Question

Diagram and describe the removal of CO2 from working tissues and release to the environment.

Answer 1

Ans- CO2 is produced in cells mainly during glycolysis and citric acid cycle in cytoplasm and mitochondria respectively.Like other molecules it always moves down its concentration gradient from sites of production to extracellular space.It diffuses much readily than oxygen.As it is produced it dissolves into water of cytoplasm and continues to build up until it reaches partial pressure greater than 40 or 45 mm Hg.This sets up a concentration gradient down which CO2 can diffuse.From that extracellular space CO2 molecules freely diffuse through capillary walls rapidly equilibrating and raising partial pressure of CO2 in blood from about 40 mm Hg on arterial side of capillary to 45 or 48 on venous side.Once the venous blood returns to the lungs, CO2 diffuses out of the bloodstream through the capillaries, and into the alveoli where it is expelled, during which time, O2 simultaneously bind with haemoglobin to carried back to the tissues.

There are ,3 ways CO2 are carried in bloodstream from peripheral tissues back to the lungs(1). As a dissolved gas (2). As bicarbonate (3).Dissoved in Hb

As CO2 diffuses from peripheral tissues into the bloodstream approx 10% of it remains dissolved in the plasma, or the extracellular fluid matrix of the blood, to a partial pressure of about 45 mmHg.[2] The majority of the carbon dioxide that diffuses through the capillaries and, ultimately, into the red blood cells combines with water in a chemical reaction catalyzed by the enzyme carbonic anhydrase to form carbonic acid. Carbonic acid almost immediately dissociates into a proton and a bicarbonate anion (HCO3-). Thus, bicarbonate is the principal means by which carbon dioxide is transported throughout the bloodstream according to the equation CO2 + H2O --> H2CO3 --> H+ + HCO3-. As carbon dioxide continues to be produced by tissues, this reaction is continually driven forward in the periphery, according to Le Chatelier's Principle. The proton formed by this reaction is buffered by hemoglobin while the bicarbonate anion diffuses out of the red blood cell and into the serum in exchange for a chloride anion through a special HCO3-/Cl- transporter. Thus, venous blood has both a higher concentration of bicarbonate and a lower concentration of chloride thanks to this so-called chloride shift. In the lungs, this process is reversed as both the HCO3-/Cl- exchanger and carbonic anhydrase enzyme reverse directions; this results in an influx of bicarbonate into red blood cells, an efflux of chloride ions, and the generation of first carbonic acid and then carbon dioxide. The carbon dioxide diffuses out of the red blood cells, through the capillary walls, and into the alveolar spaces where it is exhaled. Finally, the remaining 10% of the carbon dioxide that diffuses into the bloodstream and, subsequently, into the red blood cells, binds to the amino terminus of proteins, predominantly hemoglobin, to form carbaminohemoglobin.Of note,this site is different from the one to which O2 binds.Multiple physiologic phenomena ensure that this continuous cycle runs with maximum efficiency.