Urine Concentration
The kidneys can dilute or concentrate urine without altering the amounts of solutes reabsorbed or excreted. Kidneys can eliminate excess water in the body even by diluting urine to the lowest limit of 50 mOsm/L.
At the same time, if the body faces acute dehydration, kidneys can conserve water by increasing the osmolarity of urine excreted to about 1200-1400 mOsm/L in humans. In these two conditions, extracellular fluid osmolarity is maintained at the normal level of 300 mOsm/L. A human being of 70 kg body weight must excrete a minimum 600 mOsm of solutes every day. At the maximum osmotic concentration of 1200 mOsm/L, the obligatory minimum quantity of urine that must be produced is 600 mOsm/1200 mOsm/L = 0.5 L. A desert animal, an Australian hopping mouse, can concentrate urine up to 10,000 mOsm/L, whereas an aquatic animal beaver can concentrate only up to 500 mOsm/L and has the minimum urineconcentrating capacity.The capacity of the kidney to concentrate urine is attributed to the special arrangement of juxtamedullary nephrons with their loops of Henle dipping deep into the hyperosmotic medullary interstitium. Two mechanisms are involved in the process of urine concentration. One creates hypertonicity of the medulla by the countercurrent multiplier system, and the other maintains it by the countercurrent exchange system.
Fig. 9.8 Countercurrent multiplier system. The tubular fluid, when it enters the descending limb, becomes progressively more concentrated due to loss of water. But the ascending limb pumps out Na+, K+ and Cl- ions, and the filtrate becomes hypo-osmotic. The water removed from the tubule enters the vasa recta
9.5.1 Countercurrent Multiplier System
The descending loop of Henle carries the tubular fluid downward from the cortex to the medulla.
The tubule’s U-shaped arrangement enables the fluid to flow in opposite directions (countercurrent) in the two tubules (Fig. 9.8). In the thick ascending limb of the loop of Henle, sodium is actively transported from the tubular fluid to the peritubular space. Although chloride, potassium and other ions are co-transported with sodium, the thick ascending limb is impermeable to water. Passive reabsorption of sodium and chloride ions also takes place from the thin ascending limb of the loop of Henle. The descending limb is permeable to the water simultaneously and impermeable to solutes. Since the medullary interstitium gets more and more concentrated with the sodium chloride diffused from the ascending limb, there is increased osmotic outflow of water from the descending limb as it dips deep into the medulla. More sodium chloride will be added to the descending limb from the proximal tubule as a continuous process. The newly arrived sodium chloride gets added to the already existing sodium chloride in the interstitium, thus multiplying the osmotic concentration of the medullary interstitium. Hence, the physiological processes involved in the multiplication of medullary interstitial hypertonicity with specialised transport mechanisms of solutes and water when the countercurrent flow of tubular fluid occurs through the loop of Henle are known as countercurrent multiplier system.The fluid leaving the loop of Henle has an osmolarity of 100 mOsm/L, which is about one-third of plasma osmolarity. Suppose further reabsorption of water is not taking place under the influence of ADH. In that case, the osmolarity can even become 50 mOsm/L since additional reabsorption of solutes occurs from the distal tubule and collecting ducts. The plasma concentration of ADH influences water reabsorption from the late distal tubules, collecting tubules and ducts. ADH-dependent water reabsorption is more pronounced in the cortical collecting tubules than in the collecting ducts. The limited absorption of water in the medullary collecting duct also helps to maintain hypertonic medullary interstitium.
9.5.2 Countercurrent Exchange System
Countercurrent exchange system is a U-shaped blood vessel (vasa recta) that parallels Henle’s loop. Blood flow through the vasa recta constitutes 5% of total renal blood flow, meeting the metabolic demands of the interstitium, at the same time preserving the hypertonicity of the renal medulla. The colloid osmotic pressure of plasma proteins and hydrostatic pressure of blood flowing through these capillaries are favoured by reabsorption. As the descending limb dips deep into the medulla, it loses water and solutes, increasing its tonicity. When the blood in the ascending limb flows in the opposite direction, it gains water and loses solutes, gradually decreasing tonicity.
9.6