Respiratory Adaptations in Health and Disease:
|Created by Diane R. Karius, Ph.D.|
As you know by this point, hypoxia refers to low oxygen. However, we really haven't described where the oxygen is low with this term. Strictly speaking, the term hypoxia refers to inadequate oxygen available for use by the tissues. Anoxia then refers to the total absence of oxygen being delivered to the tissue. Hypoxemia is the proper term for low oxygen content in the blood. The animation (below) will describe the different forms of hypoxia and there is a written description below the animation.
If you think about hypoxia, you might realize that there are several way in which the tissue could experience inadequate oxygen availability - only some of which we've talked about in the respiratory section. Physiologists have defined four forms of hypoxia:
- Hypoxic hypoxia: This is the form we've talked most about in the Respiratory section. In this form of hypoxia, the PaO2 is below normal because either the alveolar PO2 is reduced (e.g environmental reasons such as altitude) or the blood is unable to equilibrate fully with the alveolar air (e.g. as would occur in lung diseases with diffusion impairments such as emphysema or fibrosis).
- Anemic hypoxia: In this form of hypoxia, the lungs are in perfect working condition, but the oxygen carrying capacity of the blood has been reduced. As the name implies, anemia is a very effective way of producing anemic hypoxia. Carbon Monoxide produces anemic hypoxia - because it binds to the Hb with such high affinity, preventing oxygen from binding, it reduces the oxygen carrying-capacity of the blood. The tissues do not get sufficient oxygen to maintain their metabolic needs because the blood is not carrying it.
- Circulatory hypoxia: In this form of hypoxia the lungs are working just fine and the blood can carry sufficient oxygen. However, the tissue is not receiving sufficient oxygen because the heart cannot pump the blood to the tissue (or the arteries leading to the tissue have been blocked by clots etc...). Sickle cell anemia can lead to circulatory hypoxia as the cells sickle in the blood vessels and block them. (Yes - It also produces an anemic hypoxia as the sickled blood cells are removed from circulation.)
- Histotoxic hypoxia: Histotoxic literally means that the cells have been poisoned. In this form of hypoxia, there is no problem getting the oxygen to the tissue - the lungs, blood and circulatory system are all working just fine. However, the tissue is unable to use the oxygen. Cyanide leads to histotoxic hypoxia by poisoning the systems that utilize oxygen to create energy and preventing them from using the oxygen. Even though there is plenty of oxygen there, the cells experience a lack of oxygen and are affected as if there was too little/no oxygen available.