Concepts of Human Anatomy & Physiology   5/e   Van De Graaff/Fox
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I. Urinary System and Kidney Structure

Concept: Each kidney contains many tiny tubules that empty into a cavity drained by the ureter. Each of the tubules receives a blood filtrate from a capillary bed called the glomerulus. The filtrate is similar to tissue fluid but is modified as it passes through the tubules and is thereby changed into urine. The tubules and associated blood vessels thus form the functional units of the kidneys, which are known as nephrons.

II. Glomerular Filtration

Concept: The glomerular capillaries have large pores in their walls, and the visceral layer of the glomerular capsule, in contact with the glomerulus, has filtration slits. Water, together with dissolved solutes (but not proteins), can thus pass from the blood plasma to the inside of the capsule and the nephron tubules. The volume of this filtrate produced by both kidneys each minute is called the glomerular filtration rate (GFR).

III. Reabsorption of Salt and Water

Concept: Most of the salt and water filtered from the blood is returned to the blood across the wall of the proximal convoluted tubule. The reabsorption of water occurs by osmosis, in which water follows the transport of NaCl from the tubule into the surrounding peritubular capillaries. Most of the water remaining in the filtrate is reabsorbed across the wall of the collecting duct in the renal medulla. This occurs as a result of the high osmotic pressure of the surrounding tissue fluid, which is produced by transport processes in the nephron loop.

IV. Renal Plasma Clearance

Concept: As blood passes through the kidneys, some of the constituents of the plasma are removed and excreted in the urine. The blood is thus "cleared" of particular solutes in the process of urine formation. These solutes may be removed from the blood by filtration through the glomeruli or by secretion by the tubular cells into the filtrate. At the same time, certain molecules in the tubular fluid can be reabsorbed back into the blood.

V. Renal Control of Electrolyte and Acid-Base Balance

Concept: The kidneys regulate the blood concentration of Na+, K+, HCO3-, and H. Aldosterone stimulates the reabsorption of Na+ in exchange for K+ in the tubule. Aldosterone thus promotes the renal retention of Na+ and the excretion of K+. Secretion of aldosterone from the adrenal cortex is stimulated directly by a high blood K+ concentration and indirectly by a low Na+ concentration via the renin-angiotensin system.

VI. Ureters, Urinary Bladder, and Urethra

Concept: Urine is channeled from the kidneys to the urinary bladder by the ureters and expelled from the body through the urethra. The mucosa of the urinary bladder permits distension, and the muscles of the urinary bladder and urethra are used in the control of micturition.

VII. Development of the Urinary System

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