When there is a large excess of water in the body, the kidney can excrete as much as 20 L/day of dilute urine, with a concentration as low as 50 mOsm/L. The kidney performs this impressive feat by continuing to reabsorb solutes while failing to reabsorb large amounts of water in the distal parts of the nephron, including the late distal tubule and the collecting ducts.

Figure 1 shows the approximate renal responses in a human after ingestion of 1 liter of water. Note that urine volume increases to about six times normal within 45 minutes after the water has been drunk. However, the total amount of solute excreted remains relatively constant because the urine formed becomes dilute and urine osmolarity decreases from 600 to about 100 mOsm/L. Thus, after ingestion of excess water, the kidney rids the body of the excess water but does not excrete excess amounts of solutes.

Fig1. Water diuresis in a human after ingestion of 1 liter of  water. Note that after water ingestion, urine volume increases and  urine osmolarity decreases, causing excretion of a large volume of  dilute urine; however, the total amount of solute excreted by the  kidneys remains relatively constant. These responses of the kidneys  prevent plasma osmolarity from decreasing markedly during excess  water ingestion. 

When the glomerular filtrate is initially formed, its osmolarity is about the same as that of plasma (300 mOsm/L). To excrete excess water, it is necessary to dilute the filtrate as it passes along the tubule. This dilution is achieved by reabsorbing solutes to a greater extent than water, as shown in Figure 2, but this occurs only in certain segments of the tubular system, as described in the following sections.

Fig2. Formation of dilute urine when antidiuretic hormone  (ADH) levels are very low. Note that in the ascending loop of Henle,  the tubular fluid becomes very dilute. In the distal tubules and collecting tubules, the tubular fluid is further diluted by the reabsorption  of sodium chloride and the failure to reabsorb water when ADH levels  are very low. The failure to reabsorb water and continued reabsorption of solutes lead to a large volume of dilute urine. (Numerical  values are in milliosmoles per liter.) 

Tubular Fluid Remains Isosmotic in the Proximal Tubule. As fluid flows through the proximal tubule, solutes and water are reabsorbed in equal proportions, so little change in osmolarity occurs; thus, the proximal tubule fluid remains isosmotic to the plasma, with an osmolarity of about 300 mOsm/L. As fluid passes down the descending loop of Henle, water is reabsorbed by osmosis and the tubular fluid reaches equilibrium with the surrounding interstitial fluid of the renal medulla, which is very hypertonic—about two to four times the osmolarity of the original glomerular filtrate. Therefore, the tubular fluid becomes more concentrated as it flows into the inner medulla.

Tubular Fluid Is Diluted in the Ascending Loop of Henle. In the ascending limb of the loop of Henle, especially in the thick segment, sodium, potassium, and chloride are avidly reabsorbed. However, this portion of the tubular segment is impermeable to water, even in the presence of large amounts of ADH. Therefore, the tubular fluid becomes more dilute as it flows up the ascending loop of Henle into the early distal tubule, with the osmolarity decreasing progressively to about 100 mOsm/L by the time the fluid enters the early distal tubular segment. Thus, regardless of whether ADH is present or absent, fluid leaving the early distal tubular segment is hypo-osmotic, with an osmolarity of only about one third the osmolarity of plasma.

Tubular Fluid in Distal and Collecting Tubules Is Further Diluted in the Absence of ADH. As the dilute fluid in the early distal tubule passes into the late distal convoluted tubule, cortical collecting duct, and collecting duct, there is additional reabsorption of sodium chloride. In the absence of ADH, this portion of the tubule is also impermeable to water, and the additional reabsorption of solutes causes the tubular fluid to become even more dilute, decreasing its osmolarity to as low as 50 mOsm/L. The failure to reabsorb water and the continued reabsorption of solutes lead to a large volume of dilute urine.

To summarize, the mechanism for forming dilute urine is to continue reabsorbing solutes from the distal segments of the tubular system while failing to reabsorb water. In healthy kidneys, fluid leaving the ascending loop of Henle and early distal tubule is always dilute, regardless of the level of ADH. In the absence of ADH, the urine is further diluted in the late distal tubule and collecting ducts and a large volume of dilute urine is excreted.

اخر الاخبار

اخبار العتبة العباسية المقدسة

شركة براق القمّة تعرض نماذج صناعتها من الشاحنات في مهرجان عين الحياة الرابع

المجمع العلمي يطلق برنامج (القرآني الصغير) ضمن فعاليات مهرجان عين الحياة الرابع

مجموعة مشاتل الكفيل تعرض تشكيلة متكاملة من النباتات الظلية والموسمية في مهرجان عين الحياة الرابع

قسم الشؤون الدينية يختتم الدورة 57 لتأهيل منتسبي العتبة العباسية المقدسة الجدد

المزيد

الآخبار الصحية

عوامل تضعف القدرات الإدراكية لدى الإنسان

طريقة جديدة لتجديد خلايا الدماغ

دراسة بريطانية: الأطعمة النباتية عالية الجودة تقلل خطر فشل القلب

7 أطعمة يومية "تسمم" جسمك بالبلاستيك دون أن تشعر

المزيد

الآخبار التكنلوجية

اكتشاف أثري في أوريغون الأمريكية قد يعيد كتابة تاريخ البشرية

اكتشاف كائن بحري مدرع بلسان "حديدي" في بحر اليابان

نظام كوكبي "مقلوب" يحير علماء الفلك

بسبب الذكاء الاصطناعي.. هذه الوظائف "ستختفي" بعد أشهر قليلة ؟!

المزيد

مواضيع ذات صلة

Osmoreceptor-ADH Feedback System

Urine Specific Gravity

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

أمراض الجهاز البولي

آلية التبول