Section 6

Question 1

What is the PO2 of arterial blood, and why is oxygen considered to be poorly soluble in liquid such as plasma?

Answer 1

The PO2 of arterial blood is 100 mmHg, representing around 200 ml O2/L. Oxygen is considered poorly soluble in liquids like plasma, with only about 3 ml O2 able to physically dissolve into 1 L.

Question 2

What is the solution to the limited solubility of oxygen in plasma, and how does it address the supply of oxygen for the body’s metabolic needs?

Answer 2

The solution is haemoglobin (Hb). Once bound to Hb, oxygen no longer contributes to the PO2, and the Hb-bound oxygen acts as a reserve, addressing the limited solubility of oxygen in plasma.

Question 3

What is the critical concept in understanding the difference between saturation and content in the context of oxygen transport in the blood?

Answer 3

The critical concept is that once oxygen is bound to Hb, it no longer contributes to the PO2, and the PO2 only represents the freely dissolved oxygen in the plasma. This understanding is crucial for differentiating between saturation and content.

Question 4

Describe the structure of human hemoglobin (Hb) and its composition.

Answer 4

Human hemoglobin is an assembly of four protein subunits. Each subunit consists of a protein chain and a heme group, which contains the iron molecules to which oxygen binds.

Question 5

What percentage of circulating oxygen is bound to hemoglobin (Hb)?

Answer 5

98.5% of circulating oxygen is bound to hemoglobin (Hb).

Question 6

Describe the process by which each iron atom in a hemoglobin (Hb) molecule can bind an oxygen molecule.

Answer 6

Each iron atom in a hemoglobin (Hb) molecule can bind an oxygen molecule, leading to the formation of HbO2, Hb(O2)2, Hb(O2)3, and Hb(O2)4, as represented by the equation: Hb + O2 ↔ Hb O2 ↔ Hb(O2)2 ↔ Hb(O2)3 ↔ Hb(O2)4.

The double arrows indicate that each reaction is fully reversible, allowing Hb to bind oxygen for transport and unbind oxygen for delivery.

Question 7

What does it mean for hemoglobin (Hb) to be fully saturated, and how is Hb saturation expressed?

Answer 7

Hemoglobin (Hb) is fully saturated when all Hb present is carrying its maximum oxygen load. Hb saturation is expressed as a percentage.

Question 8

According to the chemistry law of mass action, how does the concentration of a substance, such as PO2, affect a reversible reaction like Hb binding to oxygen?

Answer 8

According to the law of mass action, if you increase the concentration of one substance involved in a reversible reaction (such as increasing PO2), the reaction is driven to the other side. The opposite is also true.

Question 9

What is the oxygen dissociation curve, and what does it describe?

Answer 9

The oxygen dissociation curve describes the relationship between PO2 and % Hb saturation.

Question 10

How is the oxygen dissociation curve shaped, and what is notable about the steep slope and plateau regions?

Answer 10

The oxygen dissociation curve is sigmoidal, with a steep slope between 0 and 60 mmHg and a plateau beyond 60 mmHg as it approaches 100 mmHg. The steep slope below 60 mmHg indicates that a small change in PO2 can have a large effect on % Hb saturation.

Question 11

What does the plateau region of the oxygen dissociation curve represent, and in which PO2 range is it found?

Answer 11

The plateau region of the curve, from 60 mmHg to 100 mmHg, represents the PO2 range found in the pulmonary capillaries where hemoglobin (Hb) is collecting oxygen.

Question 12

What percentage of saturation does the dissociation curve indicate for hemoglobin (Hb) when the blood is leaving the lungs with a PO2 of 100 mmHg?

Answer 12

The dissociation curve shows that hemoglobin (Hb) is 97.5% saturated when the blood is leaving the lungs with a PO2 of 100 mmHg.

Question 13

Why is the plateau phase of the curve considered a margin of safety, and how does it benefit individuals with pulmonary disease or those in specific circumstances?

