Under normal conditions, erythropoiesis and erythrolysis occur at the same pace, however if the oxygen-carrying capacity of RBCs fall, a negative feedback system steps up RBC production. During the negative feedback control, the condition monitored is the amount of oxygen delivered to body tissues, not the number of circulating RBCs.
Erythropoietin: hormone produced by the kidneys that regulates RBC production.
WHITE BLOOD CELLS (LEUKOCYTES)
WBC can be classified as either:
a ) Granular leukocytes: Neutrophils, Eosinophils, Basophils,
b) Agranular leukocytes: T & B Lymphocytes and Natural Killer (NK) Cells, Monocytes
WBC’s:
Have nuclei
Do not contain Hemoglobin
approx: 5,500 – 12,500 WBCs/µL of blood. Lifespan: hours, days, weeks, months, years depending
Major Histocompatibility (MHC) Antigens : proteins protruding from their plasma membrane into the extracellular fluid called “cell identity markers” and are unique for each horse.
Functions of WBCs
In healthy mammals, WBCs may live for several months to years. However, during periods of infection, phagocytic WBCs may live only for a few hours. Chemotaxis : released chemicals from microbes and inflamed tissues that attract WBCs. These substances include toxins from microbes & kinins. Emigration : the process where WBCs leave the bloodstream where they roll along the endothelium, stick to it, and then squeeze between endothelial cells. Phagocytosis : neutrophils and macrophages can actively ingest bacteria and dispose of dead matter. As microbes invade deeper tissues, the general function of WBCs is to combat them by phagocytosis or immune responses. To accomplish this, many WBCs leave the bloodstream and collect at sites of pathogen invasion or inflammation.
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