Urinary System Overview

Overview of the Urinary System

• The main components of the urinary system are: kidneys, ureters, urinary bladder, and urethra.
• The urinary system has several functions, including:
  • Removal of metabolic wastes and toxins
  • Elimination of excess nutrients and hormones
  • Regulation of blood volume and pressure
  • Regulation of electrolytes and body pH
  • Regulation of erythropoiesis
  • Calcium absorption
  • Control of the concentration of most compounds in the extracellular fluid

Kidney Structure

• The kidney is formed of multiple lobes, each lobe made up of a renal pyramid.
• The kidney has a fibrous capsule that covers the cortex and is continuous with the supporting tissue at the hilum.
• The renal cortex extends between the medullary pyramids and contains renal corpuscles and convoluted tubules.
• The renal medulla contains multiple medullary pyramids separated by regions of cortex.
• Millions of nephrons are packed into each kidney.
• The cortex consists of convoluted tubules (distal and proximal) and renal corpuscles (glomeruli).

Nephron Structure

• Each renal corpuscle has a vascular pole and a urinary or tubular pole.
• The vascular pole is where the afferent arteriole enters and the efferent arteriole leaves.
• The urinary or tubular pole is where the proximal convoluted tubule begins.
• The parietal layer of the glomerular capsule consists of a simple squamous epithelium supported by a basal lamina and a thin layer of reticular fibers.
• The cells of this layer, called podocytes, have a cell body and several primary processes that give rise to numerous secondary processes or pedicels.
• The pedicels interdigitate, defining elongated spaces 30-40 nm wide, called filtration slits.
• Spanning adjacent processes is a thin semipermeable diaphragm of uniform thickness.

Proximal Convoluted Tubule (PCT)

• The PCT is lined by simple cuboidal epithelium.
• The lining cells have numerous long apical microvilli, basal infoldings, and lateral canaliculi.
• The PCT is the site of most of the reabsorption of useful solutes and water from the filtrate.
• Approximately 80% of solutes and water in the initial filtrate are resorbed by the PCT.

Loop of Henle

• The loop of Henle is composed of three parts: descending thick limb, thin limb, and ascending thick limb.
• The descending thick limb is lined by simple cuboidal epithelium.
• The thin limb is lined by simple squamous epithelium.
• The ascending thick limb is lined by simple cuboidal epithelium.
• The loop of Henle establishes and maintains a high salt concentration in the interstitial fluid of the deep medulla.

Distal Convoluted Tubule (DCT)

• The DCT is lined by simple cuboidal epithelium.
• The DCT is the location of hormonally regulated (by aldosterone) Na+ for K+ exchange.
• The DCT is responsible for further reabsorption of solutes and water.

Collecting Tubules and Collecting Ducts

• Collecting tubules and ducts are lined by simple epithelia that gradually change from flattened simple cuboidal epithelium to simple columnar epithelium.
• The epithelia of the collecting tubules and ducts in the medulla increase their water permeability in response to vasopressin (antidiuretic hormone).

Peritubular Capillaries

• The peritubular capillaries arise from the efferent arterioles draining the glomeruli.
• These capillaries cling closely to adjacent renal tubules and empty into nearby venules.
• They are low-pressure, porous capillaries that readily absorb solutes and water from the tubule cells.

Juxtaglomerular Apparatus (JG Apparatus)

• The JG apparatus is located in the region of the nephron where the ascending limb of the loop of Henle and the adjacent afferent and efferent arterioles are positioned next to each other.
• The macula densa consists of cells of the ascending thick limb of the loop of Henle (DCT type cells) that are next to the efferent arteriole and are much taller than adjacent DCT cells.
• The JG cells are smooth muscle cells in the tunica media of the afferent arteriole that have become modified into secretory epithelial-like cells.

Ureters

• The ureters are muscular tubes that transport urine from the kidneys to the urinary bladder.
• The mucosa consists of a transitional epithelium underlain by a lamina propria and a muscularis externa.
• The muscularis externa consists of two or three layers of smooth muscle.

Urinary Bladder

• The bladder wall has three layers: a mucosa containing transitional epithelium, a thick muscular layer, and a fibrous adventitia.
• The muscular layer, called the detrusor muscle, consists of intermingled smooth muscle fibers arranged in inner and outer longitudinal layers and a middle circular layer.
• The bladder is very distensible and uniquely suited for its function of urine storage.