Chapter 28 Urinary System

Mosby items and derived items © 2007, 2003 by Mosby, Inc.

Slide 1

Overview of the Urinary System

 Kidneys —principal organs of the urinary system; accessory organs are ureters, urinary bladder, and urethra (Figure 28-1)  Urinary system —regulates the content of blood plasma to maintain the “dynamic constancy” or homeostasis of the internal fluid environment within normal limits Mosby items and derived items © 2007, 2003 by Mosby, Inc.

Slide 2

Anatomy of the Urinary System

 Gross structure (Figure 28-2)  Kidneys (two) • Shape, size, and location  Roughly oval with a medial indentation  Approximately 11 cm by 7 cm by 3 cm • •       Left kidney often larger than right; right kidney is a little lower Located in a retroperitoneal position Lie on either side of the vertebral column between T12 and L3 Superior poles of both kidneys extend above level of twelfth rib and lower edge of thoracic parietal pleura Renal fasciae anchors the kidneys to surrounding structures Heavy cushion of fat surrounds each kidney Internal structures of kidney     Cortex and medulla Renal pyramids comprise much of the medullary tissue Renal columns —where cortical tissue dips into the medulla between the pyramids Calyx —cuplike structure at each renal papilla to collect urine; join together to form renal pelvis  Renal pelvis narrows as it exits kidney to become ureter Blood vessels of the kidneys —kidneys are highly vascular Mosby items and derived items © 2007, 2003 by Mosby, Inc.

Slide 3

Anatomy of the Urinary System

 Gross structure (cont.)  Renal artery —large branch of abdominal aorta; brings blood into each kidney  Interlobular arteries —between the pyramids of the medulla and the renal artery branches; interlobular arteries extend toward the cortex, arch over the bases of the pyramids, and form arcuate arteries; from arcuate arteries, interlobular arteries penetrate the cortex Mosby items and derived items © 2007, 2003 by Mosby, Inc.

Slide 4

Anatomy of the Urinary System

 Gross structure (cont.)  Pattern of blood flow through kidneys —abdominal aorta → renal artery → segmental arteries → lobar arteries → interlobar arteries → arcuate arteries → interlobular artery → afferent arteriole → glomerulus (glomerular capillaries) → efferent arteriole → peritubular capillaries (vasa recta) → interlobular veins → arcuate veins → interlobar veins → lobar veins → segmental veins → renal vein → inferior vena cava (Figure 28-3) Mosby items and derived items © 2007, 2003 by Mosby, Inc.

Slide 5

Anatomy of the Urinary System

 Gross structure (cont.)  Juxtaglomerular apparatus —located where afferent arteriole brushes past distal tubule; important to maintenance of blood flow homeostasis by reflexively secreting renin when blood pressure in afferent arteriole drops  Ureter —tube running from each kidney to urinary bladder; composed of three layers: mucous lining, muscular middle layer, and fibrous outer layer Mosby items and derived items © 2007, 2003 by Mosby, Inc.

Slide 6

Anatomy of the Urinary System

 Gross structure (cont.)  Urinary bladder (Figure 28-5) • Structure —collapsible bag located behind the symphysis pubis made mostly of smooth muscle tissue; lining forms rugae; can distend considerably • Functions  Reservoir for urine before it leaves the body  Aided by the urethra, expels urine from the body • Mechanism for voiding  Voluntary relaxation of external sphincter muscle  Regions of the detrusor muscle contract reflexively  Urine is forced out of the bladder and through the urethra Mosby items and derived items © 2007, 2003 by Mosby, Inc.

Slide 7

Anatomy of the Urinary System

 Gross structure (cont.)  Urethra • Small mucous membrane –lined tube, extending from trigone to exterior of the body • In female, lies posterior to symphysis pubis and anterior to vagina; approximately 3 cm long • In male, after leaving the bladder, passes through prostate gland where it is joined by two ejaculatory ducts; from prostate, extends to base of penis and then through center of penis, and ends as urinary meatus; approximately 20 cm long; male urethra is part of the urinary system, as well as part of the reproductive system Mosby items and derived items © 2007, 2003 by Mosby, Inc.

