3.2 Transport of Oxygen and Carbon Dioxide

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.