This patient is suffering from heat-induced dehydration from a biking excursion

This patient is suffering from heat-induced dehydration from a biking excursion. Since she did not have water for several hours, her kidneys began conserving water to maintain her blood pressure and other bodily functions. This was done by secreting antidiuretic hormone (ADH) which acts on the distal collecting tubule and collecting duct.

Dehydration is defined as a loss of body water, with a net loss of water. Clinical symptoms of dehydration include tachycardia, dry mucous membranes, poor skin turgor, soft/sunken eyes, decreased level of consciousness, decreased urine output, orthostatic hypotension, weight loss, decreased jugular venous pressure, increased capillary refill time, lack of tears, thirst, and weakness. Water balance is monitored by the pressure impacting on the high-pressure arterial baroreceptors located in the aortic arch and carotid sinus and the flow of blood to the renal arteries. When the baroreceptors are activated by decreased blood volume, signals are sent to the medulla to increase the sympathetic tone, leading to the release of catecholamines. The release of catecholamines cause vasoconstriction of peripheral resistance arterioles (increasing diastolic blood pressure), venoconstriction (increasing venous return to the heart), an increase in heart rate (chronotropic effect), and an increase in cardiac contractility (inotropic effect). Signals are also sent to the supraoptic and paraventricular nuclei in the hypothalamus to synthesize and release ADH (vasopressin) from nerve endings located in the posterior pituitary. ADH enhances the reabsorption of free water (without electrolytes) from the collecting tubules in the kidneys and is also a potent vasoconstrictor of the peripheral resistance arterioles. ADH acts on the V2 receptor on the basal side of the principal cells and causes insertion of AQP2 water channels in the apical cell membrane of the principal cells of the collecting duct.

Water reabsorption takes place in several areas of the nephron; however it is usually done passively due to the movement of ions into and out of the nephron. Only in the distal tubule and collecting duct does ADH play a role in water reabsorption to maintain fluid balance. Thus, the other answer options are incorrect. The proximal convoluted tubule is responsible for isosmotic reabsorption of solute and water. The thin descending loop of Henle is responsible for passive reabsorption of water via medullary hypertonicity, and is not responsible for this patient’s presentation. The thick ascending loop of Henle is the primary site of free water generation via the Na+/K+/Cl- symporter. This segment absorbs electrolytes, but does not reabsorb water. The early distal convoluted tubule is responsible for reabsorption of sodium, chloride, and calcium via Na+/Cl- symporter without water absorption.

The other answers are incorrect:

B: The early distal convoluted tubule is responsible for reabsorption of sodium, chloride, and calcium via Na+/Cl- symporter without water absorption; thus, it would not be responsible for this patient’s presentation.

C: The proximal convoluted tubule is responsible for isosmotic reabsorption of solute and water, which would not be responsible for this patient’s presentation.

D: The thick ascending loop of Henle is the primary site of free water generation via the Na+/K+/Cl- symporter. This segment absorbs electrolytes, but does not reabsorb water; thus, it would not be responsible for this patient’s presentation.

E: The thin descending loop of Henle is responsible for passive reabsorption of water via medullary hypertonicity, and would not be responsible for this patient’s presentation.

Bottom Line: Dehydration is the loss of total body water and conservation is performed by the action of antidiuretic hormone (ADH) on the distal collecting tubule of the nephron. Symptoms of dehydration include: tachycardia, dry mucous membranes, and poor skin turgor, among others.