Extracellular fluid volume is determined mainly by the balance between intake and output of water and salt. In many instances, salt and fluid intakes are dictated by a person’s habits rather than by physiological control mechanisms. Therefore, the burden of extracellular volume regulation is often placed on the kidneys, which must adapt their excretion of salt and water to match intake of salt and water under steady-state conditions.

In discussing the regulation of extracellular fluid volume, we consider the factors that regulate the amount of sodium chloride in the extracellular fluid because changes in extracellular fluid sodium chloride content usually cause parallel changes in extracellular fluid volume, provided the antidiuretic hormone (ADH)-thirst mechanisms are operative. When the ADH-thirst mechanisms are functioning normally, a change in the amount of sodium chloride in the extracellular fluid is matched by a similar change in the amount of extracellular water, and thus maintenance of osmolality and sodium concentration is relatively constant.

SODIUM INTAKE AND EXCRETION ARE BALANCED UNDER STEADY-STATE CONDITIONS

An important consideration in overall control of sodium excretion—or excretion of most electrolytes, for that matter—is that under steady-state conditions, excretion by the kidneys is determined by intake. To maintain life, a person must, over the long term, excrete almost precisely the amount of sodium ingested. Therefore, even with disturbances that cause major changes in kidney function, balance between intake and output of sodium usually is restored within a few days.

If disturbances of kidney function are not too severe, sodium balance may be achieved mainly by intrarenal adjustments with minimal changes in extracellular fluid volume or other systemic adjustments. However, when perturbations to the kidneys are severe and intrarenal compensations are exhausted, systemic adjustments must be invoked, such as changes in blood pressure, changes in circulating hormones, and alterations of sympathetic nervous system activity.

These adjustments can be costly in terms of overall homeostasis because they cause other changes throughout the body that may, in the long run, be damaging. For example, impaired kidney function may lead to increased blood pressure that, in turn, helps to maintain normal sodium excretion. Over the long term the high blood pressure may cause injury to the blood vessels, heart, and other organs. These compensations, however, are necessary because a sustained imbalance between fluid and electrolyte intake and excretion would quickly lead to accumulation or loss of electrolytes and fluid, causing cardiovascular collapse within a few days. Thus, one can view the systemic adjustments that occur in response to abnormalities of kidney function as a necessary trade-off that brings electrolyte and fluid excretion back in balance with intake.

SODIUM EXCRETION IS CONTROLLED BY ALTERING GLOMERULAR FILTRATION OR TUBULAR SODIUM REABSORPTION RATES

The two variables that influence sodium and water excretion are the rates of glomerular filtration and tubular reabsorption:

Excretion = Glomerular filtration − Tubular reabsorption

GFR normally is about 180 L/day, tubular reabsorption is 178.5 L/day, and urine excretion is 1.5 L/day. Thus, small changes in GFR or tubular reabsorption potentially can cause large changes in renal excretion. For example, a 5 percent increase in GFR (to 189 L/day) would cause a 9 L/day increase in urine volume, if tubular compensations did not occur; this increase would quickly cause catastrophic changes in body fluid volumes. Similarly, small changes in tubular reabsorption, in the absence of compensatory adjustments of GFR, would also lead to dramatic changes in urine volume and sodium excretion. Tubular reabsorption and GFR usually are regulated precisely, so excretion by the kidneys can be exactly matched to intake of water and electrolytes.

Even with disturbances that alter GFR or tubular reabsorption, changes in urinary excretion are minimized by various buffering mechanisms. For example, if the kidneys become greatly vasodilated and GFR increases (as can occur with certain drugs or high fever), this condition raises sodium chloride delivery to the tubules, which in turn leads to at least two intrarenal compensations: (1) increased tubular reabsorption of much of the extra sodium chloride filtered, called glomerulotubular balance, and (2) macula densa feedback, in which increased sodium chloride delivery to the distal tubule causes afferent arteriolar constriction and return of GFR toward normal. Likewise, abnormalities of tubular reabsorption in the proximal tubule or loop of Henle are partially compensated for by these same intrarenal feedbacks, as dis cussed in Chapter 27.

Because neither of these two mechanisms operates perfectly to restore distal sodium chloride delivery all the way back to normal, changes in either GFR or tubular reabsorption can lead to significant changes in urine sodium and water excretion. When this occurs, other feedback mechanisms may come into play, such as changes in blood pressure and changes in various hormones, and they eventually return sodium excretion to equal sodium intake. In the next few sections, we review how these mechanisms operate together to control sodium and water balance and in so doing also act to control extracellular fluid volume. All these feedback mechanisms control renal excretion of sodium and water by altering either GFR or tubular reabsorption.

اخر الاخبار

اخبار العتبة العباسية المقدسة

العتبة العباسية المقدسة تشهد تشييع جثمان المحقق الشيخ محمد رضا المامقاني

قسم الشؤون الفكرية يختتم برنامج (فتية سيد الماء) لرعاية الأيتام بنسخته الثانية

لتطوير المهارات الإدارية.. العتبة العباسية المقدسة تطلق الدورة التعليمية الأولى لمسؤولي وحداتها

قسم التطوير يقيم دورة تدريبية عن الظهور الإعلامي لملاكاته

المزيد

الآخبار الصحية

دراسة تكشف "السبب الخفي" وراء ترك التمارين الرياضية

كشف التوقيت "المثالي" لتغيير فرشاة الأسنان

بلاستيك في أنسجة بشرية.. دراسة تكشف مفاجأة طبية

دراسة تحذر من الإفراط في ممارسة الرياضة.. كيف يضر دماءك؟

المزيد

الآخبار التكنلوجية

"ديب سيك" تحجب أحدث نماذجها عن شركات الرقائق الأميركية

"سامسونغ" تطلق سلسلة هواتف "غالاكسي إس 26"

حملات سرية ورسائل تشويه.. كيف يتم استغلال "تشات جي بي تي"؟

أثر نفسي غير متوقع للرياضة.. مرتبط بالغضب

المزيد

مواضيع ذات صلة

Importance of Pressure Natriuresis and Pressure Diuresis in Maintaining Body Sodium and Fluid Balance

Osmoreceptor-ADH Feedback System

Urine Specific Gravity

Renal Mechanisms for Excreting Dilute Urine

Physiological Control of Glomerular Filtration and Renal Blood Flow

Renal Blood Flow

Determinants of The GFR

Glomerular Filtration—The First Step in Urine Formation

Urine Formation Results From Glomerular Filtration, Tubular Reabsorption, and Tubular Secretion

Multiple Functions of The Kidneys

التنظيم الحمضي – القاعدي في الجسم Acid-Base Balance

حصى الكلية Urinary Caliculi