H.6 Gas Exchange

Hemoglobin  Hemoglobin is a protein found in RBC’s composed of 4 polypeptides, each polypeptide containing a heme group. Each heme group contains an iron atom. Each iron atom can bind with one oxygen molecule (O 2 )

 Each time an oxygen molecule binds to a hemoglobin molecule the molecule changes shape.  The shape changes the hemoglobin’s affinity for oxygen.  A hemoglobin with no oxygen molecules has the least affinity for oxygen.  A hemoglobin with 3 oxygen has the highest affinity for oxygen  A hemoglobin with 4 oxygen has no affinity for oxygen.

 Hb 4 is used as an abbreviation for hemoglobin. A hemoglobin with one oxygen would be written as Hb 4 O 2

How hemoglobin’s affinity for oxygen changes (No affinity)

H.6.1  Partial pressure – the pressure exerted by a single type of gas when it is found in a mixture of gases. P total = P 1 +P 2 +P 3 …..  Can be calculated by taking the total pressure of a mixture of gases within which a gas occurs, multiplied by the percentage of the total volume the gas occupies.

H.6.2 Dissociation Curves  An oxygen dissociation curve is a graph that shows how various forms of hemoglobin and myoglobin perform under various conditions.  X axis is partial pressure of oxygen  Y axis is percent hemoglobin saturation

 Adding oxygen to hemoglobin is referred to as loading/association  Removing oxygen to hemoglobin is referred to as unloading/dissociation  Hemoglobin is fully saturated when it is bound to 4 oxygen molecules

Dissociation curve As the partial pressure of oxygen increases so the Hb becomes more saturated with oxygen. There is high affinity by each haem group and oxygen at high partial pressures. There is low affinity between each haem group and oxygen at low partial pressures.

 The oxygen dissociation curves show the tendency of hemoglobin to bind to oxygen (affinity) and separate from oxygen (dissociation). Remember: Oxygen can only move in and out of blood in either the lung capillaries or the tissue capillaries. As blood circulates its % saturation does not change, except in the two instances illustrated below

Oxygen Loading Blood in the pulmonary artery has a Po 2 = 5 kPa. This blood goes to the alveoli of the lung. The Alveoli has a Po 2 = 16 kPa Hb is 100% saturated at 10kPa. The fully saturated Hb will be carried away from the lung in the pulmonary vein

Oxygen Unloading The demand for oxygen within the tissues changes and haemoglobin responds to meet the demand for oxygen. Blood leaves the lung/ pulmonary vein/ aorta at 100% saturation Blood arrives at the tissues 100% saturated with oxygen since to exchange is not possible up to that point (thickness of the walls of blood vessels + flow rate) A typical respiring tissue would have a low Po 2 = 5- 7 kPa..Oxygen dissociates from the haemoglobin and diffuses into the cell.

Myoglobin  An oxygen binding protein found. in muscle. Myoglobin makes muscle “dark”  Myoglobin has one polypeptide and 1 heme group so can bind to one oxygen

 Myoglobin allows oxygen to be stored in muscle tissue in case of anaerobic situations at which time the oxygen will dissociate and be used for cellular respiration  This delays the onset of lactic acid fermentation

Myoglobin has a higher affinity for oxygen than haemoglobin and retains its oxygen until very low partial pressures occur. Such low partial pressures occur when the muscle is working very hard and the oxygen is used up in aerobic respiration. Myoglobin unloads its oxygen when there is a high rate of respiration such as during intense exercise. The oxygen is replaced during rest as excess post-exercise oxygen consumption

Fetal Hemoglobin  Fetal hemoglobin has a higher affinity for oxygen than adult hemoglobin.  In the capillaries of the placenta adult hemoglobin dissociates oxygen and fetal hemoglobin loads that same oxygen.

At a partial pressure of Po 2 = 5 kPa. Adult Hb can retain less than 50% Hb but Fetal HB can associate with a much higher 80 + %. The oxygen dissociates from the adult Hb and is loaded up to the fetal haemoglobin. The Po 2 in fetal tissues is very low due to the high metabolic rate associated with fetal growth rates. Therefore although fetal Hb has a higher affinity for oxygen in such a low partial pressure environment of the fetal tissue it unloads oxygen readily. At birth the fetal Hb is replaced with adult Hb

6.3 Carbon Dioxide Transport  CO 2 is the waste product of ATP production/cellular respiration  It diffuses out of cells into capillary beds where it enters the circulatory system.  It may be transported 3 ways: dissolved in plasma, bound to hemoglobin, or as bicarbonate ions in the RBC’s

 Most is transported as bicarbonate.  The RBC cytoplasm contains carbonic anhydrase that catalyzes the reaction between CO 2 and water to form carbonic acid.  Carbonic acid then dissociates into hydrogen ions and bicarbonate