Answer 13

The plateau phase represents a margin of safety because even if there is a drop in PO2 to 60 mmHg, hemoglobin (Hb) would still be 90% saturated. This is crucial for individuals with pulmonary disease and for normal healthy persons in situations like high altitudes or oxygen-deprived environments, providing a safety buffer for oxygen transport to the tissues until arterial PO2 drops below 60 mmHg.

Question 14

What does the steep portion of the oxygen dissociation curve correspond to, and in which PO2 range is it found?

Answer 14

The steep portion of the curve, between 0 and 60 mmHg, corresponds to the range of PO2 found in the systemic capillaries.

Question 15

Describe the change in oxygen saturation from the arrival of blood in the systemic capillaries (PO2 of 100 mmHg) to its departure (PO2 of 40 mmHg).

Answer 15

Blood arrives in the systemic capillaries with a PO2 of 100 mmHg and is 97.5% saturated. By the time it leaves the systemic capillaries, the PO2 has dropped to 40 mmHg, and the blood is now 75% saturated, indicating that 25% of the oxygen has been unloaded to support metabolism at rest.

Question 16

How does the steep portion of the oxygen dissociation curve facilitate oxygen unloading in metabolically active tissues?

Answer 16

In metabolically active tissues where more oxygen is needed, a drop in PO2 to 20 mmHg can release an additional 45% of the total oxygen. The steep portion of the curve allows for larger amounts of oxygen dissociation for small decreases in PO2.

Question 17

How does the steep portion of the curve benefit individuals breathing at altitude, and what happens to alveolar PO2 and arterial PO2 at high altitudes?

Answer 17

At altitude, decreased atmospheric pressure leads to a decrease in alveolar PO2 and arterial PO2. The decrease in arterial PO2 activates carotid chemoreceptors, increasing ventilation. If the person is at rest, this increased ventilation results in a small decrease in arterial PCO2, leading to a small increase in alveolar PO2. On the steep portion of the curve, this small increase in alveolar PO2 can greatly increase % Hb saturation.

Question 18

Which of the following resonate with the plateau phase of the oxygen dissociation curve?

• in the alveoli
• in the tissues
• low O2 attached to Hb
• high O2 attached to Hb

Answer 18

• In the alveoli
• High O2 attached to Hb

Question 19

Which of the following resonate with the steep portion of the oxygen dissociation curve?

• in the alveoli
• in the tissues
• low O2 attached to Hb
• high O2 attached to Hb

Answer 19

• In the tissues
• Low O2 attached to Hb

Question 20

Why could the role of hemoglobin in gas exchange be initially ignored when discussing oxygen being driven from the alveoli to the blood?

Answer 20

The role of hemoglobin in gas exchange could be initially ignored because blood PO2 depends entirely on dissolved oxygen, allowing the discussion to focus on the oxygen driven from the alveoli to the blood by a PO2 gradient.

Question 21

What crucial role does hemoglobin play in facilitating the exchange of oxygen between the alveoli and the lungs?

Answer 21

Hemoglobin plays a crucial role in permitting the transfer of large quantities of oxygen before blood equilibrates with the surrounding tissues.

Question 22

Describe the hypothetical situation when no hemoglobin is present in the blood and its impact on the equilibrium between alveolar PO2 and pulmonary capillary blood PO2.

Answer 22

In the hypothetical situation with no hemoglobin, the alveolar PO2 and pulmonary capillary blood PO2 are at equilibrium.

Question 23

How does the presence of partially saturated hemoglobin impact blood PO2, and what is the role of dissolved oxygen in this scenario?

Answer 23

As hemoglobin starts to bind with oxygen, it removes oxygen from solution. Because only dissolved oxygen contributes to blood PO2, the blood PO2 remains below that of the alveoli, favoring the net diffusion of more oxygen down its partial pressure gradient from the alveoli to the blood.

Question 24

What happens when hemoglobin is fully saturated with oxygen, and how does this affect the equilibrium between alveolar and blood PO2?

Answer 24

When hemoglobin is fully saturated with oxygen, the alveolar and blood PO2 are at equilibrium again. The blood PO2 resulting from dissolved oxygen is equal to the alveolar PO2, despite the total oxygen content in the blood being much greater than in the case of no hemoglobin.

