16.3 Digestive and Excretory Systems

The digestive system breaks down food into usable nutrients. It includes organs like the stomach and intestines, which work together to process what we eat. Enzymes play a crucial role, breaking complex molecules into simpler forms our bodies can absorb.

The excretory system removes waste and maintains balance in our bodies. Kidneys are the stars here, filtering blood and producing urine. They're made up of tiny units called nephrons that do the heavy lifting of waste removal and water balance.

Digestive System

Digestive Tract and Enzymes

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• The digestive system consists of the gastrointestinal tract (mouth, esophagus, stomach, small intestine, large intestine, rectum, anus) and accessory organs (salivary glands, liver, gallbladder, pancreas)
• The digestive tract is a long, continuous tube that breaks down food into smaller molecules that can be absorbed and used by the body
• Enzymes are proteins that act as biological catalysts to speed up chemical reactions in the body
• Digestive enzymes break down macromolecules (carbohydrates, proteins, lipids) into smaller molecules that can be absorbed by the body
• Examples of digestive enzymes include amylase (breaks down carbohydrates), pepsin (breaks down proteins), and lipase (breaks down lipids)

Stomach and Small Intestine

• The stomach is a muscular organ that stores and breaks down food using strong muscular contractions and acidic gastric juices
• The stomach lining secretes hydrochloric acid and pepsinogen (inactive form of pepsin) which help to break down food and kill bacteria
• The small intestine is a long, narrow tube (about 20 feet long) where most of the digestion and absorption of nutrients occurs
• The small intestine has three sections: duodenum (receives chyme from stomach and secretions from liver and pancreas), jejunum (middle section where most nutrient absorption occurs), and ileum (final section that absorbs remaining nutrients and bile salts)
• The inner lining of the small intestine has many finger-like projections called villi which increase the surface area for nutrient absorption (microvilli on the surface of each villus further increase surface area)

Large Intestine and Liver

• The large intestine (colon) is a wider, shorter tube (about 5 feet long) that absorbs water and electrolytes from the remaining digestive material and forms solid waste (feces)
• The large intestine has four sections: cecum (connects to ileum and appendix), ascending colon, transverse colon, and descending colon (which leads to the rectum and anus)
• The liver is the largest internal organ and has many important functions, including producing bile (helps digest fats), storing glucose as glycogen, and detoxifying harmful substances
• The liver receives blood from the digestive tract via the hepatic portal vein, processes the nutrients and toxins, and sends the filtered blood to the heart via the hepatic veins
• The gallbladder stores and concentrates bile produced by the liver, and releases it into the duodenum when needed to digest fatty foods

Excretory System

Kidneys and Nephrons

• The excretory system removes waste products from the body and maintains homeostasis of water, salt, and pH levels in the blood
• The kidneys are a pair of bean-shaped organs located in the lower back that filter the blood and produce urine
• Each kidney contains about 1 million tiny filtration units called nephrons
• Each nephron consists of a renal corpuscle (glomerulus and Bowman's capsule) and a renal tubule (proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct)
• The glomerulus is a cluster of capillaries that filters the blood under high pressure, allowing small molecules (water, glucose, amino acids, urea) to pass into the Bowman's capsule while retaining larger molecules (proteins, blood cells)

Urine Formation and Homeostasis

• The filtrate from the Bowman's capsule flows through the renal tubule, where useful substances (glucose, amino acids, water, ions) are reabsorbed back into the bloodstream while waste products (urea, excess water and ions) are secreted into the tubule
• The loop of Henle creates a concentration gradient in the medulla of the kidney, allowing the kidney to produce concentrated urine and conserve water
• The distal convoluted tubule and collecting duct are regulated by hormones (antidiuretic hormone, aldosterone) to fine-tune the reabsorption of water and ions based on the body's needs
• The final product, urine, flows from the collecting ducts into the renal pelvis, through the ureters to the bladder, and is excreted from the body through the urethra
• The kidneys help maintain homeostasis by regulating blood volume, blood pressure, electrolyte balance, and acid-base balance through the production of urine and the secretion of hormones (erythropoietin, renin, calcitriol)