Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Chapter 38 Structure and Function of the Kidney

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Structure and Location of the Kidneys Structure –Paired, bean-shaped organs –Multilobular structure, composed of up to 18 lobes –Each lobule is composed of nephrons, the functional units of the kidney. Location –Outside the peritoneal cavity in the back of the upper abdomen –One on each side of the vertebral column at the level of the 12th thoracic to 3rd lumbar vertebrae

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Function of the Nephron Glomerular filtration –Three layers of filtration Reabsorb water, electrolytes, and other substances needed to maintain the constancy of the internal environment into the bloodstream Secrete other unneeded materials into the tubular filtrate for elimination

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Capillary Systems Supplying the Nephron Glomerulus Capillary System –A unique, high-pressure capillary filtration system located between the afferent and the efferent arterioles –Selectively dilates or constricts to regulate glomerular capillary pressure Peritubular Capillary System –A low-pressure reabsorptive system that originates from the efferent arteriole

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Four Segments of the Nephron Tubule Proximal Convoluted Tubule –A highly coiled segment; drains into the Bowman capsule Loop of Henle –A thin looped structure Distal Convoluted Tubule –A distal coiled portion Collecting Tubule –Joins with several tubules to collect the filtrate

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Tubular Filtration The urine filtrate flows through the tubular component of the nephron. As the filtrate flows through the tubule, the concentration of water and electrolytes in the filtrate changes due to –Reabsorption of water and solutes by tubular cells into the peritubular capillary blood –Secretion from the blood into the tubular lumen

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Na + Transport As the Driving Force Na+/K+ ATPase maintains the concentration gradient. Symport Antiport Exchange

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Question Which of the following best describes the function of the nephron? –A. The detoxification of blood –B. The retention of important organic materials found in the filtrate –C. The resorption of water, electrolytes, and other substances from the bloodstream –D. The synthesis and storage of urine

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Answer C. The resorption of water, electrolytes, and other substances from the bloodstream Rationale: The resorption of water, electrolytes, and other substances from the bloodstream is the main function of the nephrons.

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Regulation of Urine Concentration Response to changes in interstitial osmolarity –1200 mOsm normal ADH changes the permeability of the tubules of the collecting duct.

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Elimination Functions of the Kidney Renal clearance Regulation of sodium Potassium elimination pH-dependent elimination of organic ions Uric acid elimination Urea elimination Drug elimination

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins The Juxtaglomerular Complex The juxtaglomerular complex is thought to represent a feedback control system that links changes in the GFR with renal blood flow. Because of its location between the afferent and efferent arterioles, the juxtaglomerular complex is thought to play an essential feedback role in linking the level of arterial blood pressure and renal blood flow to the GFR and the composition of the distal tubular fluid.

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Endocrine Functions of the kidney Juxtaglomerular Complex The Renin–Angiotensin–Aldosterone Mechanism –Plays important part in regulation of blood pressure Erythropoietin –Regulates the differentiation of red blood cells in bone marrow Vitamin D –Increases calcium absorption from the gastrointestinal tract –Helps regulate calcium deposition in bone

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Question Which of the following hormones is produced in the kidney? –A. ADH –B. Erythropoietin –C. Aldosterone –D. Angiotensin

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Answer B. Erythropoietin Rationale: Erythropoietin is formed in the kidney in response to decreased blood oxygenation.

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Action of Diuretics Loop Diuretics –Exert their effect in the thick ascending loop of Henle Thiazide Diuretics –Prevent the reabsorption of NaCl in the distal convoluted tubule

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Action of Diuretics (cont.) The Aldosterone Antagonists (potassium-sparing diuretics) –Reduce sodium reabsorption and increase potassium secretion in the late distal tubule and cortical collecting tubule site regulated by aldosterone

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Characteristics of Normal Urine A clear, amber-colored fluid Approximately 95% water and 5% dissolved solids The kidneys normally produce approximately 1.5 L of urine each day. Contains metabolic wastes and few or no plasma proteins, blood cells, or glucose molecules

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Renal Clearance Definition –The volume of plasma that is completely cleared each minute of any substance that finds its way into the urine Determining Factors –The ability of the substance to be filtered in the glomeruli –The capacity of the renal tubules to reabsorb or secrete the substance

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Tests for Renal Function Urinalysis Glomerular filtration rate Blood tests –Serum creatinine –Blood urea nitrogen Cystoscopy Ultrasonography Radiologic and other image studies

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Question Normal urine should _______________. –A. be cloudy –B. have a pH of 7.0 or greater –C. have a specific gravity of 1.0 –D. be sterile

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Answer D. be sterile Rationale: There will be no bacteria present in a healthy urinary tract.

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Renal Casts Casts are molds of the distal nephron lumen. Casts develop when the protein concentration of the urine is high (as in nephrotic syndrome), urine osmolality is high, and urine pH is low.

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Specific Gravity The specific gravity of urine varies with its concentration of solutes. The urine specific gravity provides a valuable index of the hydration status and functional ability of the kidneys. Healthy kidneys can produce concentrated urine with a specific gravity of to During periods of marked hydration, the specific gravity can approach

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Factors Affecting the Glomerular Filtration Rate Glomerular capillary hydrostatic pressure Glomerular capillary osmotic pressure Hydrostatic and osmotic pressures in the Bowman capsule

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Serum Creatinine Levels In addition to its use in calculating the GFR, the serum creatinine level is used in estimating the functional capacity of the kidneys. If the value doubles, the GFR (and renal function) probably has fallen to one half of its normal state. A rise in the serum creatinine level to three times its normal value suggests that there is a 75% loss of renal function. With creatinine values of 10 mg/dL or more, it can be assumed that approximately 90% of renal function has been lost.

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins New Methods of Investigation Cystoscopy provides a means for direct visualization of the urethra, bladder, and ureteral orifices. It relies on the use of a cystoscope, an instrument with a lighted lens. Ultrasonographic studies use the reflection of ultrasonic waves to visualize the deep structures of the body. The procedure is painless, is noninvasive, and requires no patient preparation.

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins New Methods of Investigation (cont.) Radiologic studies include a simple flat plate of the kidneys, ureters, and bladder that can be used to determine the size, shape, and position of the kidneys and observe any radiopaque stones that may be in the kidney pelvis or ureters. Urography is used to detect space-occupying lesions of the kidneys, pyelonephritis, hydronephrosis, vesicoureteral reflux, and kidney stones. Other diagnostic tests include computed tomography (CT) scans, magnetic resonance imaging (MRI), radionuclide imaging, and renal angiography.