Histology Chapter 19: Urinary System

Histology Chapter 19: Urinary System

• The urinary system is composed of the kidneys, ureters, urinary bladder, and urethra.
• Kidneys are bean-shaped organs with a concave hilum, where the ureter, renal artery, and vein enter.
• The ureter divides into major and minor calyces, which surround the renal sinus containing adipose tissue.
• Each minor calyx has a renal pyramid, a conical region of medulla bordered by extensions of the cortex.
• The cortex and hilum are covered by a fibrous capsule.

Renal Functions

• Regulate water and electrolyte balance (inorganic ions).
• Regulate acid-base balance, involved in urine formation.
• Produce renin, an enzyme to regulate blood pressure and proper glomerular filtration pressure.
• Synthesize erythropoietin stimulating red blood cell production in response to lack of oxygen (hypoxia).
• Excrete metabolic wastes and excess water.
• Convert vitamin D to its active form (1,25-dihydroxyvitamin D3 or calcitriol).
• Perform gluconeogenesis during prolonged fasting generating glucose from amino acids.

Nephrons

• Functional units of the kidney, each consisting of a corpuscle and a long renal tubule.
• Renal corpuscle: dilated initial part surrounding a tuft of capillaries, site of blood filtration, and always located in the cortex.
• Proximal tubule: a long convoluted section in the cortex with a shorter straight part entering the medulla.
• Loop of Henle: a U-shaped section in the medulla with a thin descending and a thin ascending limb.
• Distal tubule: a thick straight part ascending from the loop of Henle that enters the cortex.
• Connecting tubule: a short segment connecting the nephron to collecting ducts.
• Cortical nephrons are almost entirely in the cortex.
• Juxtamedullary nephrons have long loops, close to the medulla.

Renal Vasculature

• Blood supply to the kidneys: a segmental artery, renal artery, renal vein, interlobar arteries, arcuate arteries, interlobular arteries, afferent arterioles, glomerulus, efferent arterioles, peritubular capillaries, vasa recta, arcuate veins, interlobar veins, and renal vein.

Renal Corpuscle

• Fenestrated glomerular capillary endothelium, thick combined basal lamina, or glomerular basement membrane (GBM), and filtration slit diaphragms between pedicels.
• These structures regulate the filtration of blood components.

Mesangial Cells

• Cells attached to the glomerular capillaries providing structural support and functions.
• Phagocytic cells removing antigen-antibody complexes and produce interleukins in response to glomerular injury or damage.
• Regulate blood pressure as a result of their contractile function.
• Part of the juxtaglomerular apparatus.

Renal Tubules

• Proximal convoluted tubule (PCT): prominent brush border on the apical surface, large number of mitochondria, interdigitating lateral borders, basal striations, and Na+/K+-ATPase.
• Loop of Henle: simple squamous epithelium except for thick ascending limb.
• Distal convoluted tubule (DCT): smaller brush border, fewer mitochondria, and interdigitating lateral borders.

Collecting Ducts

• Several collecting ducts converge in the renal papilla, deliver urine into minor calyx.
• Tubules are organized as cortical collecting ducts, medullary collecting ducts, and papillary collecting ducts.
• Principal cells and intercalated cells are abundant. These are important for the regulation of water and electrolytes and help maintain acid-base balance.

The Renin-Angiotensin-Aldosterone System (RAAS)

• A hormonal system that regulates blood pressure by controlling sodium retention and potassium excretion in the kidneys.

Medical Applications of Urinary System Problems

• Polycystic kidney disease: fluid-filled cysts in the kidneys.
• Glomerulonephritis: inflammation of glomeruli.
• Diabetic glomerulosclerosis: thickening and loss of function of the glomerular basal membrane in diabetes.
• Sickle cell nephropathy: damage due to sickled erythrocytes in the vasa recta
• Renal calculi: kidney stones caused by calcium salts, oxalate, or uric acid.
• Bacterial infections: cystitis or inflammation of the bladder mucosa, pyelonephritis affecting the kidney

Ureters, Bladder, and Urethra

• Urine is transported from renal pelvis to the bladder via ureters.
• Bladder walls are similar to the calyces and renal pelvis with mucosal, muscular, and adventitial layers.
• Lining of bladder, ureters, and renal pelvis is transitional epithelium (urothelium).
• Cells of the transitional epithelium are organized into three layers: basal cells, intermediate cells, and superficial umbrella cells.
• Urethra is a fibromuscular tube carrying urine from the bladder to the exterior.
• Male urethra has three segments: prostatic, membranous, and spongy.
• Female urethra is exclusively urinary and transitions from transitional to non-keratinized stratified squamous epithelium.

This quiz covers the anatomy and functions of the urinary system, including the structure of the kidneys, ureters, bladder, and urethra. It highlights the renal functions such as water and electrolyte balance, acid-base regulation, and waste excretion. Test your knowledge of key concepts related to the urinary system.