Question

Part 1: Describe the amount and distribution of body water in the intracellular and extracellular compartments. Describe a cell's characteristics under hypertonic and hypotonic conditions.

Part 2: BE SURE TO ANSWER BOTH PARTS! (A) Describe the rate of water reabsorption/secretion in the collecting ducts of a person with water deprivation, as compared with a person who ingests 3 liters of water per day. (B). Schwartz-Bartter syndrome is characterized by excessive release of ADH. This will have what effects on ECF and ICF volumes?

Answer 1

Part 1:

The water in the body is contained within the numerous organs and tissues of the body. These innumerable fluids can be lumped together into larger collections which can be discussed in a physiologically meaningful way. These collections are referred to as "compartments"". The major division is into Intracellular Fluid (ICF: about 23 liters) and Extracellular Fluid (ECF: about 19 liters) based on which side of the cell membrane the fluid lies. Typical values for the size of the fluid compartments are listed in the table.

Body Fluid Compartments (70 kg male)

% of Body Weight % of Total Body Water Volume (Litres) ECF 27 45 19 Plasma 4.5 7.5 3.2 ISF 12.0 20.0 8.4 Dense CT water 4.5 7.5 3.2 Bone water 4.5 7.5 3.2 Transcellular 1.5 2.5 1.0 ICF 33 55 23 TBW 60% 100% 42 liters

The ability of an extracellular solution to make water move into or out of a cell by osmosis is know as its tonicity. A solution's tonicity is related to its osmolarity, which is the total concentration of all solutes in the solution. A solution with low osmolarity has fewer solute particles per liter of solution, while a solution with high osmolarity has more solute particles per liter of solution. When solutions of different osmolarities are separated by a membrane permeable to water, but not to solute, water will move from the side with lower osmolarity to the side with higher osmolarity.

Three terms—hypotonic, isotonic, and hypertonic—are used to compare the osmolarity of a cell to the osmolarity of the extracellular fluid around it.

Note: When we use these terms, we are considering only solutes that cannot cross the membrane.

• If the extracellular fluid has lower osmolarity than the fluid inside the cell, it’s said to be hypotonic—hypo means less than—to the cell, and the net flow of water will be into the cell.

• In the reverse case, if the extracellular fluid has a higher osmolarity than the cell’s cytoplasm, it’s said to be hypertonic—hyper means greater than—to the cell, and water will move out of the cell to the region of higher solute concentration.

• In an isotonic solution—iso means the same—the extracellular fluid has the same osmolarity as the cell, and there will be no net movement of water into or out of the cell.

Part 2.

A :- The body loses water primarily by excreting it in urine from the kidneys. Depending on the body's needs, the kidneys may excrete less than a pint or up to several gallons of urine a day. ... Usually, people can drink enough fluids to compensate for excess water loss.

This test has been used as an aid in the diagnosis of polyuric disorders.1 Individuals with diabetes insipidus (DI), due to either hypothalamic (HDI) or nephrogenic disease (NDI), cannot conserve free water. When these patients are deprived of water for extended periods they will lose weight due to dehydration to a greater extent than healthy controls. This water loss is accompanied by hypernatremia and an increase in plasma osmolality. Normal individuals and patients with psychogenic polydipsia will not lose >3% of their body weight during the water deprivation test and will maintain serum sodium and osmolality levels within normal limits. The urine osmolality of patients with DI remains low, often less than that of plasma whereas subjects that do not have DI will produce concentrated urine (osmolality typically between 300 and 400 mOsm/kg). Some patients with psychogenic polydipsia fail to produce concentrated urine unless the deprivation is prolonged. Patients with NDI show high ADH levels as the plasma osmolality exceeds 300 mOsm/kg while patients with HDI have low or normal levels.

Administration of ADH in the second phase of the test can be used to identify the cause of DI. ADH administration to patients with HDI will cause an increase in urine osmolality >10% and will fail to produce this level of urine concentration in patients with NDI.1

B. The syndrome of inappropriate antidiuretic hormone secretion or SIADH (other names: Schwartz-Bartter syndrome, SIAD—syndrome of immoderate antidiuresis) is characterized by excessive release of antidiuretic hormone from the posterior pituitary gland or another source. The result is often dilutional hyponatremia in which the sodium remains normal but total body fluid increases. It was originally described in people with small-cell carcinoma of the lung, but it can be caused by a number of underlying medical conditions. The treatment may consist of fluid intake restriction, various medicines, and management of the underlying cause. SIADH was first described in 1957.

SIADA affectes on ECF & ICF:

Excessive ADH causes an inappropriate increase in the reabsorption in the kidneys of solute-free water ("free water"): excess water moves from the distal convoluted tubules (DCT)s and collecting tubules of the nephrons - via activation of aquaporins, the site of the ADH receptors - back into the circulation. This has two consequences. First, in the extracellular fluid (ECF) space, there is a dilution of blood solutes, causing hypoosmolality, including a low sodium concentration - hyponatremia. [There is no expansion of the ECF volume because as it attempts to expand, aldosterone is suppressed and atrial natriuretic peptide (ANP) is stimulated: both of these hormones cause isotonic ECF fluid to be excreted by the kidneys sufficient to keep ECF volume at a normal level.] Also, virtually simultaneously to these ECF events, the intracellular space (ICF) volume expands. This is because the osmolality of the ECF is (transiently) less than that of the ICF; and since water is readily permeable to cell membranes, solute-free water moves from the ECF to the ICF compartment by osmosis: all cells swell. Swelling of brain cells - cerebral edema - causes various neurological abnormalities which in acute and/or severe cases can result in convulsions, coma, and death.