Question

how do the partial pressures of oxygen and carbon dioxide in blood change during alveolar gas exchange? what about during systemic gas exchange?

Answer 1

When blood passes through pulmonary capillary, RBC is exposed to oxygen only for 0.75 second at rest and only for 0.25 second during severe exercise. So, diffusion of oxygen must be quicker and effective. Fortunately, this is possible because of pressure gradient. Partial pressure of oxygen in the pulmonary capi­llary is 40 mm Hg and in the alveoli, it is 104 mm Hg. Pressure gradient is 64 mm Hg. It facilitates the diffusion of oxygen from alveoli into the blood.

Partial pressure of carbon dioxide in alveoli is 40 mm Hg whereas in the blood it is 46 mm Hg. Pressure gradient of 6 mm Hg is responsible for the diffusion of carbon dioxide from blood into the alveoli.

Systemic gas exchange means exchange of gases between blood capillaries and tissue cells.   
EXCHANGE OF RESPIRATORY GASES AT TISSUE LEVEL
Oxygen enters the cells of tissues from blood and carbon dioxide is expelled from cells into the blood.
DIFFUSION OF OXYGEN FROM BLOOD INTO THE TISSUES
Partial pressure of oxygen in venous end of pulmonary capillary is 104 mm Hg. However, partial pressure of oxygen in the arterial end of systemic capillary is only 95 mm Hg. It may be because of physiological shunt in lungs. Due to venous admixture in the shunt, 2% of blood reaches the heart without being oxygenated. Average oxygen tension in the tissues is 40 mm Hg. It is because of continuous metabolic activity and constant utilization of oxygen. Thus, a pressure
gradient of about 55 mm Hg exists between capillary blood and the tissues so that oxygen can easily diffuse into the tissues . Oxygen content in arterial blood is 19 mL% and in the venous blood, it is 14 mL%. Thus, the diffusion of oxygen from blood to tissues is 5 mL/100 mL of blood.
DIFFUSION OF CARBON DIOXIDE FROM TISSUES INTO THE BLOOD
Due to continuous metabolic activity, carbon dioxide is produced constantly in the cells of tissues. So, the partial pressure of carbon dioxide is high in the cells and is about 46 mm Hg. Partial pressure of carbon dioxide in arterial blood is 40 mm Hg. Pressure gradient of 6 mm Hg is responsible for the diffusion of carbon dioxide from tissues to the blood .Carbon dioxide content in arterial blood is 48 mL%. And in the venous blood, it is 52 mL%. So, the diffusion of carbon dioxide from tissues to blood is 4 mL/100 mL of blood.