Introduction: Processing Nutrients and Removing Waste
Welcome to the expanded Lesson 18. This lesson delves deeply into two vital organ systems responsible for maintaining homeostasis: the digestive system, which breaks down food into absorbable nutrients, and the excretory system, which filters blood and removes metabolic wastes. We will journey through the alimentary canal, explore the roles of accessory organs, understand hormonal regulation, and then examine the intricate filtration and balancing act performed by the kidneys.
Part 1: The Digestive System
The digestive system's primary goal is to convert complex macromolecules from food into monomers that can be absorbed and utilized by the body's cells. This involves four main processes: ingestion, digestion (mechanical and chemical), absorption, and elimination.
Diagram: Detailed Overview of the Human Digestive System
1.1 Journey Through the Alimentary Canal
Oral Cavity (Mouth)
Digestion begins here. Mastication (chewing) performs mechanical digestion. Salivary glands release saliva containing salivary amylase (for carbohydrates) and lingual lipase (for fats). Saliva's mucus lubricates food into a bolus.
Pharynx and Esophagus
The bolus is pushed into the pharynx. The epiglottis closes over the trachea to prevent choking. The bolus moves down the esophagus via peristalsis.
Stomach
A muscular sac that churns the bolus with gastric juice. Gastric juice contains hydrochloric acid (HCl), creating a pH of 1.5-3.5 to kill microbes and denature proteins, and pepsin, a protease that begins protein digestion. The stomach lining is protected by a thick mucus layer. The semi-liquid mixture is now called chyme.
Diagram: Stomach Anatomy and Cell Types
Small Intestine
The major site for chemical digestion and nutrient absorption. It is divided into three sections: duodenum, jejunum, and ileum. Its massive surface area is due to circular folds, villi, and microvilli (the "brush border"). In the duodenum, chyme mixes with secretions from the pancreas and liver.
Large Intestine
Primarily absorbs water, electrolytes, and vitamins (like K and B vitamins) produced by gut bacteria. It compacts undigested material into feces for elimination via the rectum and anus.
1.2 Accessory Organs and Major Enzymes
The pancreas, liver, and gallbladder are crucial accessory organs that secrete substances necessary for digestion into the duodenum.
| Enzyme/Secretion | Source | Site of Action | Function |
|---|---|---|---|
| Salivary Amylase | Salivary Glands | Mouth | Digests starch into smaller sugars. |
| Pepsin | Stomach (Chief cells) | Stomach | Digests proteins into smaller polypeptides. |
| Bile | Liver (stored in Gallbladder) | Small Intestine | Emulsifies fats (breaks large globules into small droplets). Not an enzyme. |
| Pancreatic Amylase | Pancreas | Small Intestine | Continues starch digestion. |
| Trypsin/Chymotrypsin | Pancreas | Small Intestine | Continues protein digestion into smaller peptides. |
| Pancreatic Lipase | Pancreas | Small Intestine | Digests emulsified fats into fatty acids and monoglycerides. |
| Peptidases/Disaccharidases | Small Intestine (Brush border) | Small Intestine | Complete protein and carbohydrate digestion into monomers (amino acids, monosaccharides). |
1.3 Hormonal Control of Digestion
Digestion is tightly coordinated by hormones. This is a classic example of a negative feedback system.
- Food in stomach → Gastrin released → Stimulates gastric acid (HCl) production.
- Acidic chyme in duodenum → Secretin released → Stimulates pancreas to release bicarbonate (HCO₃⁻) to neutralize acid.
- Fats/proteins in duodenum → Cholecystokinin (CCK) released → Stimulates gallbladder to release bile and pancreas to release digestive enzymes.
Clinical Correlation: Peptic Ulcers
Peptic ulcers are sores on the lining of the stomach or duodenum. They are most commonly caused by the bacterium Helicobacter pylori, which damages the protective mucous layer, allowing HCl and pepsin to erode the underlying tissue. Long-term use of NSAIDs (like ibuprofen) can also cause ulcers by inhibiting mucus production.
Part 2: The Excretory System
The excretory system, primarily the kidneys, is responsible for filtering blood, removing metabolic wastes (like urea), and maintaining water, salt, and pH balance (osmoregulation and acid-base homeostasis).
2.1 Kidney Structure and the Nephron
The kidneys are the main excretory organs. Each kidney has an outer cortex and an inner medulla. The functional unit is the nephron. There are about one million nephrons per kidney.
Diagram: Detailed Structure of the Nephron
Blood enters the glomerulus via the afferent arteriole and exits via the efferent arteriole. The filtrate passes through the tubule system where its composition is modified.
2.2 The Three Key Functions of the Nephron
Urine is formed through three main processes:
- Glomerular Filtration: Blood pressure forces water and small solutes (salts, glucose, amino acids, urea) from the glomerulus into Bowman's capsule, forming the filtrate. Blood cells and large proteins are too big to pass and remain in the blood.
- Tubular Reabsorption: As the filtrate moves through the tubule, essential substances are reclaimed into the blood. This is a highly selective process.
- Proximal Tubule: Reabsorbs ~65% of water and salt, and 100% of glucose and amino acids.
- Loop of Henle: Creates a salt gradient in the medulla, allowing for concentrated urine to be formed. Water leaves the descending limb, and salt is pumped out of the ascending limb.
- Distal Tubule & Collecting Duct: Fine-tuning of water and salt reabsorption, regulated by hormones.
- Tubular Secretion: Certain waste products (like H⁺, K⁺, and some drugs/toxins) are actively transported from the blood into the filtrate, mainly in the distal tubule. This helps regulate blood pH and eliminate toxins.
2.3 Hormonal Control of Kidney Function
The kidneys' function is fine-tuned by hormones to maintain blood pressure, volume, and osmolarity.
- Antidiuretic Hormone (ADH): Released by the posterior pituitary when the body is dehydrated (high blood osmolarity). It increases the permeability of the collecting duct to water, leading to more water reabsorption and concentrated urine.
- Aldosterone: Part of the Renin-Angiotensin-Aldosterone System (RAAS). It promotes the reabsorption of Na⁺ (and thus water follows by osmosis) in the distal tubule, which increases blood volume and pressure.
The Renin-Angiotensin-Aldosterone System (RAAS)
Interactive Practice Quiz (Expanded)
Test your understanding of the digestive and excretory systems. Choose the best answer for each question (A-E) and then submit to see your results.