5.4 - Transport of Gases by Blood

Gas transport in blood

Although CO2 is more soluble than O2 in blood, both gases require a specialized transport system to move most of their molecules between the lungs and other tissues

Dissolved oxygen in blood

Oxygen is not highly soluble in liquids; only about 1.5% of the total oxygen is dissolved in the blood and transported in the bloodstream

Erythrocytes

Red blood cells; carry most oxygen from the lungs to the body's tissues

Hemoglobin (Hb)

Special protein in erythrocytes that binds oxygen molecules to the erythrocyte

Heme

Portion of hemoglobin that contains iron; the part that binds oxygen

Oxygen-carrying capacity of hemoglobin

Each hemoglobin molecule contains four iron-containing heme molecules, so one hemoglobin molecule can carry up to four molecules of oxygen

Oxygen diffusion into red blood cells

As oxygen diffuses across the respiratory membrane from the alveolus to the capillary, it also diffuses into the red blood cell and is bound by hemoglobin

Structure of hemoglobin

Composed of four subunits arranged in a ring-like fashion, each with an iron atom bound to the heme in the center

Cooperative binding of oxygen

Binding of the first oxygen molecule causes a conformational change that allows the second to bind more readily, and each bound oxygen facilitates binding of the next until all four heme sites are occupied

Cooperative dissociation of oxygen

After the first oxygen molecule dissociates and is dropped off at the tissues, the next oxygen molecule dissociates more readily

Saturated hemoglobin

All four heme sites are occupied by oxygen

Partially saturated hemoglobin

One to three heme sites are occupied by oxygen

Hemoglobin saturation

Percent of the available heme units in the whole body that are bound to oxygen at a given time; 100% means every heme unit in all erythrocytes is bound to oxygen

Normal hemoglobin saturation

Generally 95 to 99 percent in a healthy individual with normal hemoglobin levels

Oxyhemoglobin (HbO2)

Final product formed when oxygen binds to hemoglobin (reversible reaction); bright red molecule that contributes to the bright red color of oxygenated blood

Reduced hemoglobin

Hemoglobin that does not have oxygen bound to it (Hb)

Oxygen-hemoglobin dissociation curve

Graph describing the relationship of partial pressure to the binding of oxygen to heme and its subsequent dissociation from heme

Oxygen-hemoglobin curve and PO2

As PO2 increases, a proportionately greater number of oxygen molecules are bound by heme; the lower the PO2, the fewer oxygen molecules are bound; PO2 plays a major role in binding at the respiratory membrane and dissociation at body tissues

Affinity of oxygen for heme

Increases as more oxygen molecules are bound

Factors that regulate the oxygen-hemoglobin dissociation curve

Metabolism, pH, temperature, and BPG; they act as automatic control mechanisms that regulate how much oxygen is delivered to different tissues

Metabolism and oxygen delivery (active tissues)

Highly active tissues such as muscle rapidly use oxygen to make ATP, lowering tissue PO2 to about 20 mmHg versus about 95 mmHg in capillaries (a difference of about 75 mmHg), so more oxygen dissociates from hemoglobin and enters the tissue

Metabolism and oxygen delivery (low-metabolism tissues)

Tissues such as adipose use less oxygen, so tissue PO2 stays relatively high and fewer oxygen molecules dissociate from hemoglobin and enter the interstitial fluid

Oxygen reserve in venous blood

Although venous blood is called deoxygenated, some oxygen is still bound to hemoglobin, providing a reserve for when tissues suddenly demand more oxygen

pH and metabolism

Tissues generate carbon dioxide, which increases hydrogen ions and decreases pH

pH and the oxygen-hemoglobin curve

pH affects hemoglobin's affinity for oxygen; a pH change shifts the curve left (increasing affinity) or right (decreasing affinity); pH below normal means more oxygen is released (right shift), higher pH means less oxygen is released (left shift)

Bohr effect

As more CO2 enters the bloodstream and decreases pH, oxygen dissociates from hemoglobin, causing oxygen unloading in the tissues where metabolic cells need it

