Urinary System Functions

Urinary System Components

• The urinary system consists of:
  • 2 kidneys
  • 2 ureters
  • Bladder
  • Urethra

Kidney Functions

• Regulate the balance between water and electrolytes and the acid-base balance
• Excrete metabolic wastes along with excess water and electrolytes in urine
• Excrete bioactive substances including drugs
• Secrete renin
• Secrete erythropoietin (in the proximal convoluted tubule)
• Convert vitamin D (in the proximal convoluted tubule)
• Maintain pH levels and regulate blood pressure
• Perform glycogenesis during starvation or periods

Kidney Structure

• Each kidney has a hilum, where nerves enter and the ureter exits
• The kidneys are covered by a thin fibrous layer
• The outer layer consists of fibroblasts and collagen fibers
• The inner layer is made of myofibroblasts that resist volume and pressure
• Inside the hilum, the upper end of the ureter expands and becomes the renal pelvis
• The renal pelvis divides into two or three major calyces, which then turn into minor calyces
• The area surrounding the renal pelvis and calyces has adipose tissue
• The parenchyma (functional tissue) of the kidney is divided into renal cortex and renal medulla
• The renal cortex is an outer darker region containing renal corpuscles and tubules
• The renal medulla is the inner region consisting mostly of aligned linear tubules and ducts

Nephron Structure and Functions

• Nephrons are functional units that filter blood and form urine
• Each nephron consists of a corpuscle and a long renal tubule
• The renal corpuscle contains a glomerulus and Bowman's capsule
• The glomerulus is a tuft of capillaries where blood filtration begins
• Bowman's capsule collects the filtrate from the blood
• The renal tubule is divided into several segments, each with specific functions in processing the filtrate
• The proximal convoluted tubule (PCT) reabsorbs a majority of the filtrate
• The loop of Henle is a U-shaped structure with descending and ascending limbs
• The distal convoluted tubule (DCT) continues to reabsorb ions and water, under the influence of hormones
• The collecting duct receives filtrate from multiple nephrons and reabsorbs water and concentrates the urine

Blood Flow in the Kidneys

• The renal arteries transport oxygenated blood from the heart and aorta to the kidneys for filtration
• The renal veins transport filtered deoxygenated blood from the kidneys to the heart
• The renal arteries enter each kidney at the hilum and divide into segmental arteries
• The segmental arteries branch into interlobar arteries, which extend from the renal pyramids to the corticomedullary junction

Renal Corpuscle

• The renal corpuscle contains a glomerulus surrounded by a epithelial capsule called the glomerular capsule
• The visceral layer is made of stellate epithelium called podocytes
• The podocytes have primary processes that extend and curve around the glomerular capillaries
• The filtration apparatus is made of podocytes and capillary endothelial cells
• The filtration slit pores and slit diaphragms regulate the passage of substances during filtration

Proximal Convoluted Tubule (PCT)

• The PCT reabsorbs a majority of the filtrate, including water, ions, and nutrients
• The PCT cells are highly specialized for their role in reabsorption and secretion within the nephron
• The PCT has a dense brush border, abundant mitochondria, and extensive membrane invaginations

Loop of Henle

• The loop of Henle is a U-shaped structure with descending and ascending limbs
• The thin descending limb and thin ascending limb have simple squamous epithelium and organelles
• The thick ascending limb has simple cuboidal epithelium and many mitochondria

Distal Convoluted Tubule (DCT)

• The DCT performs regulated reabsorption while influenced by the hormone aldosterone from the adrenal glands
• The DCT cells are smaller and flatter compared to those in the PCT

Ureters

• The ureters are muscular tubes that transport urine from the renal pelvis of the kidneys to the urinary bladder
• The ureters have three layers: mucosa, muscularis, and adventitia
• The ureters move urine towards the bladder via peristaltic contractions and prevent backflow into the kidneys as the bladder fills

Urinary Bladder

• The bladder is a hollow organ with a highly expandable wall consisting of mucosa, muscularis, and adventitia
• The bladder stores urine (400-600 mL in an average adult) and expels it during micturition

Urothelium (Transitional Epithelium)

• Found in the renal pelvis, ureters, bladder, and parts of the urethra
• Consists of basal cells, intermediate cells, and umbrella cells
• Protects underlying tissues from the toxic effects of urine and allows the bladder to stretch and contract