OXYGEN CARRIED IN BLOOD WHEN PERSON IS AT HIGH ALTITUDE-TUTORIAL PHYSIOLOGY

Oxygen Uptake in the Lungs is Increased About 70X by Hemoglobin in the Red Cells

• In the lungs oxygen must enter the blood
• A small amount of oxygen dissolves directly in the serum, but 98.5% of the oxygen is carried by hemoglobin
• All of the hemoglobin is found within the red blood cells (RBCs or erythrocytes)
• The hemoglobin content of the blood is about 15 gm/deciliter (deciliter = 100 mL)
• Red cell count is about 5 million per microliter

Each Hemoglobin Can Bind Four O2 Molecules (100% Saturation)

• Hemoglobin is a protein molecule with 4 protein sub-units (2 alphas and 2 betas)
  • Each of the 4 sub-units contains a heme group which gives the protein a red color
  • Each heme has an iron atom in the center which can bind an oxygen molecule (O2)
  • The 4 hemes in a hemoglobin can carry a maximum of 4 oxygen molecules
• When hemoglobin is saturated with oxygen it has a bright red color; as it loses oxygen it becomes bluish (cyanosis)

The Normal Blood Hematocrit is Just Below 50%

• Blood consists of cells suspended in serum
• More than 99% of the cells in the blood are red blood cells designed to carry oxygen
  • 25% of all the cells in the body are RBCs
• The volume percentage of cells in the blood is called the hematocrit
• Normal hematocrits are about 40% for women and 45% for men

At Sea Level the Partial Pressure of O2 is High Enough to Give Nearly 100% Saturation of Hemoglobin

• As the partial pressure of oxygen in the alveoli increases the hemoglobin in the red cells passing through the lungs rises until the hemoglobin is 100% saturated with oxygen
  • At 100% saturation each hemoglobin carries 4 O2 molecules
  • This is equal to 1.33 mL O2 per gram of hemoglobin
• A person with 15 gm Hb/deciliter can carry:
  • Max O2 carriage = 1.33 mL O2/gm X 15 gm/deciliter = 20 mL O2/deciliter
• A plot of % saturation vs pO2 gives an S-shaped “hemoglobin dissociation curve“
• At 100% saturation each hemoglobin binds 4 oxygen molecules
• The hemoglobin dissociation curves in this lecture were calculated with an equation given by J. W. Severinghaus (Simple, accurate equations for human blood O2 dissociation computations. Journal of Applied Physiology 46: 599-602, 1979)

At High Altitudes Hemoglobin Saturation May be Well Below 100%

• At the alveolar pO2 of 105 mm Hg at sea level the hemoglobin will be about 97% saturated, but the saturation will fall at high altitudes
• At 12,000 feet altitude alveolar pO2 will be about 60 mm Hg and the hemoglobin will be 90% saturated
• At 29,000 feet (Mt. Everest) alveolar pO2 is about 24 mm Hg and the hemoglobin will be only 42% saturated
• At very high altitudes most climbers must breath pure oxygen from tanks
• During acclimatization to high altitude the hematocrit can rise to about 60%- this increases the amount of oxygen that can be carried
• Hematocrits above 60% are not useful because the blood viscosity will increase to the point where it impairs circulation

Fetuses Live in a Low Oxygen Environment and Require a Special Hemoglobin

• The developing fetus cannot breathe and must get all of its blood from the placenta
• Fetal blood has a very low pO2, about 30 mm Hg, equivalent to living at 26,000 feet altitude
• The physiologist Barcroft labeled this situation “Everest in utero”
• To extract more oxygen from the mother’s blood fetuses make a special hemoglobin (hemoglobin F) which has a very high affinity for oxygen

Acid Conditions Help Release O2 in the Tissues

• Hemoglobin must bind oxygen tightly to load up efficiently in the lungs, but this makes it hard to release the oxygen in the tissues
• Some unloading occurs because the tissue pO2 is low, causing oxygen to diffuses from the blood
• Active tissues make lots of acid (carbonic and lactic) and this also helps to unload oxygen from the hemoglobin
• At low pH hemoglobin has a lower affinity for oxygen; this will cause more oxygen to come off in the tissues- important in exercise
• The increased unloading of O2 at low pH is known as the Bohr effect
• The left (black) curve is at pH 7.4, the middle (red) curve is at pH 7.1, and the right (blue) curve is at pH 6.8. For any pO2 the hemoglobin is more saturated at pH 7.4 than at pH 6.8.
• To test your understanding of these curves use the graph to determine how much O2 will be delivered to muscle tissues by a deciliter of blood under these conditions:
  • The pH where the blood is loaded (in the lungs) is 7.4 and the pO2 is 95 mm Hg
  • The pH where the blood is unloaded (in the muscle) is 6.8 and the pO2 is 35 mm Hg
  • Assume the blood has 15 gm hemoglobin per deciliter so that a deciliter carries 20 mL of O2 when it is 100% saturated
  • ANSWERS ARE AT BOTTOM OF PAGE.
    

Hypoxia Can Have Several Causes

• Hypoxia is tissue oxygen deficiency
• Brain is the most sensitive tissue to hypoxia: complete lack of oxygen can cause unconsciousness in 15 sec and irreversible damage within 2 min.
• Oxygen delivery and use can be interrupted at several sites

• Causes:
  • Hypoxic: high altitude, pulmonary edema, hypoventilation, emphysema, collapsed lung
  • Anemic: iron deficiency, hemoglobin mutations, carbon monoxide poisoning
  • Ischemic: shock, heart failure, embolism
  • Histotoxic: cyanide poisoning (inhibits mitochondria)
• Carbon monoxide (CO) poisoning:
  • CO binds to the same heme Fe atoms that O2 binds to
  • CO displaces oxygen from hemoglobin because it has a 200X greater affinity for hemoglobin.
  • Treatment for CO poisoning: move victim to fresh air. Breathing pure O2 can give faster removal of CO
• Cyanide poisoning:
  • Cyanide inhibits the cytochrome oxidase enzyme of mitochondria
  • Two step treatment for cyanide poisoning:
    • 1) Give nitrites
      • Nitrites convert some hemoglobin to methemoglobin. Methemoglobin pulls cyanide away from mitochondria.
    • 2) Give thiosulfate.
      • Thiosulfate converts the cyanide to less poisonous thiocyanate.

Oxygen Delivery to Tissues: Effect of pH

• • (Bohr Effect)
    • To test your understanding of these curves use the graph to determine how much O2 will be delivered to muscle tissues by a deciliter of blood under these conditions:
      • The pH where the blood is loaded (in the lungs) is 7.4 and the pO2 is 95 mm Hg
      • The pH where the blood is unloaded (in the muscle) is 6.8 and the pO2 is 35 mm Hg
      • Assume the blood has 15 gm hemoglobin per deciliter so that a deciliter carries 20 mL of O2 when it is 100% saturated

    Start by drawing a line (green) from pO2 = 95 to the point where it intersects the pH 7.4 curve. Then draw a line parallel to the X axis that intersects the scale on the Y axis. You will see that the % saturation is about 97%. The amount of O2 loaded by a deciliter of blood will be:

    O2 loaded = .97 X 20 mL = 19.4 mL

Related articles by Zemanta

• Are capillary blood gases coming back? (respiratorytherapycave.blogspot.com)
• Get Your Blood Sucked & Look Younger (cocoperez.com)