Temperature and the oxygen-hemoglobin curve

Higher temperature promotes faster dissociation of oxygen from hemoglobin (right shift); lower temperature inhibits dissociation (left shift)

Temperature effect in active tissues

Body temperature is tightly regulated and generally does not affect gas exchange except in pathological conditions; highly active tissues release more energy than is given off as heat, so oxygen readily dissociates from hemoglobin there

Bisphosphoglycerate (BPG)

Compound in erythrocytes that promotes the dissociation of oxygen from hemoglobin; the greater the BPG concentration, the more readily oxygen dissociates regardless of its partial pressure

Source of BPG

Byproduct of glycolysis; erythrocytes lack mitochondria, so glycolysis is their sole method of producing ATP

Factors that raise BPG levels

Certain hormones (androgens, epinephrine, thyroid hormones, growth hormone) or an increase in pH; if BPG is too low, hemoglobin will not release its oxygen

Fetal hemoglobin (HbF)

Differs slightly in structure from adult hemoglobin; at the same oxygen pressures it binds more oxygen, and saturation occurs at a lower PO2, allowing the fetus to take more oxygen from maternal blood

Three mechanisms of CO2 transport

Dissolved in plasma, as bicarbonate (HCO3-), and bound to hemoglobin

Dissolved CO2

About 7% of CO2 diffusing into the blood from the tissues dissolves directly in plasma; it diffuses across the respiratory membrane into the alveoli at the pulmonary capillaries and is exhaled

Bicarbonate transport of CO2

About 70% of CO2 is transported to the lungs as bicarbonate ion (HCO3-); most is produced in erythrocytes after CO2 diffuses into the capillaries and into red blood cells

Carbonic anhydrase (CA)

Intracellular enzyme found in high concentrations in erythrocytes that causes CO2 and water to form carbonic acid (H2CO3), which dissociates into bicarbonate (HCO3-) and hydrogen (H+) ions

Chloride shift

Bicarbonate builds up in erythrocytes and moves down its concentration gradient into the plasma in exchange for chloride (Cl-) ions; exchanging one negative ion for another leaves the electrical charge of the erythrocytes and blood unchanged

Red blood cells as a pH buffer

Increased H+ lowers pH; H+ ions are removed by buffers in the blood, most of which bind to hemoglobin within red blood cells, helping maintain normal blood pH

Bicarbonate reaction at the pulmonary capillaries

Reaction is reversed: bicarbonate in plasma re-enters erythrocytes in exchange for chloride, H+ and HCO3- join to form carbonic acid, which carbonic anhydrase converts to CO2 and water; CO2 diffuses into plasma, across the respiratory membrane, and into the alveoli to be exhaled

Carbaminohemoglobin

Formed when CO2 binds hemoglobin; about 23% of CO2 is transported this way; CO2 does not bind iron like oxygen but binds amino acid sections on the globin portions of hemoglobin

Color of hemoglobin not carrying oxygen

Tends to have a bluish-purple tone, creating the darker maroon color typical of deoxygenated blood

CO2 binding and partial pressure

Blood reaching tissues has lower PCO2 than the tissues, so CO2 enters the blood and red blood cells and binds hemoglobin; in the pulmonary capillaries PCO2 is high compared to the alveoli, so CO2 dissociates from hemoglobin and diffuses into the air

Haldane effect

Relationship between PO2 and the affinity of hemoglobin for CO2; hemoglobin saturated with oxygen does not readily bind CO2, but when oxygen is not bound to heme and PO2 is low, hemoglobin readily binds CO2

Fate of CO2 diffusing into the bloodstream (Figure 5.27)

7% remains dissolved in plasma; 93% diffuses into red blood cells, where 23% binds Hb to form carbaminohemoglobin and 70% is converted to H2CO3 by carbonic anhydrase, which dissociates into H+ and HCO3-

Fate of H+ and HCO3- in red blood cells (Figure 5.27)

H+ is removed by buffers, especially hemoglobin; HCO3- moves out of the red blood cell in exchange for Cl- (chloride shift)