The digestive system breaks food into absorbable nutrients through a muscular tube called the alimentary canal and several accessory organs that aid digestion. The stomach prepares food chemically and mechanically; its regions include the cardia surrounding the entrance from the esophagus, the fundus curving above, the main body where mixing occurs, and the pylorus leading into the small intestine. The stomach wall's rugae are folds that expand to accommodate meals and increase surface area for secretion. From the stomach, chyme enters the small intestine, where most digestion and absorption occur. The duodenum is the short first segment, about 25 cm, that receives bile and pancreatic juice. The jejunum, around 2.5 m long, is the principal site of nutrient absorption, while the ileum, about 3.5 m, absorbs vitamin B12 and bile salts. Throughout the small intestine, villi and microvilli project into the lumen, dramatically increasing absorptive surface area.
The accessory digestive organs include the salivary glands, the liver, the gallbladder, and the pancreas. The liver is the largest gland in the body, with right and left lobes organized functionally into eight segments, and its basic structural units are lobules of hepatocytes arranged around central veins. The porta hepatis serves as the entry point for vessels and bile ducts, while Glisson's capsule covers the organ. The liver produces bile that emulsifies fats, the gallbladder stores and concentrates bile between meals, and the pancreas secretes digestive enzymes and hormones such as insulin and glucagon into the duodenum.
The urinary system filters blood and eliminates waste while regulating fluid and electrolyte balance. Each kidney has an outer cortex housing the renal corpuscles and an inner medulla organized into renal pyramids that drain into minor and major calyces, all funneling into a central renal pelvis that connects to the ureter. The renal hilum is the entry and exit point for vessels and ureter, and surrounding adipose tissue and a tough renal capsule protect the organ. The nephron is the microscopic functional unit, numbering over a million per kidney. Each nephron begins with a renal corpuscle formed by the glomerulus inside Bowman's capsule, continues through the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule, and ends by joining a collecting duct. Urine formation involves three sequential processes: filtration at the glomerulus driven by blood pressure, reabsorption of most of the water, glucose, and useful solutes along the nephron (with about 65 percent returning in the proximal tubule), and secretion of wastes especially in the distal tubule, with the loop of Henle establishing a medullary concentration gradient via countercurrent multiplication. From the kidneys, urine flows through the ureters to the urinary bladder for storage and exits the body through the urethra, with micturition controlled by a reflex coordinated with voluntary sphincter activity.