The Urinary System

Question 1

What is the urinary system composed of?

Answer 1

• Kidneys, ureters, urinary bladder, and urethra
• Functions include:
  1. Storage of urine
  2. Excretion of urine
  3. Regulation of blood volume
  4. Regulation of erythrocyte production
  5. Regulation of ion levels
  6. Regulation of acid-base balance

Question 2

What is the blood supply to the kidney like?

Answer 2

• Blood enters kidneys by renal arteries
• Within the renal sinus, the renal arteries branch into
  segmental arteries
• Segmental arteries then branch off into interlobar arteries which travel in renal columns
• At corticomedullary junction, interlobar arteries branch into arcuate arteries
• Arcuate arteries give off branches called interlobular arteries which radiate out to the cortex

Question 3

How does urine formation work?

Answer 3

• Nephrons form urine through 3 processes
  1. Filtration: Water and some dissolved solutes move out of glomerulus and into capsular space of renal corpuscle due to pressure. Once fluid leaves plasma and enters capsular space it is called filtrate
  2. Tubular reabsorption: substances in filtrate return to blood where most water and all needed solutes are reabsorbed
  3. Tubular secretion: solutes are actively transported out of blood and into tubular fluid

Question 4

What is the nephron loop?

Answer 4

• The nephron loop (loop of Henle) projects into the medulla and contains two parts:
  1. Descending limb: Extends from the cortex into the medulla and is lined with simple cuboidal then simple squamous epithelium
  2. Ascending limb: Returns from medulla into cortex and is lined with simple squamous then simple cuboidal epithelia
• The loop facilitates reabsorption of water and solutes
  (especially sodium and chloride ions)

Question 5

What is the distal convoluted tubule?

Answer 5

• Sits in renal cortex and contacts the afferent arteriole wall at the vascular pole
• It is lined with simple cuboidal epithelium (but microvilli are sparse and short)
• Secretes K+ and H+ from peritubular capillaries into tubular fluid
• Responds to ADH and aldosterone by altering reabsorption: ADH increases water reabsorption and aldosterone increases sodium reabsorption

Question 6

How does tubular fluid become urine?

Answer 6

• Distal convoluted tubules lead to collecting tubules that
  empty into collecting ducts
• Collecting ducts course through medulla toward renal papilla and are lined by simple cuboidal then simple columnar epithelium
• Collecting ducts modify tubular fluid under the influence of aldosterone and ADH: higher levels of these hormones lead to retention of Na+ and H20
• Once tubular fluid leaves collecting duct it is urine:
  1. Collecting duct empties into papillary duct that opens into minor calyx
  2. Minor calyx leads to major calyx which leads to pelvis
• Calyces and pelvis are lined with transitional epithelium

Question 7

What is the juxtaglomerular apparatus?

Answer 7

• A collection of structures associated with the nephron which helps regulate blood pressure
• Juxtaglomerular cells: Modified smooth muscle cells of afferent arteriole located at vascular pole of renal corpuscle. They can release renin which causes increase in blood pressure
• Macula densa: Group of modified epithelial cells in distal convoluted tubule that contact juxtaglomerular cells. They can monitor ion concentration in tubular fluid and can stimulate juxtaglomerular cells
• Extraglomerular mesangial cells sit between juxtaglomerular cells and arterioles. They can contract and phagocytize filtered particles

Question 8

What is the urinary tract composed of?

Answer 8

• Ureters, urinary bladder and urethra

Question 9

What are ureters?

Answer 9

• Each of the 2 ureters is a fibromuscular tube that conduct urine from a kidney to the urinary bladder
• Originates at renal pelvis at the hilum of the kidney
• Enters base of urinary bladder at its posterolateral wall
• Wall of ureter has three layers:
  1. Mucosa with transitional epithelium
  2. Muscularis containing inner longitudinal layer and outer circular layer of smooth muscle
  3. Adventitia of areolar connective tissue

Question 10

What is the urinary bladder?

Answer 10

• Main function is to store urine
• Located immediately posterior to pubic symphysis
• In females, the urinary bladder lies anterior inferior to the uterus and directly anterior to the vagina
• In males, the urinary bladder lies anterior to the rectum and superior to the prostate gland
• Only its superior surface is covered with peritoneum

Question 11

What is the urethra?

Answer 11

• A fibromuscular tube originating at the urinary
  bladder that conducts urine to the exterior of the body
• Lined with mucin producing cells in mucosa
• Has smooth muscle to help propel urine
• Two sphincters help control release of urine:
  1. Internal urethral sphincter: at neck of bladder and is involuntary
  2. External urethral sphincter: in urogenital diaphragm and is voluntary
• Female urethra only transports urine
• Male urethra has urinary and reproductive functions: transports both urine and semen, but not simultaneously and is partitioned into 3 segments…
  1. Prostatic urethra: through prostate gland and is transitional epithelium lining
  2. Membranous urethra: short segment through urogenital diaphragm and is stratified columnar or pseudostratified epithelial lining
  3. Spongy urethra: long segment through erectile tissue of penis and is lined by pseudostratified epithelium proximally and stratified squamous distally
• Its opening is called the external urethral orifice