The Urinary SystemThe Urinary SystemChapter 26Chapter 26

The Urinary SystemThe Urinary SystemChapter 26Chapter 26

The organs of the urinary system include:

Functions of the urinary system (all done by kidneys):

Regulate the volume, concentration, pH and content of blood

Eliminate metabolic wastes as urine

3 processes occur within the kidneys to accomplish these goals

Blood vessels (capillaries)

Renal tubules

R

Reabsorption (R) – Movement (by passive & active means) of most fluid & many solutes from renal tubules back into the blood; approx. 99% filtate reabsorbed

F

Filtration (F) – Pressure (blood pressure) forces some fluid (plasma) and small substances from blood to renal (kidney) tubules. Results in the formation of “filtrate” in the renal tubules; approx. 180 liters filtrate/day

blood

filtrate

S

Secretion (S) – Selective movement of specific substances (e.g. H+, K+) from blood to tubules

urine

Filtered blood

Gross Anatomy of the Kidneys Retroperitoneal organs

Left kidney more superior than right (due to liver)

Notch at medial border (hilus/hilum) for renal artery & vein, ureter

Each kidney surrounded by 3 layers of CT:

renal capsule – innermost layer of dense CT

adipose capsule – middle layer protecting & insulating kidneys

renal fascia – outer layer holding kidneys in place in abdominal cavity

Internal Anatomy of the Kidneys

Cortex

Medulla

pyramids

renal papillae

renal columns

Pelvis - collecting basin

minor calyces

major calyces

renal pelvis

where urine formation occurs

Microscopic Anatomy of the KidneysFunctional unit of the kidneys is the “Nephron”

Nephron = renal corpuscle + renal tubules

Most (85%) nephrons classified as “cortical nephrons” – corpuscle & most of tubule located within cortex

Some (15%) classified as “juxtamedullary nephrons” – corpuscle at junction of cortex & medulla & loop of nephron extends into medulla

Bowman’s capsule (parietal (capsular) layer)

Visceral layer of Bowman’s capsule (podocytes)

glomerulus

capsular space

Filtration occurs across “endothelial capsular (filtration) membrane” – junction between glomerular endothelium & podocytes; results in formation of “filtrate” in capsular space

Renal corpuscle – 1st part of the nephron; site of filtration; comprised of glomerulus – capillary network Bowman’s (glomerular) capsule – double layered capsule of epithelial tissue (inner visceral layer/outer parietal layer), surrounding glomerulus

NEPHRON

proximal convoluted tubule

reabsorption

Loop of Henle

more reabsorption

distal convoluted tubule

Secretion, some reabsorption

renal corpuscle

filtration

H2O

solutes

filtrate

COLLECTING SYSTEM

collecting duct

variable secretion &/or reabsorption

papillary duct

delivery of urine to minor

calyx

Representative Nephron

Filtrate from renal corpuscle will move into proximal convoluted tubule (PCT) loop of Henle distal convoluted tubule (DCT); and then into a collecting system of tubes (connecting tubule collecting duct papillary duct minor calyx)

Connecting tubule

Efferent arteriole

Afferent arteriole

glomerulus

Blood supply to kidneys

(Cortical radiate arteries)

Efferent arterioles branch into a second capillary network, the peritubular capillaries, which surround the renal tubules. The peritubular capillaries which surround the tubules of juxtamedullary nephrons are longer & straighter, therefore known as vasa recta

Reabsorption & secretion occur between the renal tubules & peritubular capillaries

Peritubular capillaries venules interlobular veins arcuate veins interlobar veins renal vein

Juxtaglomerular Apparatus (JGA)Extremely important to regulate the rate of filtration that occurs at the glomerulus (glomerular filtration rate “GFR”)

To regulate glomerular BP, we have a feedback system – the juxtaglomerular apparatus (JGA)

JGA =

juxtaglomerular cells of the afferent arteriole – recognize if renal BP is too low & then can respond

macula densa cells of the distal convoluted tubule – recognize if decreased filtrate produced or too many solutes within filtrate & then can respond

Juxtaglomerular cells

Macula densa

Histology of the kidneys

Once urine is formed, it will move from the pelvis of the kidneys into the ureters

retroperitoneal, muscular tubes running from kidneys to urinary bladder

lined with transitional epithelium

transports urine primarily by peristalsis

Urinary bladder Hollow muscular organ that temporarily stores urine prior to “micturition”

Lined with mucosa of transitional epithelium with rugae

Muscularis of 3 layers of smooth muscle known as “detrussor muscle”

Internally have “trigone” – triangular area formed by openings of ureters & urethra

Micturition reflex – stretch receptors in wall of bladder stimulate smooth muscle of detrussor muscle (involuntary); stimulation of internal urethral sphincter (involuntary) & external urethral sphincter of skeletal muscle (voluntary)

Histology of bladder & ureters

Ureter

Urinary bladder

Transitional epithelium

muscularis

Urethra Transports urine out of body

Inner urethral sphincter of smooth muscle; external urethral sphincter of skeletal muscle

Anatomically different in men & women