Question

Describe gas exchange in the human body from inspiration (breathing in) to expiration (breathing out). How does oxygen and carbon dioxide travel throughout the body? Provide specific details of how, where, and why these gasses travel. Your details should also include the lungs, heart, blood vessels, and muscle tissues. Use concepts learned in class and key terms to ensure you have a detailed response.

Answer 1

Diffusion of Oxygen from Atmospheric  Air into Alveoli -
Partial pressure of oxygen in the atmospheric air is 159
mm Hg and in the alveoli, it is 104 mm Hg. Because of the pressure gradient of 55 mm Hg, oxygen easily enters
from atmospheric air into the alveoli .

Diffusion of Oxygen from Alveoli into Blood -
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 .

Oxygen is transported from alveoli to the tissue by
blood in two forms:
1. As simple physical solution
2. In combination with hemoglobin

Oxygen combines with hemoglobin in blood and is
transported as oxyhemoglobin. Transport of oxygen
in this form is important because, maximum amount
(97%) of oxygen is transported by this method.

Carbon dioxide is transported by the blood from cells
to the alveoli.
Carbon dioxide is transported in the blood in four
ways:
1. As dissolved form (7%)
2. As carbonic acid (negligible)
3. As bicarbonate (63%)
4. As carbamino compounds (30%)

AS DISSOLVED FORM-
Carbon dioxide diffuses into blood and dissolves in the
fluid of plasma forming a simple solution. Only about
3 mL/100 mL of plasma of carbon dioxide is transported
as dissolved state. It is about 7% of total carbon
dioxide in the blood.
AS CARBONIC ACID-
Part of dissolved carbon dioxide in plasma combines
with the water to form carbonic acid. Transport of
carbon dioxide in this form is negligible.
AS BICARBONATE-
About 63% of carbon dioxide is transported as bicarbonate. From plasma, carbon dioxide enters the
RBCs. In the RBCs, carbon dioxide combines with
water to form carbonic acid. The reaction inside RBCs
is very rapid because of the presence of carbonic
anhydrase. This enzyme accelerates the reaction.
Carbonic anhydrase is present only inside the RBCs
and not in plasma. That is why carbonic acid formation
is at least 200 to 300 times more in RBCs than in
plasma.
Carbonic acid is very unstable. Almost all carbonic
acid (99.9%) formed in red blood corpuscles, dissociates
into bicarbonate and hydrogen ions. Concentration of
bicarbonate ions in the cell increases more and more.
Due to high concentration, bicarbonate ions diffuse
through the cell membrane into plasma.

AS CARBAMINO COMPOUNDS
About 30% of carbon dioxide is transported as carbamino compounds. Carbon dioxide is transported in
blood in combination with hemoglobin and plasma
proteins. Carbon dioxide combines with hemoglobin to
form carbamino hemoglobin or carbhemoglobin. And
it combines with plasma proteins to form carbamino
proteins. Carbamino hemoglobin and carbamino
proteins are together called carbamino compounds.
Carbon dioxide combines with proteins or hemoglobin with a loose bond so that, carbon dioxide is
easily released into alveoli, where the partial pressure
of carbon dioxide is low. Thus, the combination of
carbon dioxide with proteins and hemoglobin is a
reversible one. Amount of carbon dioxide transported
in combination with plasma proteins is very less compared to the amount transported in combination with
hemoglobin. It is because the quantity of proteins in
plasma is only half of the quantity of hemoglobin.

Diffusion of Carbon Dioxide from  Blood into Alveoli -
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.

Diffusion of Carbon Dioxide from Alveoli  into Atmospheric Air -
In atmospheric air, partial pressure of carbon dioxide is
very insignificant and is only about 0.3 mm Hg whereas,
in the alveoli, it is 40 mm Hg. So, carbon dioxide enters
passes to atmosphere from alveoli easily .

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 .