Hypoxic injury is common and has many causes. For a cell to function normally it requires a constant input of oxygen and nutrients and the continuous removal of waste products. This occurs through the exchange of molecules between the extracellular fluid and the cytosol. For microscopic organisms this exchange can occur directly between the ECF and every cell of the organism. For complex organisms such as the horse, nutrient delivery is facilitated by the circulatory system. A fine meshwork of capillaries feed every cell in the body. Nutrients diffuse out of the blood into the ECF where the nutrients are available to the tissues. Reduced oxygen availability to a cell or tissue is called hypoxia. One cause of hypoxia is . Ischemia is the reduced perfusion of blood to an area or tissue. Without blood flow there is insufficient oxygen delivery to meet the tissue’s metabolic needs. Once this occurs injury to the cell begins. Chemical agents can contain molecules that interfere with membrane integrity or alter the function of membrane transport proteins, ultimately hampering the control of the content of the ICF, and thereby disrupting homeostasis. The body converts some of the chemicals into toxic metabolites such as , which need to be neutralized. If not, the cell can become overwhelmed. Viruses have two mechanisms by which they cause cellular injury. The first is . In this case the virus inserts its own genetic information into the nucleus of the host cell, thereby using the cell’s organelles to replicate viral components and produce more viruses. The second mechanism is . In this case the virus triggers the host cell to become cancerous and replicate rapidly and uncontrollably, spreading the virus as the tumour grows. Damage occurs to cells as they age. In normal cell function, metabolites known as free radicals are created. These molecules have a lone, unpaired in the valance shell of one of the atoms. This unpaired electron has an affinity for phospholipids and can disrupt the membrane of the cell and of organelles. Normally free radicals are neutralized by peroxisomes, but as the cell ages peroxisomes are reproduced more slowly and greater damage is done to the cell by free radicals. Cell Death There are two causes of cell death. Cases of extreme, irreversible cell injury leading to cell death is called necrosis, while genetically controlled cell death without cell injury is called . Necrosis is always pathological and is generally not restricted to single cells. Even if only one cell dies, the process will damage the cells neighbouring it. An inflammatory reaction always occurs with necrosis. Apoptosis is not always pathologic, meaning it is a part of the body’s normal physiological process. Apoptosis occurs without pre-existing cell damage and is generally limited to single cells, without bystander damage nor an inflammatory reaction.
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