The introduced amount of water must be equal to the excreted amount of water excreted to maintain homeostasis. Alterations in water balance cause hypo- or hypernatremia. On average, an adult introduces about 2 liters of water daily through food and produces about 0.5 liters through cellular respiration (glucose + O₂ ↔ CO₂ + H₂O), resulting in a total water daily intake of about 2.5 L. Under physiological conditions, there is an obligatory loss of water of about 1.5 L through the urinary tract; a small quantity, about 0.1 L, is lost through excretion, and, finally, a quota, corresponding to about 0.9 L, is eliminated by perspiratio insensibilis, which consists of skin and respiratory tract evaporation (Fig. 1). In this way, the balance between the volume of incoming and outcoming water is maintained. Basically, body water volume is controlled by its intake, through the stimulus of thirst, and by its excretion at the renal level.

Fig1. Water balance. (Copyright EDISES 2021. Reproduced with permission)

If total body Na⁺ is kept constant, a change in total body water alters osmolality. In other words, if there is an osmolality increase, the body’s response is to improve water intake through the thirst sensation and the diuresis contraction. Viceversa, if there is an osmolality reduction, the organism determines a reduction in the thirst sense and a diuresis increase. The osmolality alteration is detected by osmoreceptors in the hypothalamus. In case of increased plasma osmolality, even by 12%, the osmoreceptors stimulate vasopressin synthesis, also known as the antidiuretic hormone (ADH), in supraoptic and paraventricular nuclei of the hypo thalamus. Then, it is secreted by the posterior pituitary gland. The ADH exerts its action at the collector duct, making it permeable to water. Following the interaction of ADH and its receptor on the cellular membrane of the collector duct, the activation of a sequence of events leads to the insertion in the apical cellular membrane of channel proteins, known as aquaporins, allowing the water passage. The effect is the passive reabsorption of water according to an osmotic gradient, from the lumen of the collecting tubule to the hypertonic medullary interstitium, and the production of concentrated urine. The ADH also acts on the ascending tract of Henle’s loop, increasing sodium transport into the medullary interstitium. The resulting increase in interstitial osmolality contribute to the osmotic gradient for water reabsorption. In the absence of the ADH, the collector duct is impermeable to water. The most powerful stimulus for ADH release is the increase of plasma osmolality; however, other non-osmotic factors regulate ADH secretion, including blood pressure changes and volemia, nausea, pain, stress, hypoglycemia, pregnancy, and various medications. Reductions in blood volume and blood pressure are detected, respectively, by voloreceptors, stretch receptors located in the cardiac atria, and baroreceptors in the carotid sinus, which stimulate hypothalamic ADH synthesis and release (Fig. 20.3). While the osmoreceptors detect even small changes in plasma osmolality (1%), the voloreceptors and baroreceptors are less sensitive, detecting a reduction of no less than 10%.

Fig2. Factors influencing ADH release. (Copyright EDISES 2021. Reproduced with permission)

اخر الاخبار

اخبار العتبة العباسية المقدسة

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المزيد

الآخبار الصحية

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هذه الأطعمة تحمي دماغك من الشيخوخة وتزيد حدة الذكاء

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المزيد

الآخبار التكنلوجية

الأرصاد الجوية تلجأ إلى الذكاء الاصطناعي.. لأي غرض؟

في منزلك.. "تعديل بسيط" يخفض ضغط الدم بشكل طبيعي

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غوغل في مأزق.. بيانات 2.5 مليار مستخدم بقبضة القراصنة

المزيد

مواضيع ذات صلة

Albuminuria

Proteinuria

rheumatoid factor (RF)

reticulocyte count (Retic count)

red blood cell indices (RBC indices, Blood indices)

NGAL

Biomarkers of Tubulo-Interstitial Injury

Low-Molecular-Weight Plasma Proteins

Immunofluorescence Assay (IFA)

Enzyme-Linked Fluorescent Assay (ELFA)

Diagnostic Evaluation of Lyme disease

red blood cell count (RBC count, Erythrocyte count)