Basic renal processes

The nephron performs three basic renal processes (see Figure 19.2):

• Filtration

• Reabsorption

Filtration is the movement of fluid and solutes from the glomerular capillaries into Bowman's capsule. Filtration is a nonselective process, so everything in the plasma except for the plasma proteins is filtered. Approximately 20% of the plasma is filtered as it passes through the glomerulus. On average, this results in a glomerular filtration rate (GFR) of 125 ml/min or 180 l of filtrate per day. Reabsorption is the movement of filtered substances from the renal tubule into the peritubular capillaries for return to the vascular compartment. This process takes place throughout the tubule. Approximately 178.5 l of filtrate are reabsorbed, resulting in an average urine output of 1.5 l per day. Secretion is the movement of selected unfiltered substances from the peritubular capillaries into the renal tubule for excretion. Any substance that is filtered or secreted, but not reabsorbed, is excreted in the urine.

The maintenance of plasma volume and plasma osmolarity occurs through regulation of the renal excretion of sodium, chloride, and water. Each of these substances is freely filtered from the glomerulus and reabsorbed from the tubule; none is secreted. Because salt and water intake in the diet may vary widely, the renal excretion of these substances is also highly variable. In other words, the kidneys must be able to produce a wide range of urine concentrations and urine volumes. The most dilute urine produced by humans is 65 to 70 mOsm/l and the most concentrated the urine can be is 1200 mOsm/l (recall that the plasma osmolarity is 290 mOsm/l). The volume of urine produced per day depends largely upon fluid intake. As fluid intake increases, urine output increases to excrete the excess water. Conversely, as fluid intake decreases or as an individual becomes dehydrated, urine output decreases in order to conserve water.

On average, 500 mOsm of waste products must be excreted in the urine per day. The minimum volume of water in which these solutes can be dissolved is determined by the ability of the kidney to produce a maximally concentrated urine of 1200 mOsm/l:

1200 mOsm/L y

This volume, referred to as obligatory water loss, is 420 ml water/day. In other words, 420 ml of water will be lost in the urine each day in order to excrete metabolic waste products regardless of water intake.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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