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Fluid balance is maintained by insuring that the amount of water consumed via food and drink is equal to the amount of water excreted. One way our body keeps us putting in the effort to maintain water balance is through thirst and salt cravings. The kidneys are the most important factor in excretion of water, although we lose water elsewhere, it is primarily lost through the kidneys. Our kidneys keep control of the volume of fluids via controlling the amount of water excreted via urine, the kidneys are able to conserve water by producing urine that is concentrated relative to plasma or they can insure we get rid of excess water by producing urine that is dilute relative to plasma.
Vasopressin (Antidiuretic hormone, ADH) is a peptide hormone produced by the hypothalamus and is responsible for direct control of water excretion via the kidneys. ADH is responsible for the insertion of water channels into the membranes of cells where it will line the collecting ducts which allows water reabsorption to occur, with a lack of ADH little water will be reabsorbed in the collecting ducts and dilute urine will be produced. Anything that stimulates ADH secretion also stimulates thirst. A couple of factors that influence ADH secretion is the following.
Special receptors within the hypothalamus that are sensitive when plasma gets too concentrated and will stimulate ADH secretion. There are also stretch receptors found in the atria of the heart that can be activated by a larger than normal volume of blood returning from the heart to the veins, this will inhibit ADH secretion as the body will have excess fluid it wants to get rid of. Stretch receptors are also present in the aorta and carotid arteries and are stimulated when blood pressure begins to fall, it stimulates ADH secretion because your body needs to maintain enough volume in order to generate the blood pressure necessary to maintain a normal blood pressure which is able to effectively deliver blood to the tissues.
Total volume is not the only thing tightly controlled, osmolarity (amount of solute per unit volume) is also regulated tightly. Variation in osmolarity will cause cells to shrink or swell thereby damaging or even destroying the cellular structure and disrupt normal cellular function. In order to control this your body balances the intake and excretion of sodium with that of water as sodium is the major solute in extracellular fluids effectively determining the osmolarity of extracellular fluids. Osmolarity must be integrated with regulation of volume as changes in water volume will have a diluting or concentrating effect on the body fluids. When you become dehydrated you lose more water than solute and the osmolarity of your bodily fluids increases as a result, your body wishes to conserve water and not sodium which causes the rise in osmolarity. If you however lose a lot of blood from trauma your losses of sodium and water are going to be proportionate to the composition of bodily fluids and therefore your body will need to conserve both water and sodium as it is necessary.
ADH also plays a role in reducing sodium concentration by increasing the amount of water reabsorption in the kidneys helping the body to dilute bodily fluids. The kidneys have a regulated mechanism responsible for reabsorbing sodium in the distal nephron just in case osmolarity decreases below normal. The mechanism is controlled by the use of aldosterone which is a steroid hormone produced in the adrenal cortex. Firstly the adrenal cortex senses plasma osmolarity because when osmolarity is above normal aldosterone secretion is stopped and the lack of aldosterone will cause sodium to be reabsorbed in the distal tubule. ADH secretion increases alongside this allowing the body to conserve water providing a complementary effect to the low aldosterone levels in order to decrease the osmolarity of body fluids. The overall effect on urine excretion is a decrease in the amount of urine excreted as well as an increase in the osmolarity of said urine.
Additionally the kidneys are able to sense low blood pressure, which will cause lower filtration rates and lower flow through the tubule, triggering a response to raise blood pressure and conserve volume. Juxtaglomerular cells located in the afferent and efferent arterioles will produce renin which is a peptide hormone that initiates a hormonal response which ultimately leads to the production of angiotensin II which will stimulate the adrenal cortex and produce aldosterone.
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