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The Urinary System

 

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

 

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

 

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

 

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

 

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

 

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

 

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

 

 

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

 

Functional anatomy of the nephron

•Distal convoluted tubule (DCT)

•Actively secretes ions, toxins, drugs

•Reabsorbs sodium ions from tubular fluid

 

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

 

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 

 

Glomerular filtration rate (GFR)

•Amount of filtrate produced in the kidneys each minute

•Factors that alter filtration pressure change GFR

 

Factors controlling the GFR

•A drop in filtration pressure stimulates Juxtaglomerular apparatus (JGA)

•Releases renin and erythropoietin

 

Sympathetic activation

•Produces powerful vasoconstriction of afferent arterioles

•Decreases GFR and slows production of filtrate

•Changes the regional pattern of blood flow

•Alters GFR

•Stimulates release of renin by JGA

 

 

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

 

The loop of Henle and countercurrent multiplication

•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

 

Reabsorption and secretion at the DCT

•DCT performs final adjustment of urine

•Active secretion or absorption

•Absorption

•Tubular cells actively resorb Na+ and Cl-  

•In exchange for potassium or hydrogen ions (secreted)

 

Reabsorption and secretion along the collecting system

•Water and solute loss is regulated by aldosterone and ADH

•Reabsorption

•Sodium ion, bicarbonate, and urea are resorbed

•Secretion

•pH is controlled by secretion of hydrogen or bicarbonate ions

 

Control of urine volume and osmotic concentration

•Urine volume and osmotic concentration are regulated by controlling water reabsorption

•Precise control allowed via facultative water reabsorption

 

Function of the vasa recta

•Removes solutes and water

•Balances solute reabsorption and osmosis in the medulla

 

Composition of normal urine

•Varies with the metabolic and hormonal events of the body

•Reflects filtration, absorption and secretion activity of the nephrons

•Urinalysis is the chemical and physical analysis of urine

 

Summary of renal function:

•Each segment of nephron and collecting system contribute

•Glomerulus

•PCT

•Descending limb

•Thick ascending limb

•DCT and collecting ducts

•Concentrated urine produced after considerable modification of filtrate


Urine Transport, Storage, and Elimination

Urine production ends with fluid entering the renal pelvis

•Rest of urinary system transports, stores and eliminates

•Ureters

•Bladder

•Urethra

 

The ureters

•Pair of muscular tubes

•Extend from renal pelvis to the bladder

•Peristaltic contractions force urine toward the urinary bladder

 

The urinary bladder

•Hollow, muscular organ

•Reservoir for the storage of urine

•Contraction of detrusor muscle voids bladder

•Internal features include

•Trigone

•Neck

•Internal urethral sphincter

•Ruggae

 

The urethra

•Extends from the urinary bladder to the exterior of the body

•Passes through urogenital diaphragm (external urinary sphincter)

•Differs in length and function in males and females

 

Micturition reflex and urination

•Urination coordinated by micturition reflex

•Initiated by stretch receptors in wall of bladder

•Urination requires coupling micturition reflex with relaxation of external urethral sphincter


Aging and the Urinary System

Changes with aging include:

You should now be familiar with:

•The components of the urinary system and their functions

•The location and structural features of the kidneys

•The structure of a nephron, and the processes involved in the formation of urine

•The normal characteristics, composition, and solute concentrations of a representative urine sample