Question 25

What is the primary determinant of % hemoglobin saturation, and how can the dissociation curve shift?

Answer 25

The primary determinant of % hemoglobin saturation is PO2. The dissociation curve can shift due to factors such as changes in pH, carbon dioxide levels, 2,3-bisphosphoglycerate (BPG) production, and the presence of carbon monoxide (CO).

Question 26

How does pH affect the oxygen dissociation curve, and what is this phenomenon known as?

Answer 26

The combination of H+ and lactic acid, which can decrease pH, enhances the dissociation of oxygen from hemoglobin in a phenomenon known as the Bohr effect.

Question 27

What role does 2,3-bisphosphoglycerate (BPG) play in affecting the oxygen dissociation curve, and when is its production increased?

Answer 27

2,3-bisphosphoglycerate (BPG) within red blood cells affects the oxygen dissociation curve by shifting it to the right. BPG production is increased when oxygen saturation in arterial blood is below normal.

Question 28

What is the Haldane effect, and how does carbon dioxide (CO2) impact the oxygen dissociation curve?

Answer 28

The Haldane effect occurs when carbon dioxide (CO2) binds to hemoglobin, shifting the oxygen dissociation curve to the right (decreasing % saturation for a given PO2). This mechanism increases oxygen unloading in metabolically active tissues where PCO2 is increasing.

Question 29

How does carbon monoxide (CO) impact hemoglobin and the oxygen dissociation curve, and why is it problematic?

Answer 29

Carbon monoxide (CO) competes with oxygen for the same binding site on hemoglobin, forming carboxyhemoglobin (Hb CO). CO has a much greater binding affinity than oxygen, decreasing % saturation. It also shifts the oxygen dissociation curve to the left, requiring larger drops in PO2 to unload oxygen, and even low levels of CO can make a large number of oxygen-binding sites on hemoglobin unavailable. This can lead to severe consequences, including death.

Question 30

What are the three ways in which carbon dioxide is transported in the blood?

Answer 30

Carbon dioxide is transported in the blood through three methods: physically dissolved in the plasma, bound to hemoglobin, and as bicarbonate (HCO3-).

Question 31

What fraction of total blood CO2 is physically dissolved, and what is its significance?

Answer 31

Only about 5-10% of total blood CO2 is physically dissolved in the plasma. This small fraction accounts for the 46 mmHg partial pressure of CO2 when blood leaves the systemic capillaries.

Question 32

How does CO2 bind to hemoglobin, and what is the significance of this binding?

Answer 32

CO2 does not bind to the haem-O2 binding sites; rather, it binds to the globin part of the hemoglobin molecule. Hemoglobin without oxygen has a greater affinity for CO2, and the unloading of O2 in the systemic capillaries enhances the uptake of CO2. Only another 5-10% of the total CO2 is transported in this manner.

Question 33

What is the bicarbonate (HCO3-) method of CO2 transport, and what percentage of circulating CO2 does it make up?

Answer 33

Bicarbonate makes up 80-90% of circulating CO2. The reaction involves CO2 combining with water to form carbonic acid (H2CO3), which spontaneously dissociates into hydrogen ions and bicarbonate. Red blood cells contain the enzyme carbonic anhydrase (CA), which accelerates this reaction.

Question 34

What happens in the systemic capillaries as more CO2 enters the red blood cells?

Answer 34

In the systemic capillaries, as more CO2 enters the red blood cells, bicarbonate and hydrogen ions accumulate.

Question 35

How is the exchange of bicarbonate and chloride ions facilitated in red blood cells?

Answer 35

Red blood cells have a bicarbonate-chloride carrier that passively allows the exchange of these ions across the cell membrane.

Question 36

What is the result of the bicarbonate-chloride exchange in red blood cells, and what is this phenomenon known as?

Answer 36

The result of the bicarbonate-chloride exchange is that bicarbonate leaves the cells, and chloride enters the cell, down its electrochemical gradient. This inward shift of chloride, in exchange for bicarbonate, is known as the chloride (Hamburger) shift.

