Hypoxia, Hypoxemia & Hemoglobin-Oxygen Saturation Curve

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How much oxygen is normally dissolved in the plasma of arterial blood? How much is normally transported by hemoglobin?

1.5% of oxygen is dissolved in plasma and the rest is bound to hemoglobin.

What is meant by the percent saturation of hemoglobin? What is the most important factor influencing this saturation?

Percentage saturation of hemoglobin is the percentage of oxygen bound to hemoglobin in blood. The most important factor is the partial pressure of oxygen (PO2).

What is the oxygen saturation and partial pressure of oxygen in your pulmonary arteries at rest?

Oxygen saturation is 98.5%, i.e., this percentage of oxygen is bound to hemoglobin in blood. Partial pressure of oxygen depends on two factors:

When oxygen is moving from muscle to capillaries: PO2 = 40 mm Hg due to poor oxygen content of blood.

When oxygen is moving from capillaries to muscle: PO2 = 100 mm Hg due to rich oxygen content of blood.

Why is only 25% of available oxygen split from hemoglobin in tissue capillaries in resting condition? Why is this important?

This is important because it creates the oxygen buffer in blood. The 25% of oxygen is kept as back-up incase a fight-of-flight scenario occurs and a large demand of oxygen is required. It is important because our bodies then have a substantial reserve of oxygen.

Oxygen Saturation Curve

Oxygen Saturation Curve

What are the main factors affecting hemoglobin's affinity for oxygen?

The factors affecting hemoglobin's affinity for oxygen are:

Lowered pH

• Increased temperature

• More 2,3-diphosphoglycerate

• Increased levels of CO2

Which way do they shift the oxygen-hemoglobin dissociation curve?

Curve moves to the right due to:

• Decrease in pH (Bohr shift).

• Increased temperature.

• Increased DPG.

• Increased CO2 levels.

What are the implications of this shift?

A right shift means that less and less oxygen is binding hemoglobin for a particular partial pressure of oxygen, and more is mobilized in tissues. The opposite is true for a left shift.

What does this actually mean, that the oxygen-hemoglobin curve is "shifted"?

The sigmoid shape of the oxygen dissociation curve is a result of the cooperative binding of oxygen to the four polypeptide chains. Cooperative binding is the characteristic of a hemoglobin to have a greater ability to bind oxygen after a subunit has bound oxygen (Oxygen Dissociation Curve). Thus, hemoglobin is most attracted to oxygen when three of the four polypeptide chains are bound to oxygen.

How does fever affect the affinity of hemoglobin for oxygen? Why might this be important?

Fever raises the body temperature causing the curve to shift right. This means that more oxygen is flowing into tissue and less is binding to hemoglobin. It is important because this helps the body fight infection as more blood is flowing into tissue.

Why is the sigmoid shape of the hemoglobin-oxygen dissociation curve so important clinically?

It is important because it demonstrates cooperative binding. Cooperative binding is the characteristic of a hemoglobin to have a greater ability to bind oxygen after a subunit has bound oxygen (Oxygen Dissociation Curve). Thus, hemoglobin is most attracted to oxygen when three of the four polypeptide chains are bound to oxygen.

Patients with respiratory distress are often monitored with an "oximeter" in the hospital, a device attached to their finger which measures hemoglobin oxygen saturation.

(a) If a patient's oximeter reading is 90, we get worried. Why?

This means that hemoglobin is transporting a lower than normal amount of oxygen in blood. The reading should be 98.5%.

(b) A medical student puts the oximeter on his finger and finds it reads 99. He puts on an oxygen mask, turns on the oxygen, breathes for a few minutes, and finds the reading on the meter hasn't changed. Explain.

Now, the buffer system comes into play. Even though the students is breathing in pure oxygen, the oxygen level in blood remains fairly constant hemoglobin is uploading oxygen into tissues and keeping its saturation number constant.

Classify hypoxia, and explain each category.

Hypoxic hypoxia:

Caused by low partial pressure of oxygen.

Anemic hypoxia:

Reduced transport of O2 to tissue cells.

Ischemic hypoxia:

Blood flow to tissue is reduced.

Histotoxic hypoxia:

Tissues are not using the adequate oxygen supplied to them. Caused by cyanide poisoning.

How does carbon monoxide cause hypoxia?

CO has a much higher affinity for hemoglobin than does oxygen. This causes less oxygen to be delivered to tissues causing hypoxia.

How does anemia cause hypoxia?

Anemia means a reduction in RBC count. This causes less oxygen to be delivered to tissues causing hypoxia.

How does cyanide poisoning cause hypoxia?

Cyanide blocks cytochrome oxidase, which is an enzyme needed to uptake oxygen by tissues.

We occasionally hear news reports of athletes being banned from sports competitions after testing positive for EPO (erythropoietin). What is erythropoietin? Explain why EPO would be considered a "performance-enhancing drug". Some athletes train in the mountains to achieve a similar effect. Explain how training at high altitude would produce a similar effect.

Erythropoietin increases the rate of RBC formation. Erythropoietin use is banned because it gives users an advantage over other athletes. Erythropoietin causes more blood cell production and more oxygen is transported to tissues resulting in greater stamina and stronger muscular contraction. Training at higher altitudes causes the rate of production of RBC to go up naturally, thus, producing the same effect.

Additional Reading:

Basic Pulmonology

1. Lung Mechanics
2. Alveolar-Blood Gas Exchange
3. Gas Transport and Regulation of Respiration
4. Four Causes of Hypoxemia
5. Control of Respiration
6. Systemic vs Pulmonary Circulation FAQ
7. Principles of Gas Exchange in Lungs
8. Hypoxia, Hypoxemia & Hemoglobin-Oxygen Saturation Curve
9. FAQ on Mechanics of Breathing
10. FAQ on Control of Breathing
11. Criteria for Transudate Pleural Effusion
12. Light's Criteria for Exudate Pleural Effusion
13. Notes on Lung Sounds
14. Patient with Acute Respiratory Distress Syndrome (ARDS)
15. Management of Acute Deep Venous Thrombosis
16. Notes on Asthma Treatment

Pulmonology Videos

1. Video of Pulmonology Examination in a Clinical Setting

Related Topics

1. Histology of the Respiratory System
2. Upper and Lower Respiratory Disorders
3. Pulmonary Examination for Internal Medicine

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