Problem 2

Problem 2. Approximately 90 percent of the CO2 produced by metabolic processes in the body is carried in the blood as bicarbonate. Yet, at the lungs CO2 gas is transported from the blood to the lung air.

Describe the processes involved in CO2 loading from the tissues to the blood, and CO2

unloading from the blood to the lung air. What is this process called?

Tissues to blood:

1. CO2 diffusion: tissue plasma RBC 2. 5-8% of CO2 directly combines with haemoglobin Hb

CO2 (carbamino-Hb)3. 92-95% of CO2 combines with H2O via the action of the

carbonic anhydrase enzyme to produce H2CO3

4. H2CO3 dissociates immediately into HCO3- & H+

5. Chloride shift: an anion exchange antiporter allows for the exchange of Cl- for HCO3 in the RBC HCO3

- moves into the plasma and Cl- moves into the RBC

6. Haemoglobin is used to buffer the H+ produced by H2CO3 dissociation Hb H

Blood to Lungs:

1. Chloride shift: an anion exchange antiporter allows for the exchange of Cl- for HCO3 in the RBC HCO3

- moves into the RBC and Cl- moves into the plasma

2. H+ is released from Hb3. HCO3- combines with H+ to produce H2CO3

4. Carbonic anhydrase catalyzes the release of CO2 from H2CO3 and production of H2O

5. CO2 is released from Carbamino-Hb (promoted by O2

binding)6. CO2 Diffusion: RBC plasma alveolus

Use diagrams to describe the Haldane effect.

Lungs: As PO2 increases, greater O2-Hb binding means that Hb has a lower affinity for CO2 and CO2 unloading occurs at the lungs

Tissues: As PO2 decreases, lesser O2-Hb binding means that Hb has a higher affinity for CO2 and CO2 loading occurs at the tissues

Also using diagrams, discuss how the Bohr Shift can benefit an individuals during exercise. What other factor(s) promote oxygen delivery to the tissues during exercise.

The Bohr Shift

- caused by an increase CO2 and H+

- the pH decreases in the blood, making it more acidic - this causes the Hb-O2 curve to shift to the right- the affinity haemoglobin has for oxygen to decrease- releasing more oxygen to the tissues

During Exercise

When people exercise, there is an increase in production of metabolic acid via anaerobic glycolysis and ATP hydrolysis. These processes increase the amount of CO2 in the blood. This causes the Bohr Shift to occur. This is beneficial as now more oxygen is being able to be released at the working tissues.

Describe how the body senses CO2 content in the body. How does the body respond when PCO2 in the ECF increases? Why is this response important?

Peripheral Chemoreceptors

Peripheral chemoreceptors are located in the carotid bodies, leading to the medulla oblongata, the respiratory control center. They sensitive to H+ concentration, produced when H2CO3 dissociates to form HCO3

- and H+ in the blood. They are thus indirectly sensitive to CO2 as CO2 was required to produce H2CO3. These receptors are also sensitive to O2, but not to the same magnitude as H+. They only respond to decreased PO2 when pressure is below 60mmHg, with normal PO2 being 100 mmHg.

Central chemoreceptors

Central chemoreceptors are located in the regulatory brain stem, specifically in the medulla oblongata. These chemoreceptors are also H+ sensitive, and thus depend indirectly on CO2. CO2 diffuses across the blood brain barrier, combines with H2O via the action of carbonic anhydrase producing H2CO3, which immediately dissociates into HCO3

- + H+.

A small PCO2 increase results in a large increases in ventilation, where an large decrease in PO2 is required to produce a change in ventilation. It is clear then that alterations in the partial pressure of CO2 is what drives most changes in ventilation, as opposed to decreases in O2 partial pressure.

Importance

CO2 is toxic to body tissues, and thus needs to be expelled from the body. The body can continue to produce energy via anaerobic respirations during times when adequate O2 is not available, however it cannot detoxify CO2 in the tissues.