Question 37

What does the chloride (Hamburger) shift facilitate, and how does it contribute to gas transport?

Answer 37

The chloride (Hamburger) shift facilitates the movement of CO2 from the tissues into the blood, contributing to its transport to the lungs for expiration.

Question 38

What is the Haldane effect, and how does it affect the oxygen dissociation curve?

Answer 38

The Haldane effect is a phenomenon where the binding of CO2 to hemoglobin facilitates the release of oxygen at the tissue level, resulting in a rightward shift of the oxygen dissociation curve.

Question 39

What is the reverse Haldane effect, and when does it occur?

Answer 39

The reverse Haldane effect occurs when there are increases in arterial PO2, such as when breathing supplemental oxygen. The increased PO2 prevents hemoglobin from binding carbon dioxide, forcing the CO2 to travel back to the lungs either dissolved in the plasma or as bicarbonate.

Question 40

How does an increase in PO2 affect the binding of CO2 to hemoglobin, and what does it result in?

Answer 40

An increase in PO2 prevents CO2 from binding to hemoglobin, resulting in an increase in carbon dioxide that is transported as dissolved CO2 within the blood. This may lead to an increase in blood acidity, explaining the associated increased ventilation rates with breathing supplemental oxygen.

Question 41

What is hypoxia, and what are the four categories of hypoxia?

Answer 41

Hypoxia is described as insufficient oxygen at the cellular level. The four categories of hypoxia are:

1. Hypoxic Hypoxia: Characterized by low arterial PO2 with inadequate Hb saturation. It can be caused by inadequate gas exchange or exposure to high altitude.
2. Circulating Hypoxia: Occurs when too little oxygenated blood is delivered to the tissues, often due to vascular spasms or blockages.
3. Anemic Hypoxia: Results from a reduced oxygen-carrying capacity of the blood, caused by a decrease in circulating red blood cells, decreased Hb within red blood cells, or carbon monoxide poisoning.
4. Histotoxic Hypoxia: Oxygen delivery to the tissues is normal, but something within the tissues prevents oxygen usage, such as cyanide poisoning.

Question 42

What is hyperoxia, and how can it occur?

Answer 42

Hyperoxia is characterized by an abnormally high arterial PO2. It can occur in someone breathing supplemental oxygen. While it may raise arterial PO2, it can cause oxygen toxicity, leading to dangerous levels. Increased dissolved O2 in some tissues can result in the formation of reactive oxygen species, damaging cells and causing issues like brain and retina damage.

Question 43

What is hypercapnia, and what causes it?

Answer 43

Hypercapnia is the excess of carbon dioxide in the blood and is caused by hypoventilation. Decreased ventilation affects both carbon dioxide and oxygen, resulting in decreased PO2.

Question 44

What is hypocapnia, and what are its causes?

Answer 44

Hypocapnia is below-normal arterial PCO2 and is caused by hyperventilation. It can be triggered by factors like anxiety, fever, aspirin poisoning, and exercise, especially if there is a shift to anaerobic metabolism. Hyperventilation increases alveolar PO2, but little extra oxygen is added to the blood due to near-maximal partial pressure of dissolved oxygen and % saturation of Hb with normal alveolar PO2.

Question 45

Which conditions cause a left shift in the oxygen dissociation curve, indicating increased affinity for O2?

Answer 45

Decreased temperature (↓ Temp), increased pH (↑ pH), decreased partial pressure of carbon dioxide (↓ pCO2), and decreased 2,3-bisphosphoglycerate (BPG) (↓ BPG) cause a left shift, indicating increased affinity for O2.

Question 46

Which conditions cause a right shift in the oxygen dissociation curve, indicating decreased affinity for O2?

Answer 46

Increased temperature (↑ Temp), decreased pH (↓ pH), increased partial pressure of carbon dioxide (↑ pCO2), and increased 2,3-bisphosphoglycerate (BPG) (↑ BPG) cause a right shift, indicating decreased affinity for O2.