The Urinary System

Functions of the urinary system• Homeostatic regulation of blood plasma

• Regulating blood volume and pressure• Regulating plasma ion concentrations• Stabilizing blood pH• Conserving nutrients

• Filter many liters of fluid from blood• Excretion - The removal of organic waste products

from body fluids• Urea• Uric acid• Creatinine

• Elimination - The discharge of waste products into the environment

Figure 26.1

Urinary System

• Kidneys – produce urine

• Ureters –transport urine to bladder

• Urinary bladder - stores urine

• Urethra transports urine to exterior

Location and External Anatomy of Kidneys• Located retroperitoneally

• Lateral to T12–L3 vertebrae

• Average kidney• 12 cm tall, 6 cm wide, 3 cm

thick

• Hilus• On concave surface

• Vessels and nerves enter and exit

• Renal capsule surrounds the kidney

Figure 26.3

Urinary System in Gross Dissection

Internal Gross Anatomy of the Kidneys

• Frontal section through the kidney• Renal cortex

• Renal pyramids

• Renal pelvis• Major calicies

• Minor calicies

• Gross vasculature• Renal arteries

• Branch into segmental arteries

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

Nephron – The Functional Unit of Kidney

• Nephron consists of:• Renal corpuscle• Renal tubule:

• Proximal convoluted tubule (PCT)

• Loop of Henle• Distal convoluted

tubule (DCT)• Nephron empties

tubular fluid into a system of collecting ducts and papillary ducts

Renal Corpuscle• Consists of:

• Glomerulus – tuft of fenestrated capillaries• Glomerular (Bowman’s) capsule

• Parietal layer – simple squamous epithelium• Visceral layer – consists of podocytes

• Blood travels from efferent arteriole to peritubular capillaries• Blood leaves the nephron via the efferent arteriole

Renal Corpuscle and the Filtration Membrane

Figure 23.6a

Glomerulus anatomy

• Podocytes cover lamina densa of capillaries• Project into the capsular space• Pedicels of podocytes separated by

filtration slits

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

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

Nephron• Distal convoluted

tubule (DCT)• Actively secretes

ions, toxins, drugs• Reabsorbs sodium

ions from tubular fluid

Figure 23.8

Collecting Tubules (Collecting ducts)

• Collecting tubules - Receive urine from distal convoluted tubules

Uriniferous Tubule

Figure 23.5a

Types Of Capillary Beds In Nephron

• Glomerulus - Fed and drained by afferent and efferent arterioles

• Peritubular capillaries• Arise from efferent

arterioles

• Low-pressure, porous capillaries

• Absorb solutes

• Vasa recta• Thin-walled looping vessels

• Part of the kidney’s urine-concentrating mechanism

Mechanisms of Urine Production• Filtration - filtrate of

blood leaves kidney capillaries

• Reabsorption – most nutrients, water, and essential ions reclaimed

• Secretion - active process of removing undesirable molecules

Microscopic Anatomy of the Kidney• Juxtaglomerular apparatus

• Functions in the regulation of blood pressure

• Juxtaglomerular cells – secrete renin

• Macula densa

• A portion of distal convoluted tubule

• Tall, closely packed epithelial cells

• Act as chemoreceptors

Summary of Nephron 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 Excretion

• Leaves Collecting System

• Enters 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 from the kidneys to the

urinary bladder• Oblique entry into bladder prevents backflow of urine

Histology of Ureter

• Mucosa – transitional epithelium

• Muscularis – two layers• Inner longitudinal layer

• Outer circular layer

• Adventitia – typical connective tissue

Urinary Bladder• A collapsible muscular

sac

• Stores and expels urine• Full bladder – spherical

• Expands into the abdominal cavity

• Empty bladder – lies entirely within the pelvis

Figure 23.13

Urinary Bladder• Wall of bladder

• Mucosa - transitional epithelium

• Muscular layer - detrusor muscle

• Adventitia

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

• Internal urethral sphincter - involuntary smooth muscle

• External urethral sphincter - voluntarily inhibits urination, relaxes when one urinates

Urinary Bladder and Urethra - Male

Figure 23.16a

• Males – 20 cm in length

• Three named regions• Prostatic urethra -

passes through the prostate gland

• Membranous urethra - through the urogenital diaphragm

• Spongy (penile) urethra passes through the length of the penis

Urinary Bladder and Urethra - Female• In females - length of 3–4 cm

• The smooth triangular region of the base is is called the trigone - many bladder infections persist in this region

Urethra

• Epithelium of urethra

• Transitional epithelium at the proximal end (near the bladder)

• Stratified and pseudostratified columnar – mid urethra (in males)

• Stratified squamous epithelium at the distal end (near the urethral opening)

Micturition• Bladder can hold 250 -

400ml• Greater volumes stretch

bladder walls initiates micturation reflex:

• Urination coordinated by micturition reflex• Initiated by stretch receptors

in wall of bladder• Urination requires coupling

micturition reflex with relaxation of external urethral sphincter

Blood Flow Through the Kidney