Water Balance

Body water is divided into two main [1]: about 65% is [2] and the rest is [3]. Water movement from one compartment to another is influenced mainly by the osmolarity that results from dissolved electrolytes, especially [4] salts in the 2 and [5] salts in the 3. The body's daily water budget comes from two sources: water ingested in the food and drink and [6] water produced by the body's chemical reactions. Water is lost not only through urine and feces, but also through sweat, cutaneous [7], and [8] air. Water loss that we are unaware of is called [9].

Dehydration raises the osmolarity of the blood, thereby stimulating a nucleus of the hypothalamus called the [10]. Sympathetic output from this nucleus reduces the secretion of [11], thus contributing to the sense of thirst. By moistening and cooling the mouth and filling the stomach, drinking quenches the thirst for a short time, but long-term satiety depends on a reduction of the [12] of the blood.

Fluid imbalances fall into four categories according to whether there is a deficiency or excess of total body water (TBW) and whether the fluid is of normal or abnormal osmolarity. In the state of [13], TBW is low but isotonic because proportional amounts of water and salt have been lost. This can result from hemorrhaging and severe burns. In a state of [14], TBW is low and hypertonic because the body has lost more water than salt. This occurs in cases of exposure where a person lacks drinking water. In [15], TBW is high and isotonic because the body has retained proportionate amounts of water and salt–for example, because of aldosterone hypersecretion. Finally, in [16], TBW is high and hypotonic because the body has lost salt but retained water, as when one sweats profusely and replaces this with plain drinking water. The hormone [17] can raise TBW and keep it isotonic because it promotes Na⁺ reabsorption and a proportionate amount of water follows the Na⁺ by osmosis. This hormone does not change the osmolarity of the ECF. [18] hormone, however, can reduce blood osmolarity because it promotes water reabsorption while Na⁺ is excreted in the urine. It works by stimulating cells of the renal collecting ducts to produce water channels called [19] in their plasma membranes.

Another form of fluid imbalance is [20], which does not involve an abnormality of TBW or osmolarity, but involves the abnormal accumulation of fluid somewhere in the body. The most common form of this is [21], when tissues become swollen with ECF. The various causes of 21 all come down to three major mechanisms: increased capillary [22], reduced capillary [23], or obstructed [24] drainage.

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Electrolyte Balance

The most abundant cation of the ECF, accounting for most of its osmolarity, is [25]. An abnormally low concentration of 25 in the blood, called [26], is rare because most diets include an excess of it and if 25 is retained or lost, water goes with it and keeps its concentration normal. An excess of 25, called [27], is more common. The amount of 25 in the body is regulated mainly by [28], which stimulates the distal convoluted tubules of the kidneys to reabsorb it.

The most abundant cation of the ICF is [29]. It plays a major role in cytoplasmic osmolarity and plasma membrane potentials. An excess of 29 in the blood, called [30], makes nerve and muscle cells hyperexcitable and may cause cardiac arrest; a deficiency, called [31], results in muscle weakness and sluggish reflexes because muscle and nerve cells become less excitable than normal. Aldosterone has opposite effects on this ion and [32], so the more 32 the body excretes or retains, the less 29, and vice versa.

The most abundant anion in the ECF is [33]. An excess is called [34] and a deficiency is called [35]. This ion follows 25 because of their opposite charges, so its concentration is regulated mainly as a side effect of 25 homeostasis. Parathyroid hormone regulates the homeostasis of [36] and [37] ions, both of which are involved in bone deposition.

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Acid-Base Balance

Blood and tissue fluid normally have a pH in the range of [38]. Because even small deviations from this range can be fatal, pH is carefully regulated by physiological and chemical [39]. The two physiological 39s are the [40] and [41]. The most concentrated chemical 39 in the ECF is the [42] buffer system, which serves effectively mainly because the respiratory system continually expels the CO₂ it generates. Gram for gram, the [43] buffer system is more effective than the foregoing, but it functions more in the ICF than in the ECF. The most effective buffer of all, however, is [44], which accounts for about three-quarters of the buffering capacity of both the ECF and ICF.

Although the urinary system is slower than others to react to disturbances of pH, it can neutralize more excess acid or base than any other buffer system. When there is an excessively low pH, called [45], the kidneys secrete more H⁺ ions into the tubular fluid. If the blood pH is too high–a state called [46]–the kidneys excrete [47] ions instead. When they secrete H⁺ into the renal tubules, they exchange it for [48] ions from the tubular fluid. Hydrogen ions in the tubular fluid normally neutralize all of its [49], so none of this ion appears in the urine. This ion cannot neutralize all the H⁺, however; two other buffers in the tubular fluid that neutralize additional acid are [50] and [51].

There are four categories of acid-base imbalance, named in the following four answers (two words each). If the respiratory system expels CO₂ faster than the body produces it, [52] will occur, whereas if CO₂ is produced faster than the lungs expel it, the result will be [53]. A state of [54] occurs if the body generates excess organic acids, as in alcoholism and diabetes mellitus. Chronic vomiting can produce a state of [55] by expelling stomach acid from the body. Such imbalances are described as [56] if the body's buffer systems are able to restore normal pH.

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