Urinary System Overview

Overview of the Urinary System

• The urinary system consists of the kidney, ureters, urinary bladder, and urethra.
• The kidney's functions include excreting nitrogenous waste, conserving body fluids and electrolytes, reabsorbing solutes and water, and regulating electrolyte homeostasis and acid-base balance.

Functions of the Kidney

• The kidney regulates water and electrolyte homeostasis through filtration, selective reabsorption, and regulation of fluid balance.
• It excretes metabolic waste products, bioactive substances, and excess water.
• The kidney also produces hormones such as renin and erythropoietin, regulates blood pressure, and activates vitamin D.

Structure of the Kidney

• The kidney is located retroperitoneally and has a unique structure depending on the species.
• The kidney has lobes, which are unilobular in carnivores, multilobular with deep grooves in large ruminants, and multilobar with smooth surfaces in pigs.

Important Tissues of the Kidney

• The kidney has a capsule made of collagen fibers, smooth muscle, and blood vessels.
• The cortex contains renal corpuscles, convoluted tubules, and peritubular capillary plexuses.
• The medulla has loops of Henle, collecting ducts, and vasa recta.

Nephron

• The nephron is the functional unit of the kidney and is responsible for osmoregulation.
• It consists of renal corpuscles, convoluted tubules, and loops of Henle.
• The nephron regulates water and electrolyte balance through filtration and selective reabsorption.

Renal Corpuscle

• The renal corpuscle is composed of the glomerulus and Bowman's capsule.
• The glomerulus is a tuft of capillaries with fenestrated endothelium within the Bowman's capsule.
• The Bowman's capsule has a visceral epithelium, glomerular basement membrane, urinary space, and parietal epithelium.

Ultrafiltration

• Ultrafiltration occurs at the glomerulus, where blood is pushed through a filtration barrier to produce an ultrafiltrate.
• The filtration barrier consists of the endothelium, glomerular basement membrane, and podocyte foot processes.

Formation of Urine

• The primary/glomerular filtrate is produced by ultrafiltration of blood in the renal corpuscle.
• The composition of the ultrafiltrate is similar to blood plasma, but without most proteins.
• Reabsorption of most substances occurs, and waste molecules and some water remain in the tubular system to form urine.

Renal Tubules

• The renal tubules consist of the proximal convoluted tubule, loop of Henle, and distal convoluted tubule.
• The proximal convoluted tubule is highly metabolically active, reabsorbs glucose, Na+/H2O, and amino acids, and has microvilli and a brush border.
• The loop of Henle is U-shaped, has thick descending and thin segments, and is located in the medulla.
• The distal convoluted tubule is involved in the regulation of sodium and water reabsorption.

Collecting Ducts

• The collecting ducts connect the distal convoluted tubule to the renal papillae and contain primitive urine.
• They are lined with cuboidal to low columnar epithelium and have site-specific functions, such as the regulation of sodium and water reabsorption.

Renal Papilla and Vasculature

• The renal papilla is the terminal portion of the inner medulla and contains papillary ducts.
• The vasculature of the kidney is a high blood supply, with a terminal artery system, renal artery, and interlobar artery.

Urinary Bladder and Urethra

• The urinary bladder stores urine and has a tunica mucosa, tunica submucosa, and tunica muscularis.
• The urethra is lined with transitional epithelium, has a similar structure to the ureters and urinary bladder, and has accessory sex glands.

Development of the Kidney

• The kidney develops from intermediate mesoderm.
• The pronephros regresses in mammals, while the mesonephros forms nephros that secrete fluid into the amnion.
• The metanephros develops from the ureteric bud, and the collecting tubules form, bifurcate, and lead to lobular architecture.