Digestive and Excretory Systems: Structure and Function

The Digestive System

The digestive system is made up of the organs that break down food. Nutrients cannot be used directly by cells, so food must be transformed into simple substances that the body can absorb.

The Gastrointestinal Tract

The gastrointestinal tract is a tube about 10 meters long, with walls of muscle, that runs from the mouth to the anus. Along this tract, different parts perform different functions.

• Mouth: Digestion begins here. Teeth grind food, the tongue mixes it with saliva, and the result is a mass that is swallowed.
• Pharynx: A cavity that connects the mouth to the esophagus.
• Esophagus: A tube of about 25 cm that conducts food down to the stomach.
• Stomach: A muscular, expandable organ with a capacity of about 2.5 liters. When empty, its folds disappear. It has a cardia and a pylorus valve at its entrance and exit.
• Small intestine: A thin tube of 7 to 8 meters, divided into three regions: the duodenum, the jejunum and the ileum. It connects the stomach to the large intestine.
• Large intestine: A tube of about 1.5 meters. It is wider than the small intestine and is divided into three regions by the ileocecal valve. It includes the cecum, the appendix, the colon and the rectum. It ends in the anus.

Digestive Glands

Digestive glands are organs that produce digestive juices, which break down food. They are classified into two groups:

• Glands of the digestive tube: These are found within the walls of the digestive tube and secrete their products directly into its cavities. They include the gastric glands and the intestinal glands.
• Accessory glands: These lie adjacent to the digestive tube and send their secretions through ducts. They include the salivary glands, the pancreas and the liver.

The liver is a large gland that produces bile. Bile is stored in the gallbladder and poured into the small intestine, where it emulsifies lipid droplets. The liver also helps metabolize carbohydrates, stores iron and vitamins, and contributes to removing toxins and alcohol from the blood.

The salivary glands are three pairs of glands that release saliva into the mouth, where digestion of complex carbohydrates begins.

The gastric glands produce gastric juice, which digests proteins.

The intestinal glands produce intestinal juices that digest carbohydrates, fats and proteins.

The pancreas is a gland shaped like a flat leaf, located under the stomach. It produces pancreatic juice, which pours into the small intestine and digests carbohydrates, proteins and fats. It also secretes insulin and glucagon, hormones that regulate blood glucose.

Digestion Process

Digestion is the transformation of food inside the digestive tube into nutrients that cells can use directly. It involves two types of process:

• Mechanical digestion: Physical actions that reduce the size of food and modify its texture, without changing its chemical composition. Examples include crushing in the mouth by the teeth, and swallowing, which moves food from the mouth to the esophagus and then to the pharynx.
• Peristalsis: Rhythmic contractions of the muscles of the digestive tube that push food along the tract.
• Chemical digestion: Digestive juices and enzymes break large molecules into small ones that can be absorbed.
  • Amylase (mouth), from saliva, starts the digestion of complex carbohydrates and converts them into simple sugars.
  • Hydrochloric acid and pepsin (stomach), from gastric juice, begin the digestion of proteins, which are broken into smaller amino acid chains.
  • Lipase (small intestine) breaks fats into fatty acids and glycerol. Together with other enzymes, it completes the digestion of carbohydrates and proteins.

Absorption is the passage of nutrients from the digestive tract into the blood, which takes place mainly in the small intestine. Nutrients are then carried to all body cells.

The Excretory System

The excretory system eliminates substances that cells no longer need, a process called excretion. These waste substances are dissolved in water and expelled from the body through urine, sweat and other routes. The main organs of this system are the kidneys.

• Renal cortex: The outer region of the kidney, which has a granular appearance.
• Renal artery: Carries blood to the kidneys.
• Renal vein: Carries blood away from the kidneys.
• Renal medulla: The inner region, formed by sections shaped like pyramids.
• Urinary bladder: A muscular sac that stores urine. It can hold up to about 200 cm³ and expands as it fills, producing the urge to urinate.
• Kidneys: Two organs located in the back of the abdomen, one on each side of the spine.
• Ureters: Two tubes that carry urine from each kidney to the bladder.
• Urethra: The tube that carries urine from the bladder to the outside of the body.

How the Kidneys Work

The kidneys filter the blood and form urine. Each kidney contains about one million functional units called nephrons. Each nephron consists of a capsule that surrounds a cluster of blood capillaries, and a long, twisted tube that ends in a collecting duct that drains into the ureter.

Urine formation begins in the capsule of the nephron. Blood flowing through the capillaries is filtered through their thin walls into the capsule. This filtered liquid contains water, mineral salts, glucose and urea, and then passes through the tubule, where useful substances are reabsorbed and waste is kept for elimination.