Slide 8

Microscopic Structure of the Nephron

 Nephrons, the microscopic functional units, make up the bulk of the kidney; those located in renal cortex called cortical nephrons; those near junction of cortical and medullary layers called juxtamedullary nephrons; each nephron is made up of various structures (Figure 28-4)   Renal corpuscle (Figure 28-10) Bowman’s capsule—cup-shaped mouth of nephron • • Formed by parietal and visceral walls with a space between them Pedicels in visceral layer packed closely together to form filtration slits; slit diaphragm prevents filtration slits from enlarging under pressure (Figure 28-11 and Figure 28-12) • Glomerulus —network of fine capillaries in Bowman’s capsule; together called renal corpuscle; located in cortex of kidney (Figure 28-10) • • Basement membrane lies between glomerulus and Bowman’s capsule Glomerular-capsular membrane filtration (Figure 28-12) —formed by glomerular endothelium, basement membrane, and visceral layer of Bowman’s capsule; function is Mosby items and derived items © 2007, 2003 by Mosby, Inc.

Slide 9

Microscopic Structure of the Nephron

 Nephrons (cont.)  Proximal tubule —first part of renal tubule nearest to Bowman’s capsule; follows a winding, convoluted course; also known as proximal convoluted tubule  Loop of Henle (Figure 28-4) • • Renal tubule segment just beyond proximal tubule Consists of a thin descending limb, a sharp turning, and a thick ascending limb • Juxtamedullary nephron —a nephron with a loop of Henle that dips far into the medulla • Cortical nephron —a nephron with a loop of Henle that does not dip into the medulla but remains almost entirely within the cortex; constitute about 85% of total nephron numbers Mosby items and derived items © 2007, 2003 by Mosby, Inc.

Slide 10

Microscopic Structure of the Nephron

 Nephrons (cont.)  Distal tubule —convoluted tubule beyond the loop of Henle; also known as distal convoluted tubule  Collecting duct • Straight tubule joined by the distal tubules of several nephrons • Joins larger ducts; larger collecting ducts of one renal pyramid converge to form one tube that opens at a renal papilla into a calyx Mosby items and derived items © 2007, 2003 by Mosby, Inc.

Slide 11

Physiology of the Urinary System

 Overview of kidney function  Chief functions of kidney are to process blood and form urine  Basic functional unit of kidney is nephron; forms urine through three processes (Figure 28-14) • Filtration —movement of water and protein-free solutes from plasma in glomerulus into capsular space of Bowman’s capsule • Tubular reabsorption —movement of molecules out of tubule and into peritubular blood • Tubular secretion —movement of molecules out of peritubular blood and into tubule for excretion Mosby items and derived items © 2007, 2003 by Mosby, Inc.

Slide 12

Physiology of the Urinary System

 Filtration —first step in blood processing that occurs in the renal corpuscles  From blood in the glomerular capillaries, about 180 liters of water and solutes filter into Bowman’s capsule each day; takes place through the glomerular-capsular membrane  Filtration occurs as a result of existence of a pressure gradient  Glomerular capillary filtration occurs rapidly as a result of the increased number of fenestrations  Glomerular hydrostatic pressure and filtration are directly related to systemic blood pressure Mosby items and derived items © 2007, 2003 by Mosby, Inc.

Slide 13

Physiology of the Urinary System

 Reabsorption —second step in urine formation; occurs as a result of passive and active transport mechanisms from all parts of the renal tubules; major portion of reabsorption occurs in proximal tubules (Figure 28-15)  Reabsorption in proximal tubule to move on to the loop of Henle —most water and solutes are recovered by the blood, leaving only a small volume of tubule fluid • Sodium —actively transported out of tubule fluid and into blood (Figure 28-17) • Glucose and amino acids —passively transported out of tubule fluid by means of the sodium cotransport mechanism • Chloride, phosphate, and bicarbonate ions passively move into blood because of an imbalance of electrical charge • Water —movement of sodium and chloride into blood causes an osmotic imbalance, moving water passively into blood • Urea —approximately one half of urea passively moves out of tubule with the remaining urea moving on to the loop of Henle Mosby items and derived items © 2007, 2003 by Mosby, Inc.

