L9 Gas Transport in the Blood

Question 1

True or false: Haemoglobin makes up one third of a red blood cell’s weight

Answer 1

True

Question 2

What molecule is made of four polypeptides, each one bound to a heme molecule?

Answer 2

Haemoglobin

Question 3

Where on a red blood cell does oxygen bind?

Answer 3

Each erythrocyte contains haemoglobin, which is made of four globin molecules, each one bound to a heme molecule. Each heme molecule binds one iron molecule. It is this iron molecule that binds oxygen.

Question 4

What chains make up adult haemoglobin?

Answer 4

2 alpha chains and 2 beta chains

Question 5

What is oxygen-rich haemoglobin called?

Answer 5

Oxyhaemoglobin

Question 6

What is oxygen-deficient haemoglobin called?

Answer 6

Deoxyhaemoglobin

Question 7

How long does erythropoiesis take? And where does it occur?

Answer 7

4 days, occurs in the bone marrow

Question 8

Which of the following factors affect transport of respiratory gases?

A) Oxygen diffusion gradients
B) Carbon dioxide diffusion gradients
C) Haemoglobin’s oxygen dissociation relationship
D) Transport of carbon dioxide as bicarbonate ions
E) All of the above

Answer 8

E) All of the above

Question 9

How does the oxygen diffusion gradient affect transport of respiratory gases?

Answer 9

PO2 in the alveoli is normally about 104 mmHg, in the blood PO2 is 40 mmHg. Thus O2 follows the oxygen diffusion gradient and diffuses into the blood until the blood also has a PO2 of 104 mmHg.

Blood leaving alveolar capillaries should have a PO2 of 104 mmHg.

Question 10

Blood leaving the alveolar capillaries has a PO2 of 104 mmHg, but by the time it has reached the end of the pulmonary vein it will have a PO2 of 95 mmHg. Why is this?

Answer 10

The pulmonary vein receives some deoxygenated blood from the bronchial veins, which draws oxygen along the oxygen diffusion gradient.

Question 11

Why does oxygen leave the blood and flow into the interstitial spaces?

Answer 11

The interstitial fluid will have a PO2 of about 40 mm Hg and the cells will have a PO2 closer to 20 mm Hg.

Because the blood has a PO2 of around 95 mmHg, oxygen will flow from the blood capillaries into the interstitial fluid, and then into the cells.

Question 12

How does the carbon dioxide diffusion gradient affect the transport of respiratory gases?

Answer 12

The cells produce CO2. This results in cells having an intracellular PCO2 of about 46 mm Hg, but the interstitial fluid has a PCO2 of about 45 mm Hg, and the capillaries supplying the tissues with blood will have a PCO2 of about 40 mm Hg.

CO2 diffuses into blood so that by the time the now deoxygenated blood is pumped back to the lungs through the pulmonary artery it has a PCO2 of about 45 mm Hg. The PCO2 of the alveoli is about 40 mm Hg, so CO2 diffuses from the blood into the alveoli. The blood that passes into the pulmonary vein now has a PCO2 of about 40 mm Hg.

Question 13

What effect does haemoglobin’s oxygen dissociation relationship have on the transport of respiratory gases?

Answer 13

Haemoglobin caries about 97% of the oxygen in the blood, the remaining 3% is carried in solution in the blood plasma. The binding of O2 with haemoglobin is reversible, O2 binds to the haemoglobin in the lungs dissociates from the haemoglobin into the tissues.

How much O2 is carried by haemoglobin is affected by temperature and pH.

Question 14

An increase in temperature would shift the oxygen dissociation curve to the…

A) Left
B) Right
C) No effect

Answer 14

B) Right

This means that more O2 is required to saturate the haemoglobin.

Question 15

An increase in pH (i.e. less acidic) would shift the oxygen dissociation curve to the…

A) Left
B) Right
C) No effect

Answer 15

A) Left

The haemoglobin will become saturated at lower blood PO2 levels

Question 16

What effect does pH have on the ability of haemoglobin to bind O2?

Answer 16

Ability of haemoglobin to bind O2 declines with decreasing pH.

i.e. more acidic = less Hg saturation

H ions bind to haemoglobin, causing a conformational change that decreases its affinity for O2.

O2 dissociation curve shifts to the right (Bohr effect).

Question 17

Why might blood pH decrease as the levels of CO2 increase?

Answer 17

As PCO2 increases, CO2 combines with water to form carbonic acid. Carbonic acid dissociates into H ions and bicarbonate ions. This raises the blood pH.

Question 18

What effect will exercise have on haemoglobin’s affinity for oxygen?

Answer 18

Exercise generates more CO2, lactic acid and heat. This will warm the blood and decrease the pH, shifting the O2 dissociation curve to the right.

This can cause as much as 85% of the O2 to be released from the haemoglobin.

However, the lungs will compensate by increasing respiration rate, decreasing the CO2 in the lungs, shifting the curve to the left and allowing haemoglobin to saturate more readily.

Question 19

What effect does 2,3-bisphosphoglycerate have on haemoglobin’s O2 affinity?

Answer 19

BPG or 2,3-bisphosphoglycerate also modifies the affinity of haemoglobin for O2. BPG is produced by erythrocytes and decreases the affinity of haemoglobin for O2.

People in high altitudes have high concentrations of BPG in the blood and this is thought to increase oxygen delivery to the tissues at high altitudes.

Question 20

In what forms is CO2 transported in the blood?

Answer 20

Majority as bicarbonate ions (72%)

Combined with blood proteins, called carbamino compounds (20%)

Some as CO2 dissolved in the plasma (8%)

Question 21

What is formed when haemoglobin binds with CO2?

Answer 21

Carbaminohaemoglobin

Question 22

What is the Haldane effect?

Answer 22

The affinity of haemoglobin for CO2 is greater if that haemoglobin molecule has just given up oxygen

Question 23

True of false: In the tissues, haemoglobin binds to CO2 upon giving up O2. In the alveoli the opposite occurs.

Answer 23

True

Question 24

Where does CO2 react with water to form carbonic acid?

Answer 24

In the erythrocytes

Question 25

CO2 diffuses into the erythrocytes and reacts with water to form carbonic acid. What does carbonic acid dissociate into?

Answer 25

Hydrogen ions and bicarbonate ions.

The negative bicarbonate ions diffuse out of the RBC into the plasma.

Question 26

What happens to chloride as the negative bicarbonate ions move out of the erythrocyte and into the plasma?

Answer 26

As a result of the movement of bicarbonate ions out of the RBC, chloride ions move into the cell to maintain the electrical balance.

This exchange of bicarb for chloride is called the chloride shift.

Both ions are negative.

Question 27

What is the chloride shift?

Answer 27

As negative bicarb ions leave the RBC, negative chloride ions move in to maintain the electrical balance.

Question 28

In addition to bicarbonate ions, hydrogen ions are also formed when carbonic acid dissociates. What prevents these H+ ions from dropping the pH of the blood?

Answer 28

Most of the hydrogen ions bind to haemoglobin.

Question 29

When deoxygenated blood reaches the alveoli, what happens to the CO2 in the blood?

(In all three forms: free, bound to proteins, and as bicarbonate)

Answer 29

CO2 in solution diffuses into the alveoli.

Protein-bound CO2 dissociates from the carbamino compound and diffuses into the alveoli.

Bicarbonate diffuses back into the erythrocytes as the PCO2 of the blood decreases. At the same time, haemoglobin releases hydrogen ions, which then bind with the bicarb to form carbonic acid. Carbonic acid dissociates into water and CO2 and the CO2 diffuses into the alveoli.