Ch 5 Flashcards
Water makes up what percentage of the body?
- 60% of our body weight is water, or about 2/3
- Water is important for everything, sweat, urine, circulation etc.
What is extracellular thirst?
- hypovolemic thirst, makes up 1/3 of water in the body
- accounts for water outside of cells (blood, CSF, body cavities)
- induced by perspiration, blood loss, diarrhea, heavy menstrual bleeding
- blood volume and blood pressure decrease
- requires replacement of electrolytes and water
Explain Extracellular Thirst Mechanisms
drop in blood pressure -> activates baroreceptors in kidneys -> release of renin -> renin converts angiotensinogen to angiotensin in blood -> angiotensin causes vasoconstriction and production of aldosterone and vasopressin -> aldosterone leads to increased sodium reabsorption by kidneys; vasopressin leads to increased water reabsorption!
*This process relies on the subfornical organ, located outside the blood-brain barrier. It has osmoreceptors and responds to angiotensin. Its neurons project to the HT. Located near the ventricles.
Explain Cellular Thirst
- osmotic/sodium; accounts for 2/3 of water in body
- induced by excessive salt consumption or severe thirst
- cellular thirst usually comes after extracellular thirst
- extracellular sodium increases and osmotic force draws water out from the cells
The Ventricular System…
Is responsible for the production, transport, and removal of CSF which bathes the CNS
Prandial drinking and the cessation of drinking
Prandial drinking is water intake associated with food intake. Food intake causes fluids to enter the digestive tract and often causes sodium intake. We anticipate the need for water.
Cessation of drinking occurs before extracellular and cellular fluids are rebalanced. There are species-specific mechanisms- dogs drink very much and very rapidly. Cessation comes from receptors in the mouth, esophagus, stomach and the swallowing reflex.
Feeding and Fasting
Feeding (absorptive phase)- insulin release from pancreas, glucose from blood moved to cells and becomes stored energy as glycogen. Parasympathetic.
Fasting (utilization phase)- glucagon release from pancreas, stored glycogen released as glucose, liberation of stored energy. Sympathetic.
Ghrelin
Ghrelin is released by epithelial cells lining the empty stomach. Levels in the blood rise during fasting. Ghrelin signals hunger and stimulates feeding!
CCK
CCK is released when food reaches the intestines. It signals the digestion of fat and proteins. Injecting CCK reduces feeding behaviour!
Orexin
Orexin is produced in the lateral HT. It causes the craving of food and increases meal size by suppressing inhibitory feedback!
Importance of Liver Receptors
Liver receptors are critical. Information on blood glucose from liver sent to HT via the vagus nerve. This is used to regulate hunger, not for energy.
When can glucose be used as energy??
WHEN IT’S IN OUR CELLS!
Intracellular glucose can be used as energy, blood glucose cannot.
Leptin
Secreted by adipose tissue to decrease food intake. Increases metabolism. Involved in long-term regulation of body weight and fat stores.
Leptin is not secreted by adipose tissue in OB mice -> obese; diabetic; can be reversed with leptin injections.
Brain Physiology and Eating
The natural role of the ventromedial HT is to reduce feeding behaviours. The natural role of the lateral HT is to promote feeding behaviours.
(Lesion to VMH -> hyperphagia or over-eating)
(Lesion to LHT -> aphagia or under-eating)
Neuropeptide Y
NPY neurons are found in the arcuate nucleus near the base of the HT near the third ventricle. NPY active when hungry. Obesity can be associated with excessive NPY.
