The Loading and Unloading Reactions

Deoxyhemoglobin and oxygen combine to form oxyhemoglo-bin; this is called the loading reaction. Oxyhemoglobin, in turn, dissociates to yield deoxyhemoglobin and free oxygen molecules; this is the unloading reaction. The loading reaction occurs in the lungs and the unloading reaction occurs in the systemic capillaries.

Loading and unloading can thus be shown as a reversible reaction:


Deoxyhemoglobin + O2 < z>: Oxyhemoglobin (tissues)

The extent to which the reaction will go in each direction depends on two factors: (1) the PO2 of the environment and (2) the affinity, or bond strength, between hemoglobin and oxygen. High PO2 drives the equation to the right (favors the loading reaction); at the high PO2 of the pulmonary capillaries, almost all

506 Chapter Sixteen

Table 16.7 Relationship Between Percent Oxyhemoglobin Saturation and Po2 (at pH of 7.40 and Temperature of 37° C)

PO2 (mmHg) 100 80 61 45 40 36 30 26 23



Percent Oxyhemoglobin 97 95 90 80 75 70 60 50 40



Arterial Blood Venous Blood

Oxyhemoglobin Dissociation Curve
Po2 (mmHg)

■ Figure 16.34 The oxyhemoglobin dissociation curve. The percentage of oxyhemoglobin saturation and the blood oxygen content are shown at different values of PO2. Notice that the percent oxyhemoglobin decreases by about 25% as the blood passes through the tissue from arteries to veins, resulting in the unloading of approximately 5 ml of O2 per 100 ml of blood to the tissues.

the deoxyhemoglobin molecules combine with oxygen. Low PO2 in the systemic capillaries drives the reaction in the opposite direction to promote unloading. The extent of this unloading depends on how low the PO2 values are.

The affinity between hemoglobin and oxygen also influences the loading and unloading reactions. A very strong bond would favor loading but inhibit unloading; a weak bond would hinder loading but improve unloading. The bond strength between hemoglobin and oxygen is normally strong enough so that 97% of the hemoglobin leaving the lungs is in the form of oxyhemoglo-bin, yet the bond is sufficiently weak so that adequate amounts of oxygen are unloaded to sustain aerobic respiration in the tissues.

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  • ralf winkel
    What is theloading / unloading of oxygen?
    8 years ago
  • ranugad hornblower
    Is deoxyhemoglobin produced during the unloading reaction?
    8 years ago
  • Sofia Ryyn
    What is loading and unloading In Physiology?
    8 years ago
  • Anya
    What reaction occurs that the systemic capillaries?
    8 years ago
    What changes promote unloading of O2 from blood for muscle?
    7 years ago
  • mebrat mustafa
    What is meant by a loading reaction and unloading reaction during respiration?
    5 years ago
  • holman
    What is meant by loading and unloading oxygen?
    5 years ago
  • ponto
    Is 60mmHg of O2 unloaded in tissues?
    4 years ago
  • Terttu Haapakoski
    What is the loading and unloading of oxygen in the lungs?
    3 years ago
  • fosco
    When do oxygen unload?
    3 years ago
  • henrik
    How is oxygen loaded and unloaded in the lungs and body tissues?
    3 years ago
  • hollie
    What is ankle unloading reaction?
    2 years ago
  • Michelle Howe
    How oxgen is loaded in the lungs?
    2 years ago
  • katie-leigh
    How loading and unloading occur at tissue and lung level in oxygen hb dissociation curve?
    2 years ago
  • gianetto
    What is meant by the term oxygen ofloading?
    2 years ago
  • zainab
    How is oxygen loaded transported and unloaded in bloodstream?
    2 years ago
  • Theodoric
    How is oxygen loaded in the lungs and unloaded in the body tissue?
    2 years ago
  • eberardo
    How is oxygen is load and unoad in the lung?
    2 years ago
  • lauren hunter
    Is the bond between oxygen and hemoglobin strong or weak?
    11 months ago
  • kerstin theissen
    What is meant by the terms loading and unloading with respect to oxygen carrying?
    11 months ago
  • florian theissen
    What does high altitudes do to oxygen loading and unloading from hemoglobin?
    11 months ago
  • Nick
    What does poor loading of O2 to HB in lungs?
    10 months ago
  • ralf
    Why do conditions in the lungs favor oxygen loading?
    10 months ago
  • abaalom
    How to manipulate unloading of oxygen from hemoglobin?
    10 months ago
  • jasmine
    What factors promote oxygen loading and unloading?
    10 months ago
  • federico palerma
    How does the oxygen dissacociation curve favor unloading and loading of oxygen?
    7 months ago
  • Crassus
    Which conditions will encourage hemoglobin/oxygen unloading?
    6 months ago
  • Orlando
    What favours oxygen loading in the lungs?
    6 months ago
  • patrick
    How do high altitudes affect oxygen loading and unloading from hemoglobin?
    5 months ago
  • diego
    Does high pressure of oxygen encourage oxygen unloading?
    5 months ago
  • Biniam
    How does haemoglobin load and unload oxygen?
    5 months ago
  • lucas
    What is it meant loading and unloading of oxygen haemoglobin?
    4 months ago
  • Michael
    Which 2 factors are needed for hemoglobin to load or unload oxygen?
    3 months ago

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