3.2 Transport of Oxygen and Carbon Dioxide

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What is the main form of transport for oxygen in the blood?

Combined with hemoglobin as oxyhemoglobin

Which factor does NOT influence the shape of the oxyhemoglobin dissociation curve?

Hemoglobin structure

How is most of the carbon dioxide transported in the blood?

Mainly as bicarbonate ions

What is described by the term 'hemoglobin saturation'?

The maximum amount of oxygen that can bind to hemoglobin

Which physiological effect is associated with the Bohr effect?

Increased oxygen release in tissues with high CO2 levels

What is the main role of the protoporphyrin ring in hemoglobin?

To hold the iron atom for oxygen binding

Which molecule does oxygen normally combine with when transported in the blood?

A ferrous atom

What condition shifts the oxyhemoglobin dissociation curve to the left?

Carbon monoxide

When the pH of blood falls, what effect leads to a decrease in the oxygen affinity of hemoglobin?

Bohr effect

Which of the following is responsible for promoting oxygen release at the tissue level?

Lower pH

What is the effect of most hemoglobinopathies on the oxygen dissociation curve?

Shift it to the left

What causes a shift to the right in the CO2 dissociation curve by decreasing its affinity for CO2?

An increase in oxygen levels

Which gas binds to hemoglobin with an affinity 210 times greater than oxygen?

Carbon monoxide

Which effect is related to CO2 transport in the blood?

Haldane effect

What is the function of myoglobin in muscle cells?

Store oxygen in muscle cells

What happens to the oxygen dissociation curve in the presence of carbon monoxide?

Shifts to the left

What effect does decreased 2,3-diphosphoglycerate (2,3 DPG) have on the oxygen dissociation curve?

Shift it to the left

What is the impact of packed cells/banked blood having lower 2,3-DPG levels?

Shifts the curve to the left

What is the main determinant of how much oxygen combines with hemoglobin?

Partial pressure of oxygen

What is the normal arterial oxygen content (CaO2) in mL/dL?

20

Which equation is used to calculate the oxygen content of the blood?

(CaO2) = (1.34 x Hb x SaO2) + (PaO2 x 0.003)

What is the component of the blood's oxygen content that is measured by the PaO2?

Dissolved oxygen in plasma

How many molecules/atoms of oxygen can one molecule of Hb combine with?

4 molecules/8 atoms

What causes acute intermittent porphyria (AIP)?

Accumulation of toxic heme precursors

What is the solubility coefficient of O2 in blood?

$0.003$ ml O2/100 ml blood/mmHg

In what state must the iron molecule be to bind oxygen in hemoglobin?

$Fe^2+$ state

What factors cause a right shift in the oxygen-hemoglobin dissociation curve?

Increased PCO2 and decreased pH

What is the primary effect of the Bohr effect on hemoglobin?

Facilitates oxygen release

What causes a left shift in the oxygen-hemoglobin dissociation curve?

Decreased PCO2 and increased pH

What is the primary role of 2,3 diphosphoglycerate (2,3 DPG) in the context of hemoglobin?

Facilitates offloading of oxygen

In which location does a left shift (reverse Bohr effect) predominantly occur?

Lungs

Which of the following is responsible for promoting oxygen release at the tissue level?

Bohr effect

What is described by the term 'hemoglobin saturation'?

The percentage of heme groups in hemoglobin that contain bound oxygen

Which condition shifts the oxyhemoglobin dissociation curve to the left?

Alkalosis

The Bohr effect leads to an increase in oxygen affinity of hemoglobin at lower pH levels.

False

The Haldane effect is primarily associated with oxygen transport in the blood.

False

An increase in oxygen levels will shift the CO2 dissociation curve to the left, increasing the affinity for CO2.

False

Carbon monoxide (CO) shifts the oxyhemoglobin dissociation curve to the right.

False

Hypoventilation causes a right shift in the oxyhemoglobin dissociation curve.

True

Study Notes

Transport of Oxygen by the Blood

  • Volume of oxygen delivered to the systemic vascular bed per minute = cardiac output x arterial O2 concentration
  • Content or concentration of O2 (CaO2) is expressed in ml of O2/100 mL of blood
  • Normally expressed as volumes percent = ml O2 per 100 mL of blood
  • Oxygen content (concentration):
    • 3% O2 dissolved in plasma
    • 97% O2 reversibly combined with hemoglobin (Hb)

Calculating Oxygen Content of the Blood

  • Oxygen content is composed of hemoglobin bound + dissolved
  • Equation for calculating oxygen content of the blood is: (CaO2) = (1.34 x Hb x SaO2) + (PaO2 x 0.003)

Amount of O2 Dissolved in Plasma

  • Measured by the PaO2 (partial pressure or oxygen tension)
  • Insufficient to fulfill the body's oxygen requirements at rest (normal FiO2 at sea level)
  • Solubility coefficient of O2 = 0.003 ml O2/100 ml blood/mmHg (or per deciliter (dL))
  • Henry's Law: At a temperature of 37°C, 1 mL of plasma contains 0.00003 mL O2/mm Hg PO2

