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?
    6 years ago
  • holman
    What is meant by loading and unloading oxygen?
    6 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?
    4 years ago
  • fosco
    When do oxygen unload?
    4 years ago
  • henrik
    How is oxygen loaded and unloaded in the lungs and body tissues?
    3 years ago
  • hollie
    What is ankle unloading reaction?
    3 years ago
  • Michelle Howe
    How oxgen is loaded in the lungs?
    3 years ago
  • katie-leigh
    How loading and unloading occur at tissue and lung level in oxygen hb dissociation curve?
    3 years ago
  • gianetto
    What is meant by the term oxygen ofloading?
    3 years ago
  • zainab
    How is oxygen loaded transported and unloaded in bloodstream?
    3 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?
    2 years ago
  • kerstin theissen
    What is meant by the terms loading and unloading with respect to oxygen carrying?
    2 years ago
  • florian theissen
    What does high altitudes do to oxygen loading and unloading from hemoglobin?
    2 years ago
  • Nick
    What does poor loading of O2 to HB in lungs?
    2 years ago
  • ralf
    Why do conditions in the lungs favor oxygen loading?
    2 years ago
  • abaalom
    How to manipulate unloading of oxygen from hemoglobin?
    2 years ago
  • jasmine
    What factors promote oxygen loading and unloading?
    2 years ago
  • federico palerma
    How does the oxygen dissacociation curve favor unloading and loading of oxygen?
    1 year ago
  • Crassus
    Which conditions will encourage hemoglobin/oxygen unloading?
    1 year ago
  • Orlando
    What favours oxygen loading in the lungs?
    1 year ago
  • patrick
    How do high altitudes affect oxygen loading and unloading from hemoglobin?
    1 year ago
  • diego
    Does high pressure of oxygen encourage oxygen unloading?
    1 year ago
  • Biniam
    How does haemoglobin load and unload oxygen?
    1 year ago
  • lucas
    What is it meant loading and unloading of oxygen haemoglobin?
    12 months ago
  • Michael
    Which 2 factors are needed for hemoglobin to load or unload oxygen?
    11 months ago
  • Selina
    What prevents oxygen uploading fron blood to muscles?
    7 months ago
  • Uta Kalb
    What drives the unloading of oxygen from hemoglobin molecules in the tissues sapling?
    7 months ago
  • yerusalem
    Is unloading in oxyhemoglobin good?
    7 months ago
    How oxygen is loaded and unloaded with reference to how hemoglobin’s affinity for oxygen varies?
    6 months ago
  • Antero
    What about oxygen loading and unloading from hemoglobin during hyperventialtion?
    6 months ago
  • Frodo
    What changes in muscles occur so o2 can be unloaded from hemoglobin?
    6 months ago
  • Mirabella
    How is oxygen unloaded and loaded in the blood?
    5 months ago
  • sampsa
    How is oxygen off loaded from hemaglobin?
    5 months ago
  • hilda
    What is the function of hemoglobin account for loading and unloading oxygen?
    4 months ago
  • Catone
    How hemoglobin is able to load oxygen in the lung and unload it to the tissues?
    3 months ago
  • mezan
    What influences oxygen loading and unloading of hemoglobin?
    2 months ago
  • medhanie
    How do localised changes in po2 affect oxygen in loading and unloading?
    2 months ago
  • jimmy
    How does hyperventilation affect oxygen loading and unloading from hemoglobin?
    2 months ago
  • flynn white
    When oxygen is unloaded by hemoglobin that is expected to be the highest?
    2 months ago
  • James
    How does hemoglobin unload unload oxygen to myoglobin at the muscle?
    1 month ago
  • karoliina
    Why does haemoglobin not unload its oxygen until it reaches the capillaries in the tissues.?
    1 month ago
    What factors determine whether hemoglobin will load or unload oxygen?
    1 month ago
  • cora
    Why is oxygen unloaded in tissues that are highly active?
    1 month ago
  • Sophia
    What facilitates the unloading of oxygen to active muscle?
    1 month ago
  • eleanor
    When blood reaches the lungs, what does it unload?
    22 days ago
    Is co2 binding favoured in the tissues?
    11 days ago

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