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Up to this point, the discussion has focused primarily on the general structure and function of the endocrine system. Normal bodily function depends on the timely release and inhibition of a variety of hormones as they work together to bring about important actions at their target organs. The endocrine system plays a significant role in controlling physiological functions during exercise as well. The significant hormonal actions that are most responsive to an acute bout of exercise are presented in this section.
Posterior Pituitary Hormones
The posterior pituitary lobe stores and secretes ADH and oxytocin, which are transported from the hypothalamus. Little information is available about the effects of exercise on oxytocin. However evidence supports the idea that exercise is a potent stimulus for ADH secretion.
During periods of heavy sweating and intense exercise, ADH works to minimise the extent of water loss from the kidneys, thereby decreasing the risk of severe dehydration. This response helps the body conserve fluids, especially during exercise in the heat, when a person is most at risk for dehydration. By increasing the water permeability of the kidneys’ collecting ducts, ADH facilitates the conservation of water, allowing less water to be excreted in the urine.
With intense muscular work and heavy perspiration, the electrolytes become more concentrated in the blood plasma, which increases the plasma osmolality (the ionic concentration of dissolved substances, such as electrolytes, in the plasma). Additionally, sweating causes water to be drawn out of the blood, resulting in a lower plasma volume. The hypothalamus can sense increased plasma osmolality and lowered plasma volume, and responds by stimulating the posterior pituitary lobe to secrete ADH (see figure 10.6). In contrast, ADH secretion is minimised when fluid intake increases and the blood volume expands, resulting in more dilute urine.
Antidiuretic hormone’s influence on the conservation of body water during exercise.
Reprinted, by permission, from W.L. Kenney, J.W. Wilmore, and D.L. Costill, 2015, Physiology of sport and exercise, 6th ed. (Champaign, IL: Human Kinetics), 112.
The thyroid gland secretes T3 and T4 (general metabolic hormones) and calcitonin, which facilitates calcium metabolism. T3 and T4 are involved in the following important functions:
- Regulation of basal metabolic rate
- Protein and enzyme synthesis
- Increasing the size and number of mitochondria in most cells
- Rapid cellular uptake of glucose
- Glycolysis and gluconeogenesis
- Lipid mobilisation, increasing FFAs for use in aerobic metabolism
TSH stimulates the thyroid and controls the release of T3 and T4. During exercise plasma T4 concentrations do increase, but a delay occurs between T4 elevations and TSH concentrations, so it is unclear whether there is a direct causal relationship between these two hormones during exercise. During submaximal aerobic endurance exercise there is a sharp initial increase in T4 concentrations, but then T4 remains relatively constant at a lower level throughout the rest of the bout. During prolonged submaximal exercise, T3 concentrations tend to decrease.
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