Oxygen Chemically Combined with Hemoglobin

  • 97% of oxygen delivered to tissues is combined with hemoglobin (Hb)
  • Adult hemoglobin, HbA is a tetramer composed of:
    • 4 heme molecules + 4 globin chains
    • Cells must synthesize all parts except Fe2+
    • Four globin polypeptide chains: 2 α-globin subunits and 2 β-globin subunits
  • A heme molecule is a protoporphyrin ring with a suspended iron molecule
  • Fe in the Fe2+ state binds O2 - Not the Fe3+ state
  • One molecule of Hb combines with 4 molecules/8 atoms of oxygen

Porphyrins

  • The heme group is a prosthetic group consisting of a protoporphyrin ring and a central, suspended iron atom
  • Acute intermittent porphyria (AIP) is caused by a gene mutation that interferes with cytoplasmic heme synthesis and causes the accumulation of toxic heme precursors
  • Any drug that induces ALA synthase can accelerate this process (barbiturates, etomidate, glucocorticoids, hydralazine)

Transport of Carbon Dioxide by the Blood

  • Not discussed in this text, but will be covered in the next section

Bohr and Haldane Effects

  • Right shift = Bohr effect: CO2 and H+ cause Hb to release oxygen
  • Factors that cause a right shift:
    • Increased PCO2 from metabolism
    • Decreased pH - acidosis (ketoacidosis, lactic acidosis, etc)
  • Left shift is due to less [H+] interacting with Hb inside erythrocytes
  • Factors that shift the curve to the left:
    • Decreased CO2
    • Increased pH
    • Decreased Temperature
    • Decreased 2,3 DPG
    • Most hemoglobinopathies

Myoglobin

  • Myoglobin is a single-polypeptide heme protein that stores O2 in muscle cells
  • Combines with a single molecule O2
  • Structurally similar to a single Hb subunit
  • Hyperbolic dissociation curve
  • P50 far left of HbA
  • O2 remains bound for conditions with lower PO2### Transport of Oxygen by the Blood
  • The volume of oxygen delivered to the systemic vascular bed per minute is calculated by multiplying cardiac output (CO) by arterial O2 concentration.
  • The body has limited stores of oxygen, so limiting cardiac output or pulmonary oxygenation is fatal within minutes.
  • Oxygen content (concentration) is expressed in ml of O2/100 mL of blood and is normally around 20 mL/dL.

Calculating Oxygen Content of the Blood

  • Oxygen content is composed of hemoglobin-bound and dissolved O2.
  • The equation for calculating oxygen content of the blood is: (CaO2) = (1.34 x Hb x SaO2) + (PaO2 x 0.003).

Oxygen Delivery to Tissues

  • Oxygen delivery to tissues is approximately 5 mL O2/100 mL blood at rest.
  • Cardiac output is normally 5 L/min, and normal arterial O2 content is 20 mL/dL.
  • Normal venous O2 content is 15 mL/dL.

Amount of O2 Dissolved in Plasma

  • The amount of O2 dissolved in plasma is measured by the PaO2 (partial pressure or oxygen tension).
  • The solubility coefficient of O2 is 0.003 ml O2/100 mL blood/mmHg.
  • At a temperature of 37°C, 1 mL of plasma contains 0.00003 mL O2/mmHg PO2.

Oxygen Chemically Combined with Hemoglobin

  • 97% of oxygen delivered to tissues is combined with hemoglobin (Hb).
  • Adult hemoglobin, HbA, is a tetramer composed of 4 heme molecules and 4 globin chains.
  • One molecule of Hb combines with 4 molecules/8 atoms of oxygen.

Heme Molecule

  • A heme molecule is a protoporphyrin ring with a suspended iron atom (Fe2+).
  • Acute intermittent porphyria (AIP) is caused by a gene mutation that interferes with cytoplasmic heme synthesis.

Oxygen Transport and Carbon Dioxide

  • Oxygen is transported in the blood as oxyhemoglobin (HbO2) and dissolved O2.
  • Carbon dioxide is transported in the blood as carbamino compounds with blood proteins, bicarbonate, and dissolved CO2.
  • The oxyhemoglobin dissociation curve shows the relationship between the partial pressure of oxygen and the amount of oxygen combined with hemoglobin.

Porphyrin

  • A porphyrin is a heterocyclic compound containing four pyrrole rings arranged in a square with a metal atom in the central cavity.
  • Porphyrin is a prosthetic group consisting of a protoporphyrin ring and a central, suspended iron atom.

Cooperative Binding of Hemoglobin (Hb)

  • Hb forms a reversible bond with oxygen, allowing O2 to be released to the tissues.
  • Hemoglobin occurs in two forms: T or "Tense" form (deoxyhemoglobin) and R or "Relaxed" form (oxyhemoglobin).

O2 Carrying Capacity of Hb

  • The O2 carrying capacity of Hb is the maximal amount of O2 that can bind with Hb.
  • One gram of Hb can combine with 1.34 ml O2.

Pulse Oximetry

  • Pulse oximetry measures the oxygen saturation of Hb, which is the proportion of oxygenated hemoglobin compared to total hemoglobin.
  • The calculation to find % Hb saturation is: O2 combined with Hb ÷ O2 carrying-capacity of Hb x 100% = % Hb saturation.

Bohr Effect and Haldane Effect

  • The Bohr effect is the decrease in O2 affinity of Hb when the pH of blood falls, causing Hb to release oxygen.
  • The Haldane effect is the increase in CO2 affinity of Hb when the pH of blood falls, causing CO2 to combine with Hb.
  • The Bohr effect and Haldane effect work in synchrony for uptake and release of O2 and CO2.

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