 R-NHCOO- + H+

15

Transport of CO2 in the blood:
Fates of CO2

  • Majority of CO2
  • A small quantity of CO2
  • Haldane effect
  • Bohr effect

  • Majority of CO2
    • Enters the RBC
  • A small quantity of CO2
    • Dissolved within the RBC
  • Haldane effect
    • CO2 can combine directly with hemoglobin to form carbamino groups
    • Formation of these compounds is enhanced by the presence of unsaturated hemoglobin
  • Bohr effect
    • Since RBCs contain the enzyme carbonic anhydrase, CO2 is readily hydrated to form H+ and HCO3-
    • Most of the bicarbonate moves from the RBC into the plasma, and chloride shifts into the RBCs to maintain electrical neutrality
      • This is the main mechanism for CO2 transport in the blood
    • Hemoglobin combines with or buffers the free H+ generated, and this results in a conformational change, which decreases the affinity of hemoglobin for oxygen

16

Bohr effect

  • Describes the effect of PCO2 on the affinity of hemoglobin for O2
    • In the tissues, PCO2 rises, thereby lowering pH and decreasing the affinity of hemoglobin for O2
    • In the lungs, the reverse occurs as PCO2 falls
  • This facilitates the loading of O2 in the lungs and the release of O2 in the tissues

17

Haldane effect

  • Describes the effect of PO2 on the affinity of hemoglobin for CO2
  • That is, deoxygenated hemoglobin has a greater affinity for CO2 than does oxyhemoglobin
  • This results from the enhanced ability of deoxygenated hemoglobin to both form carbamino compounds and to accept the H+ released by the hydration of CO2
  • This enhances the ability to load CO2 in the tissues and release it in the lungs

18

Relationship between the Bohr and Haldane effects

  • Both
  • RBC
    • Haldane
    • Bohr
  • Tissue
    • Haldane
    • Bohr

  • Both
    • Important reactions that are complimentary and facilitate oxygen and carbon dioxide exchange in the tissues and the lungs
  • RBC
    • Haldane
      • Hgb oxygenation facilitates CO2 unloading
    • Bohr
      • Decreased CO2 facilitates O2 loading
  • Tissue
    • Haldane
      • Unoxygenated Hgb facilitates CO2 loading
    • Bohr
      • Increased CO2 facilitates O2 unloading

19

O2 vs. CO2 carrier systems

  • O2 carrier system
  • CO2 carrier system
  • PaCO2 and CO2

  • The oxygen carrier system (hemoglobin) is readily saturated once a critical partial pressure of oxygen is achieved (> 60 mm Hg)
  • In contrast, the multiple carrier systems for CO2 in the blood assure that saturation of the carrier system does not occur even in the presence of significant anemia
  • PaCO2 and CO2 content are nearly linearly related over the entire physiologic range

20

Clinical applications:
Hemoglobin saturation

  • Main way we can figure out how much O2 content there is
  • O2 + Hgb Hgb-O2
  • Measured by ABG and optical sensors
  • Calculation
    • (O2 combined with Hgb) / (O2 capacity to combine with Hgb) x 100

21

Pulse oximeter

  • Absorption spectra of oxyhemoglobin and deoxyhemoglobin differ
  • Oxyhemoglobin
    • Lower absorption of the 660 nm wavelength
    • Higher absorption of the 940 nm wavelength
  • Ratio of absorption at these different wavelengths is used for measurement

22

Clinical applications of SaO2

  • Diagnostically
  • Safety monitoring

  • Diagnostically
    • Sleep apnea
    • Screen for lung or cardiac disease
  • Safety monitoring
    • Hospital - ICU and operating rooms
    • At home - monitor chronic disease

23

Physiologic examples

  • Exercise at sea level
  • Breathing at 12,000 ft

  • Exercise at sea level
    • Muscle generate acid --> rightward shift in the curve
    • Oxygen released more effectively at the tissues
  • Breathing at 12,000 ft
    • ~490 torr --> PAO2 ~43
    • What do you do
      • Hyperventilate --> decrease CO2
      • Alkalosis --> shifts O2 dissociation curve to the left
        • Allows saturation of hemoglobin at lower PaO2 levels

24

Key points

  • Hemoglobin allows for...
  • DO2 ≈ 
  • CO2 transport depends mostly on...
  • Bohr and Haldane effects explain...

  • Hemoglobin allows for enriched carrying capacity of oxygen in the blood
  • DO2 ≈   1.34   X   Hgb   X   (SaO2/100)  X   CO
  • CO2 transport depends mostly on the erythrocyte
  • Bohr and Haldane effects explain the interaction between O2 / CO2 transport  
     

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