Chapter 26 - Urinary System

What are the main organs of the urinary system

Kidneys, ureters, urinary bladder, and urethra

What is the primary function of the kidneys

To produce urine by filtering blood and removing wastes

What is the function of the ureters

To transport urine from the kidneys to the urinary bladder

What is the role of the urinary bladder

To temporarily store urine prior to urination

What is the function of the urethra

To conduct urine to the exterior; in males, it also transports semen

Where are the kidneys located

Retroperitoneal, between T12 and L3 vertebrae, protected by the 11th and 12th ribs

What are the three connective tissue layers surrounding the kidneys

Fibrous capsule, perinephric fat, and renal fascia

What is the hilum of the kidney

The medial indentation where the renal artery, vein, and ureter enter or exit

What are the two main internal regions of the kidney

Renal cortex and renal medulla

What structures are found within the renal medulla

Renal pyramids and renal columns

What is a renal lobe composed of

A renal pyramid, the overlying renal cortex, and adjacent renal columns

What is the renal pelvis 

A funnel-shaped cavity that collects urine from major calyces and drains into the ureter

Describe the blood supply to the kidneys

Renal artery → segmental arteries → interlobar arteries → arcuate arteries → cortical radiate arteries → afferent arterioles → glomerulus → efferent arteriole → peritubular capillaries/vasa recta → veins in reverse order → renal vein

What is the functional unit of the kidney

The nephron

What are the two major components of a nephron

The renal corpuscle and the renal tubule

What structures make up the renal corpuscle

The glomerulus and the glomerular (Bowman’s) capsule

What is the function of the glomerulus

A capillary network that filters blood plasma into the nephron

What is the capsular space

The space between the visceral and parietal layers of the glomerular capsule that collects filtrate

What are the three main regions of the renal tubule

Proximal convoluted tubule (PCT), nephron loop (Loop of Henle), and distal convoluted tubule (DCT)

What are the two types of nephrons

Cortical nephrons (85%) and juxtamedullary nephrons (15%)

What is the main difference between cortical and juxtamedullary nephrons

Juxtamedullary nephrons have longer loops of Henle that extend deep into the medulla and play a key role in water conservation

What type of cells line the PCT

Cuboidal cells with abundant microvilli (for reabsorption)

What type of cells line the DCT

Cuboidal cells with fewer microvilli (for secretion and selective reabsorption)

What structures are part of the collecting system

Collecting ducts and papillary ducts

What are the two main cell types in the collecting duct

Intercalated cells (acid-base balance) and principal cells (water/sodium balance)

What is the juxtaglomerular complex (JGC)

A structure that regulates blood pressure and filtration rate; includes macula densa, juxtaglomerular cells, and mesangial cells

What are the three layers of the filtration membrane

Fenestrated endothelium, basement membrane, and podocyte foot processes (filtration slits)

What is glomerular hydrostatic pressure (GHP)

The blood pressure in glomerular capillaries (~50 mm Hg) that pushes water and solutes out of the plasma

What is blood colloid osmotic pressure (BCOP)

The pressure (~25 mm Hg) that opposes filtration by drawing water back into plasma

What is capsular hydrostatic pressure (CsHP)

The pressure (~15 mm Hg) opposing GHP due to resistance from filtrate already in the nephron

What is the net filtration pressure (NFP)

The sum of all pressures acting across glomerular capillaries; typically ~10 mm Hg driving filtration into the capsule

What is the histology of the ureter

Mucosa with transitional epithelium, lamina propria, smooth muscle, and outer connective tissue

What is the histology of the urinary bladder

Transitional epithelium, lamina propria, detrusor muscle, and visceral peritoneum

What is the histology of the female urethra

Stratified squamous epithelium with smooth muscle and mucous glands

What muscle contracts to expel urine from the bladder

The detrusor muscle

What are the internal and external urethral sphincters made of

Internal sphincter – smooth muscle (involuntary); External sphincter – skeletal muscle (voluntary)

What does the pontine storage center do

Inhibits detrusor contraction and keeps both urethral sphincters closed during urine storage

What does the pontine micturition center do

Stimulates detrusor contraction and relaxes urethral sphincters to allow urination

What are the two reflexes of urination

Urine storage reflex and urine voiding reflex

What is a pyelogram

An X-ray image of the urinary tract after injection of contrast dye to visualize kidneys, ureters, and bladder

What does RAAS stand for

Renin-Angiotensin-Aldosterone System

When do the kidneys release renin

When blood flow or blood pressure drops

What does angiotensin do

Constricts arteries to increase blood pressure

What does aldosterone do

Increases sodium and water retention

What is the overall function of RAAS

Increases blood pressure and perfusion to the kidneys

What is the key concept to remember about water and salt

Water follows salt 

What does filtrate in the renal corpuscle contain

Both wastes and resources

What happens to resources in the nephron

They are reabsorbed (water, electrolytes, etc.)

What happens to wastes in the nephron

They are concentrated and excreted as urine

What are the main functions of the PCT

Reabsorbs sodium, water, and glucose; secretes hydrogen ions; makes bicarbonate

What type of transporters are present in the nephron

Exchange pumps, cotransporters, countertransporters, and leak channels

What does an exchange pump do

Uses energy to move ions against their gradient

What does a cotransporter do

Moves one solute along with another

What does countertransport mean 

Exchanges one solute for another

What does a leak channel do

Allows passive diffusion

What does the DCT reabsorb

Sodium and chloride ions

What does the DCT secrete

Potassium, ammonium, and hydrogen ions

What hormone affects DCT function

Aldosterone

What does aldosterone do in the DCT 

Adds a sodium/potassium exchange pump to increase sodium reabsorption and potassium secretion

What does the nephron loop reabsorb

Sodium, chloride, potassium, and water

What determines nephron loop function

The osmolarity of the medulla and countercurrent flow with vasa recta

What is the thin descending limb permeable to

Water

What is the thick ascending limb permeable to

Solutes; it transports sodium and chloride

What does the collecting duct reabsorb

Water and sodium (variable, under hormonal control)

What hormone controls water reabsorption in the collecting duct

Antidiuretic hormone (ADH)

What is diuresis

Water loss through urine

What characterizes diuresis 

Large volume of dilute (clear) urine

What is anti-diuresis

Small volume of concentrated (dark) urine

What is the purpose of ADH

Reduces urine output by increasing water reabsorption

What is countercurrent flow

The opposite movement of filtrate in the nephron loop and blood in the vasa recta that helps concentrate urine

What is the overall effect of countercurrent mechanisms

Reabsorb large amounts of salt and water back into the bloodstream while concentrating wastes in urine

What does normal urinalysis indicate

Functional kidneys and adequate hydration

What do abnormal urinalysis results indicate

Possible infection, dehydration, kidney damage, diabetes, or bladder/urethral trauma

What are normal characteristics of urine

Clear to light yellow color, pH around 6, and specific gravity around 1.003–1.030

What does a high specific gravity indicate 

Concentrated urine (dehydration)

What does a low specific gravity indicate

Dilute urine (overhydration or impaired renal concentration ability)

What substances should not be present in normal urine

Glucose, protein, ketones, blood, bilirubin, and bacteria

What are the main functions of the kidneys 

Filter blood, remove wastes, regulate blood pressure, balance electrolytes, and maintain acid-base balance

What happens as blood moves through the nephron

Filtration, reabsorption, secretion, and concentration occur to form urine