Urinary System Overview

Urinary System Functions

• The urinary system's major functions include excreting organic wastes like urea from bodily fluids
• It eliminates these wastes into the external environment
• It regulates the volume and solute concentration of blood

Urinary System Organs

• The urinary system consists of two kidneys
• Kidneys produce urine, which passes through the urinary tract
• The urinary tract includes:
  • Two ureters, which carry urine from the kidneys to the bladder
  • The urinary bladder, which stores urine
  • The urethra, which carries urine from the bladder to the exterior of the body
• Urination, or micturition, is the process of eliminating urine

Kidney Location and Positioning

• Kidneys are located on either side of the vertebral column, between the last thoracic and third lumbar vertebrae
• The right kidney is slightly lower than the left
• Kidneys are somewhat displaced by the liver
• They are positioned behind the peritoneum (retroperitoneal)
• Adrenal glands sit on the superior surface of each kidney
• Kidneys are held in place by the overlying peritoneum, contact with adjacent organs, and supportive connective tissue
• Fibrous capsule surrounds the kidney, containing adipose tissue
• Damage to the suspensory fibers of the outer layer can cause kidney displacement or a floating kidney

Kidney Anatomy (Superficial)

• Kidneys are bean-shaped, roughly 10 cm long, 5.5 cm wide, and 3 cm thick
• The indentation on one side is the hilum
• The hilum is where the renal artery and renal nerves enter, and the renal veins and ureters exit
• Fibrous capsule covers the outer surface
• The fibrous capsule contains the renal sinus, an internal cavity

Kidney Anatomy (Sectional)

• Renal cortex: the outer layer, in contact with the fibrous capsule, and projects into the medulla as renal columns
• Renal medulla: the inner layer, containing 6-18 cone-shaped renal pyramids
• Renal papilla: the tip of each renal pyramid, projecting into the renal sinus
• Kidney Lobe: includes a renal pyramid, overlying cortex, and renal columns
• Urine production begins in nephrons in the cortex of each kidney lobe
• There are approximately 1.25 million nephrons in each kidney
• Nephron ducts carry urine into a cup-like structure called a minor calyx
• 4-5 minor calyces merge to form a major calyx
• 2-3 major calyces form a funnel-shaped chamber called the renal pelvis
• Renal pelvis connects to the ureter

Kidney Blood Supply

• Kidneys receive 20-25% of total cardiac output (about 1200 mL of blood per minute)
• Blood flow begins with the renal artery
• Blood travels through interlobar arteries, arcuate arteries, cortical radiate arteries, afferent arterioles, glomerular capillaries, efferent arterioles, peritubular capillaries, cortical radiate veins, arcuate veins, interlobar veins, and finally exits through the renal veins

Blood Flow from Peritubular Capillaries

• Blood from peritubular capillaries flows in two ways:
  • In cortical nephrons, blood flows directly into cortical radiate veins
  • In juxtamedullary nephrons, blood travels through vasa recta, which run parallel to nephron loops and flow into cortical radiate veins

Nephron Structure

• The nephron is the basic functional unit of the kidney
• It consists of two main parts: the renal corpuscle and the renal tubule

Renal Corpuscle

• Spherical structure containing a glomerular (Bowman's) capsule and a network of capillaries (glomerulus)
• Blood flows into the glomerulus via the afferent arteriole and leaves via the efferent arteriole
• The process of forcing fluid and dissolved solutes from the glomerular capillaries into the capsular space is called filtration
• It produces a solution called filtrate

Renal Tubule

• Filtrate flows through the following segments in order:
  • Proximal convoluted tubule (PCT)
  • Nephron (loop of Henle)
  • Distal convoluted tubule (DCT)
  • The filtrate is called tubular fluid at this point
  • The tubular fluid flows into a collecting duct
  • Collecting ducts lead to papillary ducts and minor calyces
  • Fluid at this stage is called urine

Functions of the Nephron

• The renal corpuscle produces filtrate via a passive process.
• The filtrate includes valuable nutrients, ions, and water.
• The tubules reabsorb useful molecules, ions, and water from the filtrate back into the blood. More than 90% of the water is reabsorbed
• Any waste products missed during filtration are secreted

Glomerular Capsule

• Forms the outer wall of the renal corpuscle.
• Encloses glomerular capillaries.
• Composed of two layers of cells: Outer (parietal) and Inner (visceral) layer.
• Visceral layer cells are called podocytes, which have foot processes (pedicels) that wrap around capillaries.

Metabolic Wastes in Urine

• Urea is the most abundant organic waste, formed from the breakdown of amino acids in skeletal muscle tissue
• Creatinine is generated from skeletal muscle creatine phosphate breakdown
• Uric acid is derived from RNA breakdown and recycling

Ureters

• Paired muscular tubes that transport urine from the kidneys to the urinary bladder
• Begins at the funnel-shaped renal pelvis.
• Ends at the posteroinferior bladder wall.
• Features slit-like ureteral openings to prevent urine backflow.
• Wall contains 3 layers: inner transitional epithelium, middle smooth muscle layer (peristalsis), and outer connective tissue.

Kidney Stones

• Also called calculi, kidney stones are solid substances consisting of calcium deposits, magnesium salts, or uric acid crystals.
• Can form in the kidney, ureter, or urinary bladder.
• Cause a painful condition known as nephrolithiasis
• Can block urine flow, potentially reducing or preventing filtration.

Urinary Bladder

• Stores urine.
• Size varies with distension and can hold up to 1 liter.
• Held in place by peritoneal folds (umbilical ligaments) and connective tissue.
• In males, it’s located between the rectum and pubic symphysis.
• In females, it's inferior to the uterus and anterior to the vagina.

Internal Urinary Bladder Anatomy

• Base includes the trigone, formed by two ureteral openings and an urethral entrance.
• Surrounding the urethral entrance is the neck of the bladder, also possessing an involuntary internal urethral sphincter.
• The bladder wall contains transitional epithelium and layers of smooth muscle (detrusor muscle), which contracts to expel urine into the urethra.

Urethra

• Extends from the urinary bladder neck to the exterior of the body
• In males, extends approximately 18-20 cm and opens at the tip of the penis.
• In females, it’s shorter, about 2.5-3.0 cm, and opens near the anterior wall of the vagina.
• A circular band of skeletal muscle (external urethral sphincter) surrounds the urethra, providing voluntary control of urination.

The Collecting System

• Several collecting ducts merge to form papillary ducts
• Papillary ducts empty into minor calyces
• Functions of the collecting system include:
  • Transporting tubular fluid from the nephron to renal pelvis
  • Adjusting final fluid composition
  • Determining final osmotic concentration of urine

Juxtaglomerular Complex

• A combination of closely associated cells in the distal convoluted tubule and afferent arteriole
• The macula densa is a region of clustered cells in the DCT, and juxtamedullary cells form part of the complex
• Unusual smooth muscle fibers are present in the afferent arteriole wall, important in regulating blood volume and blood pressure
• Secretion of erythropoietin and renin are integral kidney functions

Proximal Convoluted Tubule (PCT)

• The first segment of the renal tubule
• The majority of reabsorption occurs here.
• This is where cells lining this tubule reabsorb organic nutrients, ions, and blood plasma proteins from the tubular fluid.
• Substances are moved from the PCT tubule to the interstitial fluid or peritubular fluid, and re-enter the blood by osmosis.
• The PCT reduces tubular fluid volume.