52. Histology - Microanatomy of the Urinary System

Kidney Structure

• The kidney has an outer cortex and an inner medulla
• The cortex is composed of renal corpuscles and tubules
• The medulla contains tubules and collecting ducts grouped into pyramids
• The pyramids converge to form a papilla which projects into a minor calyx
• The renal sinus contains the renal pelvis, major and minor calyces, blood vessels and nerves

Nephron Structure

• The functional unit of the kidney is the nephron
• Each kidney contains approximately 1 million nephrons
• The nephron is composed of a renal corpuscle and a tubule
• The renal corpuscle is composed of Bowman's capsule and a glomerulus

Glomerulus

• The glomerulus is a branched fenestrated capillary
• The glomerular capillary endothelium has open fenestrae
• The glomerular capillary is surrounded by Bowman's capsule

Bowman's Capsule

• Bowman's capsule encloses the urinary space which contains provisional urine
• The parietal layer of Bowman's capsule is made of simple squamous epithelium
• The visceral layer of Bowman's capsule is made of podocytes with interdigitating pedicels, separated by slit pores spanned by slit membranes

Filtration Barrier

• The filtration barrier is composed of the glomerular capillary endothelium, the basement membrane and podocyte pedicels with slit pores
• The filtration barrier prevents the passage of large molecules from the blood into the urine

Mesangial Cells

• Mesangial cells are specialized contractile cells located within the glomerulus
• Mesangial cells participate in the maintenance of the basement membrane, provide structural support and regulate blood flow through the glomerulus

Proximal Tubule (PT)

• The proximal tubule is responsible for the reabsorption of about 2/3 of the glomerular filtrate
• The proximal tubule reabsorbs water, ions, glucose, amino acids and small proteins and secretes creatinine, some dyes and drugs
• The proximal tubule is lined by simple cuboidal epithelium with a brush border and basal striations
• The brush border contains peptidases which aid in the digestion of proteins
• The basal striations are folds in the plasma membrane which increase the surface area for reabsorption

Loop of Henle

• The loop of Henle is a U-shaped structure that extends from the proximal tubule into the medulla
• The loop of Henle is divided into a descending limb, a thin limb and an ascending limb
• The descending limb of the loop of Henle is permeable to water but impermeable to solutes
• The thin limb of the loop of Henle is permeable to both water and solutes
• The ascending limb of the loop of Henle is impermeable to water but permeable to solutes, including sodium and chloride
• The ascending limb of the loop of Henle contains a Na+, K+, ATPase pump which helps to create the osmotic gradient necessary for the concentration of urine

Distal Tubule (DT)

• The distal tubule is responsible for the reabsorption of sodium chloride and water
• The distal tubule is lined by simple cuboidal epithelium
• The distal tubule also plays a role in the regulation of potassium and calcium levels in the blood

Collecting Duct

• The collecting duct is responsible for the final concentration of urine
• The collecting duct is lined by simple cuboidal epithelium
• The collecting duct is permeable to water but impermeable to solutes
• The wall of the collecting duct contains aquaporins which allow water to pass from the tubule into the interstitial fluid, resulting in a concentrated urine.

Urethra Structure

• The male urethra is about 20 cm long and is part of the urinary and genital systems
• The female urethra is about 3-5 cm long and is part of the urinary system
• The female urethra is lined by stratified squamous epithelium and has patches of stratified or pseudostratified columnar epithelium
• The female urethra has mucous glands of Littre
• The male urethra will be examined later together with the Male Reproductive System

Test your knowledge on the intricate structures of the kidney and nephron, including their key components such as the renal corpuscle, medulla, and glomerulus. This quiz covers essential details about how these structures function together in the urinary system.