Chapter 26 The Urinary System

Learning Objectives Identify the components and functions of the urinary system Describe the location and structural features of the kidneys Describe the structure of a nephron, and outline the processes involved in the formation of urine List and describe the factors that influence filtration pressure and the rate of filtrate formation

SECTION 26-1 An Overview of the Urinary System

Functions of the urinary system Excretion The removal of organic waste products from body fluids Elimination The discharge of waste products into the environment Homeostatic regulation of blood plasma Regulating blood volume and pressure Regulating plasma ion concentrations Stabilizing blood pH Conserving nutrients

Urinary system includes: The kidneys Produce urine The ureters The urinary bladder Stores urine The urethra

Figure 26.1 An introduction to the Urinary System

SECTION 26-2 The Kidneys

The kidneys Left kidney extends slightly more superiorly than right Both kidneys and adrenal glands are retroperitoneal Hilus Entry for renal artery and renal nerves Exit for renal veins and ureter

Figure 26.2 The Position of the Kidneys Figure 26.2a, b

Figure 26.3 The Urinary System in Gross Dissection

Sectional anatomy of the kidneys Superficial outer cortex and inner medulla The medulla consists of 6-18 renal pyramids The cortex is composed of roughly 1.25 million nephrons Major and minor calyces along with the pelvis drain urine to the ureters

Figure 26.4 The Structure of the Kidney Figure 26.4a, b

Blood supply and innervation of the kidneys Renal arteries branch repeated Renal artery Segmental artery Interlobar artery Arcuate artery Interlobular artery Afferent arterioles Renal venules follow similar opposing pattern ending with renal veins

Figure 26.5 The Blood Supply to the Kidneys Figure 26.5a, b

Figure 26.5 The Blood Supply to the Kidneys Figure 26.5c, d

The nephron consists of a renal corpuscle and renal tubule The renal corpuscle is composed of Bowman’s capsule and the glomerulus The renal tubule consists of Proximal convoluted tubule (PCT) Loop of Henle Distal convoluted tubule (DCT)

Filtrate is produced at the renal corpuscle Nephron empties tubular fluid into collecting system Collecting ducts and papillary ducts form the collecting system

Figure 26.6 A Representative Nephron

Nephron functions include: Production of filtrate Reabsorption of organic nutrients Reabsorption of water and ions Secretion of waste products into tubular fluid

Two types of nephron Cortical nephrons ~85% of all nephrons Located in the cortex Juxtamedullary nephrons Closer to renal medulla Loops of Henle extend deep into renal pyramids Responsible for greatest amount of water reabsorbed

Figure 26.7 Cortical and Juxtamedullary Nephrons Figure 26.7a

Figure 26.7 Cortical and Juxtamedullary Nephrons Figure 26.7b, c

Renal tubule and blood flow Blood travels from efferent arteriole to peritubular capillaries Vasa recta Renal tubule begins at renal corpuscle Includes glomerulus and Bowman’s capsule Blood leaves the nephron via the efferent arteriole

Glomerulus anatomy Podocytes cover lamina densa of capillaries Project into the capsular space Pedicels of podocytes separated by filtration slits

Figure 26.8 The Renal Corpuscle Figure 26.8a, b

Figure 26.8 The Renal Corpuscle Figure 26.8c, d

Functional anatomy of the nephron Proximal convoluted tubule (PCT) Actively reabsorbs nutrients, plasma proteins and ions from filtrate Released into peritubular fluid Loop of Henle Descending limb Ascending limb Each limb has a thick and thin section PLAY Animation: Urinary System Anatomy

Functional anatomy of the nephron Distal convoluted tubule (DCT) Actively secretes ions, toxins, drugs Reabsorbs sodium ions from tubular fluid PLAY Animation: Urinary System Dissection and Flythrough

SECTION 26-3 Principles of Renal Physiology

Urine production maintains homeostasis Regulating blood volume and composition Excreting waste products Urea Creatinine Uric acid

Basic processes of urine formation Filtration Blood pressure Water and solutes across glomerular capillaries Reabsorption The removal of water and solutes from the filtrate Secretion Transport of solutes from the peritubular fluid into the tubular fluid

Carrier Mediated Transport Filtration in the kidneys modified by carrier mediated transport Facilitated diffusion Active transport Cotransport Countertransport Carrier proteins have a transport maximum (Tm) Determines renal threshold

Reabsorption and secretion Accomplished via diffusion, osmosis, and carrier-mediated transport Tm determines renal threshold for reabsorption of substances in tubular fluid

Renal function Most regions of the nephron perform a combination of functions General functions can be identified Filtration in the renal corpuscle Nutrient reabsorption along the PCT Active secretion at PCT and DCT Loops of Henle regulate final volume and solute concentration

Figure 26.9 An Overview of Urine Formation

SECTION 26-4 Renal Physiology: Filtration and the Glomerulus

Filtration pressures - Glomerular filtration Occurs as fluids move across the glomerulus In response to glomerular hydrostatic pressure (GHP) and blood pressure in the glomerular capillaries Capsular hydrostatic pressure (CsHP) opposes GHP Blood colloid osmotic pressure (BCOP) opposes GHP Net hydrostatic pressure (NHP) = GHP – CsHP Filtration (FP) = NHP – BCOP

Figure 26.10 Glomerular Filtration

Figure 26.10 Glomerular Filtration Figure 26.10a, b

Glomerular filtration rate (GFR) Amount of filtrate produced in the kidneys each minute Factors that alter filtration pressure change GFR

SECTION 26-5 Renal Physiology: Reabsorption and Secretion

Reabsorption and secretion at the PCT Glomerular filtration produces fluid similar to plasma without proteins The PCT reabsorbs 60-70% of the filtrate produced Reabsorption of most organic nutrients Active and passive reabsorption of sodium and other ions Reabsorption of water Secretion also occurs in the PCT PLAY Animation: Early Filtrate Processing PLAY Animation: Glomerular filtration

Figure 26.12 Transport Activities at the PCT PLAY Animation: Proximal Convoluted Tubule Figure 26.12

The loop of Henle and countercurrent multiplication Between ascending and descending limbs of loop Creates osmotic gradient in medulla Facilitates reabsorption of water and solutes before the DCT Permits passive reabsorption of water from tubular fluid

Figure 26.13 Countercurrent Multiplication and Concentration of Urine Figure 26.13a

Figure 26.13 Countercurrent Multiplication and Concentration of Urine Figure 26.13b

Figure 26.13 Countercurrent Multiplication and Concentration of Urine