Slide 14

Physiology of the Urinary System

 Reabsorption (cont.)  Reabsorption in the loop of Henle (Figure 28-19) • Water is reabsorbed from the tubule fluid, and urea is picked up from the interstitial fluid in the descending limb • Sodium and chloride are reabsorbed from the filtrate in the ascending limb, where the reabsorption of salt makes the tubule fluid dilute and creates and maintains a high osmotic pressure of the medulla’s interstitial fluid Mosby items and derived items © 2007, 2003 by Mosby, Inc.

Slide 15

Physiology of the Urinary System

 Reabsorption in the distal tubules and collecting ducts  Distal tubule reabsorbs sodium by active transport but in smaller amounts than in the proximal tubule  ADH is secreted by the posterior pituitary and targets the cells of distal tubules and collecting ducts to make them more permeable to water  With the reabsorption of water in the collecting duct, urea concentration of tubule fluid increases, causing urea to diffuse out of collecting duct into medullary interstitial fluid  Urea participates in a countercurrent multiplier mechanism that, along with countercurrent mechanisms of the loop of Henle and vasa recta, maintains the high osmotic pressure needed to form concentrated urine and avoid dehydration Mosby items and derived items © 2007, 2003 by Mosby, Inc.

Slide 16

Physiology of the Urinary System

 Tubular secretion  Tubular secretion —the movement of substances out of blood and into tubular fluid  Descending limb of loop of Henle secretes urea via diffusion  Distal and collecting tubules secrete potassium, hydrogen, and ammonium ions  Aldosterone —hormone that targets the cells of distal and collecting tubule cells, causes increased activity of sodium potassium pumps  Secretion of hydrogen ions increases with increased blood hydrogen ion concentration Mosby items and derived items © 2007, 2003 by Mosby, Inc.

Slide 17

Physiology of the Urinary System

 Regulation of urine volume (Figure 28-23)  ADH influences water reabsorption; as water is reabsorbed, total volume of urine is reduced by amount of water removed by tubules; ADH reduces water loss  Aldosterone, secreted by adrenal cortex, increases distal tubule absorption of sodium, raising sodium concentration of blood and thus promoting reabsorption of water  Atrial natriuretic hormone (ANH), secreted by specialized atrial muscle fibers, promotes loss of sodium via urine; opposes aldosterone, causing the kidneys to reabsorb less water and thereby produce more urine Mosby items and derived items © 2007, 2003 by Mosby, Inc.

Slide 18

Physiology of the Urinary System

 Regulation of urine volume (cont.)  Tubuloglomerular feedback mechanism maintains constant glomerular filtration rate (GFR) by regulating resistance in afferent arterioles; protects kidney GFR function from rapid blood pressure variations; dependent on macula densa cells and juxtaglomerular apparatus; may influence renin-angiotensin mechanism  Myogenic mechanism —rapid and effective regulation of GFR via changes in afferent arteriole smooth muscle contraction and relaxation  Urine volume —also related to total amount of solutes other than sodium excreted in the urine; generally, the more solutes, the more urine Mosby items and derived items © 2007, 2003 by Mosby, Inc.

Slide 19

Physiology of the Urinary System

 Urine composition —approximately 95% water with several substances dissolved in it; the most important are the following:  Nitrogenous wastes —result of protein metabolism; e.g., urea, uric acid, ammonia, and creatinine  Electrolytes —mainly the following ions: sodium, potassium, ammonium, chloride, bicarbonate, phosphate, and sulfate; amounts and kinds of minerals vary with diet and other factors  Toxins —during disease, bacterial poisons leave the body in urine  Pigments —especially urochromes  Hormones —high hormone levels may spill into the filtrate  Abnormal constituents —such as blood, glucose, albumin, casts, or calculi Mosby items and derived items © 2007, 2003 by Mosby, Inc.

Slide 20

The Big Picture: Urinary System and the Whole Body

 Homeostasis of water and electrolytes in body fluids relies on proper functioning of the kidneys; nephrons process blood to adjust its content to maintain a relatively constant internal environment  Urinary and cardiovascular systems are interdependent  Endocrine and nervous systems must operate properly to ensure efficient kidney function Mosby items and derived items © 2007, 2003 by Mosby, Inc.

Slide 21