The digestive system breaks down food into nutrients your cells can actually use. The excretory system then removes the metabolic waste that builds up and keeps your internal environment stable. These two systems work together to maintain homeostasis: one brings in what the body needs, the other gets rid of what it doesn't.
Digestive System
Digestive Tract and Enzymes
The digestive system has two main components: the gastrointestinal (GI) tract and the accessory organs. The GI tract is one continuous tube running from mouth to anus (mouth → esophagus → stomach → small intestine → large intestine → rectum → anus). The accessory organs, including the salivary glands, liver, gallbladder, and pancreas, aren't part of the tube itself but secrete substances into it that aid digestion.
The key players in chemical digestion are enzymes, proteins that act as biological catalysts to speed up the breakdown of macromolecules. Each enzyme is specific to a particular substrate:
- Amylase breaks down carbohydrates (starch → maltose). It's found in saliva and pancreatic secretions, so carbohydrate digestion actually begins in your mouth.
- Pepsin breaks down proteins into smaller peptides. It works in the highly acidic environment of the stomach.
- Lipase breaks down lipids (fats) into fatty acids and glycerol. Pancreatic lipase does most of this work in the small intestine.
Stomach and Small Intestine
The stomach is a muscular organ that both stores food and begins serious chemical digestion. Its walls churn food through strong muscular contractions (mechanical digestion), while gastric glands in the stomach lining secrete hydrochloric acid (HCl) and pepsinogen. The HCl drops the stomach pH to around 2, which kills most bacteria and activates pepsinogen into its active form, pepsin. The resulting mixture of partially digested food is called chyme.
The small intestine is where most digestion and absorption happen. Despite being only about an inch in diameter, it's roughly 20 feet long, giving it enormous processing capacity. It has three sections:
- Duodenum — receives chyme from the stomach along with bile from the liver/gallbladder and enzymes from the pancreas. Most chemical digestion is completed here.
- Jejunum — the middle section where the majority of nutrient absorption occurs.
- Ileum — the final section, which absorbs remaining nutrients (like vitamin B12) and bile salts for recycling.
The inner wall of the small intestine is covered in finger-like projections called villi, and each villus is further covered in microvilli. This creates a massive surface area for absorption. If you flattened out all the villi and microvilli, the total surface area would be roughly the size of a tennis court.
Large Intestine and Liver
The large intestine (colon) is wider but shorter than the small intestine (about 5 feet). Its primary job is absorbing water and electrolytes from whatever remains after the small intestine has done its work. Bacteria in the colon also produce certain vitamins (like vitamin K) and help ferment undigested material. What's left is compacted into feces. The colon has four main sections: cecum (where the appendix attaches), ascending colon, transverse colon, and descending colon, which leads to the rectum and anus.
The liver is the largest internal organ and performs hundreds of functions. For digestion, the most relevant are:
- Producing bile, which emulsifies fats (breaks large fat globules into smaller droplets so lipase can work more efficiently)
- Storing glucose as glycogen and releasing it when blood sugar drops
- Detoxifying harmful substances like alcohol and drugs
- Receiving nutrient-rich blood directly from the GI tract via the hepatic portal vein, processing those nutrients, and sending filtered blood to the heart through the hepatic veins
The gallbladder stores and concentrates bile between meals. When fatty food enters the duodenum, the gallbladder contracts and releases bile into the small intestine.
Excretory System
Kidneys and Nephrons
The excretory system removes metabolic waste products from the blood and maintains homeostasis of water, salt, and pH levels. The kidneys are a pair of bean-shaped organs in the lower back, and they're the central organs of this system.
Each kidney contains about 1 million nephrons, the functional filtration units. Understanding nephron structure is critical because each part has a distinct role:
- Renal corpuscle — made up of the glomerulus (a ball of capillaries) and Bowman's capsule (a cup-shaped structure surrounding it). Blood enters the glomerulus under high pressure, which forces small molecules (water, glucose, amino acids, urea, ions) through the capillary walls into Bowman's capsule. Large molecules like proteins and blood cells are too big to pass through and stay in the blood.
- Proximal convoluted tubule (PCT) — reabsorbs most of the useful filtered substances (glucose, amino acids, and a large portion of water and ions) back into the blood.
- Loop of Henle — dips down into the kidney's medulla and back up, creating a concentration gradient that allows the kidney to produce concentrated urine.
- Distal convoluted tubule (DCT) — fine-tunes reabsorption and secretion under hormonal control.
- Collecting duct — carries the final urine toward the renal pelvis; also regulated by hormones.
Urine Formation and Homeostasis
Urine formation involves three processes:
- Filtration — occurs at the glomerulus. Blood pressure forces small molecules out of the blood and into Bowman's capsule, producing filtrate.
- Reabsorption — as filtrate flows through the renal tubule, useful substances (glucose, amino acids, water, ions) are transported back into the surrounding capillaries. About 99% of the filtrate is reabsorbed; you don't want to lose all that water and glucose.
- Secretion — additional waste products and excess ions are actively transported from the blood into the tubule. This is how the kidneys fine-tune what ends up in urine.
Two hormones play major roles in regulating water and ion balance:
- Antidiuretic hormone (ADH) — released by the posterior pituitary gland when you're dehydrated. It makes the collecting duct more permeable to water, so more water is reabsorbed and you produce less, more concentrated urine.
- Aldosterone — released by the adrenal cortex. It increases sodium reabsorption in the DCT and collecting duct, and water follows sodium, which raises blood volume and blood pressure.
The final urine flows from the collecting ducts → renal pelvis → ureters → bladder → urethra.
Beyond waste removal, the kidneys help maintain homeostasis in several broader ways: regulating blood volume and blood pressure, maintaining electrolyte balance, controlling acid-base (pH) balance, and secreting hormones like erythropoietin (stimulates red blood cell production), renin (helps regulate blood pressure), and calcitriol (active form of vitamin D, aids calcium absorption).