Question

CO2 is removed from the body through the lungs in a process that is mediated by red blood cells, as summarized in the figure below. Transport of CO2 is coupled to the transport of O2 through hemoglobin.

-To what extent does the intracellular pH or the red blood cell vary during its movement from the tissues to the lungs and back, and why is this so?

-In what form, and where is the CO2 during its movement from the tissues of the body to the lungs?

-How is it that the CL-/HCO3- exchanger operates in one direction in tissues and in the opposite direction in the lungs?

Answer 1

a. ANS: Tissue cells produce carbon dioxide as a waste product during cellular respiration. The normal pH of tissue cells is about 7.4. This is the pH at which nerve cells best operate and cellular enzymes act to control cellular metabolism. If the carbon dioxide produced in cellular respiration accumulated within the cells, the pH of the cells would fall, hence making them more acidic, eventually resulting in death.

1. Carbon dioxide diffuses down a concentration gradient in the same manner as oxygen. Blood entering the tissue capillaries carries Carbon dioxide at low levels. Blood leaving the tissue capillaries has a higher level.

2. Carbon dioxide leaves the blood and diffuses down the concentration gradient across the capillary and alveoli walls. Blood leaving the lung capillaries has low levels of CO2.

3. Low amounts of carbon dioxide can be carried in the blood due to its low solubility.

4. Carbon dioxide diffuses into blood in tissue capillaries from surrounding cells and most enters the red blood cells. About seven per cent remains dissolved in the plasma and istransported back to the lungs in that form. Within red blood cells the following reaction occurs: The bicarbonate ions are very soluble and diffuse from the red blood cell into the blood plasma. The CO2 that remains in the red blood cells combines with the haemoglobin. So blood carries carbon dioxide from body tissues to lungs in three forms: bicarbonate ions, in the haemoglobin or dissolved in plasma. In lung capillaries the processes that occur with relation to carbon dioxide are the reverse of what occur in the blood.

b. ANS: the CO2 is in the form of the bicarbonate being converted by carbonic anhydrase when it enters from the tissues to the lungs. The CO2 is present in the blood plasma.

c. ANS: Transmembrane anion exchange is essential to an important function of the erythrocyte the transport of waste carbon dioxide (CO2), which is generated in peripheral tissues, to the lungs for excretion by respiratory exhalation .Waste CO2 released from cells into the capillary blood diffuses across the erythrocyte membrane. In its gaseous form, CO2 dissolves poorly in aqueous solutions, such as the cytosol or blood plasma, but the potent enzyme carbonic anhydrase inside the erythrocyte converts CO2 to the water-soluble bicarbonate (HCO3−) anion:

The release of oxygen from hemoglobin into the peripheral capillaries induces a conformational change in the globin polypeptide that enables a histidine side chain to bind the proton produced by the carbonic anhydrase reaction. Meanwhile, the HCO3− formed by carbonic anhydrase is transported out of the erythrocyte in exchange for an entering Cl− via Anion Exchanger 1 protein If anion exchange did not occur, HCO3− would accumulate inside the erythrocyte to toxic levels during periods of exercise, when much CO2 is generated. About 80 percent of the CO2 in blood is transported as HCO3− generated inside erythrocytes; anion exchange allows about two-thirds of this HCO3− to be transported by blood plasma external to the cells, increasing the amount of CO2 that can be transported from tissues to the lungs. Also, without anion exchange, the increased HCO3− concentration in the erythrocyte would cause the cytosol to become alkaline. The exchange of HCO3− for Cl− causes the cytosolic pH to remain near neutrality.

The overall direction of this anion-exchange process is reversed in the lungs. CO2 diffuses out of the erythrocyte and is eventually expelled in breathing. The lowered concentration of CO2 within the cytosol drives the carbonic anhydrase reaction, as written above, from right to left: HCO3− reacts to yield CO2 and OH−. At the same time, oxygen binding to hemoglobin causes a proton to be released from hemoglobin; the proton combines with the OH− to form H2O. The lowered intracellular HCO3− concentration causes HCO3− to enter the erythrocyte in exchange for Cl− .

REFERENCES:

Crandall ED, Mathew SJ, Fleischer RS, Winter HI, Bidani A (1981). "Effects of inhibition of RBC HCO3-/Cl- exchange on CO2 excretion and downstream pH disequilibrium in isolated rat lungs". J. Clin. Invest. 68 (4): 853–62.

Westen EA, Prange HD (2003). "A reexamination of the mechanisms underlying the arteriovenous chloride shift". Physiol. Biochem. Zool. 76 (5): 603–14.

Nigen AM, Manning JM, Alben JO (25 June 1980). "Oxygen-linked binding sites for inorganic anions to hemoglobin". J. Biol. Chem. 255 (12): 5525–9.

If you are satisfied with my answer please rate it.