Digestive & Excretory Systems: Processing and Waste: Seventh Grade Science
The digestive and excretory systems work together to break down food macromolecules into bioavailable nutrients, absorb them into the bloodstream, and filter metabolic nitrogenous wastes to maintain fluid and electrolyte homeostasis.
The Human Digestive System
The human digestive system consists of the gastrointestinal (GI) tract—a continuous muscular tube extending roughly nine meters from mouth to anus—along with vital accessory organs (salivary glands, liver, gallbladder, and pancreas). Digestion transforms food through two coordinated processes: mechanical digestion (physical breakdown of food into smaller particles) and chemical digestion (enzymatic hydrolysis of polymers into absorbable monomers).
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| ANATOMY OF THE HUMAN DIGESTIVE TRACT |
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| ORAL CAVITY: Teeth masticate; Salivary amylase breaks starch |
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| ESOPHAGUS: Peristaltic smooth muscle contractions push bolus down |
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| STOMACH: Rugae churn food; HCl & Pepsin hydrolyze proteins into chyme |
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| SMALL INTESTINE: Duodenum receives bile & pancreatic enzymes; |
| Jejunum & Ileum absorb nutrients via microvilli into bloodstream |
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| LARGE INTESTINE: Colon reabsorbs water/salts; compacts fecal feces |
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| RECTUM & ANUS: Stores and eliminates solid undigested waste |
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Enzymatic Breakdown of Macromolecules
Enzymes hydrolyze complex dietary polymers into monomers that can cross intestinal cell membranes:
- Carbohydrate Digestion: Begins in the mouth with salivary amylase, which cleaves starches into maltose. In the duodenum, pancreatic amylase continues this process, and brush-border enzymes (maltase, sucrase, lactase) break disaccharides into absorbable monosaccharides like glucose.
- Protein Digestion: Initiates in the acidic environment of the stomach (pH 1.5 - 2.0). Hydrochloric acid denatures coiled proteins and activates pepsinogen into pepsin, which cleaves proteins into smaller polypeptides. In the small intestine, pancreatic proteases (trypsin, chymotrypsin) and peptidases dismantle peptides into free amino acids.
- Lipid Digestion: Fat globules enter the duodenum undissolved in water. The liver synthesizes bile, which is stored in the gallbladder and released into the duodenum. Bile salts emulsify large lipid droplets into micro-droplets, allowing water-soluble pancreatic lipases to hydrolyze triglycerides into fatty acids and monoglycerides.
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| DIGESTIVE ENZYME AND MACROMOLECULE DIRECTORY |
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| Enzyme | Production Source | Site Oper.| Target & Product |
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| Salivary Amylase | Salivary Glands | Mouth | Starch into Maltose|
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| Pepsin | Gastric Chief Cells | Stomach | Protein to Peptides|
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| Pancreatic Lipase | Pancreas (Exocrine) | Duodenum | Lipids into FA + MG|
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| Trypsin | Pancreas (Exocrine) | Duodenum | Peptides to AA |
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The Small Intestine: Maximizing Absorption Surface Area
The small intestine is the primary site of nutrient absorption. It is divided into the duodenum (chemical neutralization and digestion), the jejunum (vigorous nutrient absorption), and the ileum (vitamin B12 and bile salt reabsorption). To maximize absorption efficiency, the interior lining features three nested levels of surface expansion:
- Plicae Circulares: Deep, permanent circular folds in the mucosa that slow the transit of chyme.
- Villi: Finger-like mucosal projections measuring roughly one millimeter in length, each containing an arteriole, a venule, a capillary mesh, and a central lymphatic lacteal.
- Microvilli: Microscopic folds of the plasma membrane along the apical surface of individual enterocyte cells, creating a brush border that expands the total absorbing surface area to roughly 250 square meters.
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| INTESTINAL VILLUS CROSS-SECTION |
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| Enterocyte Brush Border (Microvilli) |
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| | | Capillary Blood Mesh | | |
| | | (Absorbs Glucose & Amino) | | |
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| | | CENTRAL LACTEAL | | |
| | | (Lymphatic: Absorbs Fats) | | |
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The Human Excretory (Urinary) System
While the large intestine compacts and eliminates solid, undigested food fiber, the excretory system eliminates soluble chemical wastes produced by cellular metabolism. The breakdown of amino acids produces toxic ammonia, which the liver converts into less toxic urea. The urinary system filters urea, excess salts, and hydrogen ions from the blood, maintaining water and electrolyte balance.
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| THE HUMAN URINARY TRACT ARCHITECTURE |
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| RENAL ARTERIES: Deliver 20% of resting cardiac output to kidneys |
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| KIDNEYS: Cortex and Medulla contain 1 million nephrons each |
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| URETERS: Smooth muscular tubes transport urine via peristalsis |
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| URINARY BLADDER: Distensible muscular reservoir stores urine |
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| URETHRA: Canal that conveys urine out of body during micturition |
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The Nephron: The Microscopic Functional Engine
Each human kidney contains roughly one million microscopic functional units called nephrons. Urine formation proceeds through three sequential stages:
- Glomerular Filtration: High blood pressure inside the glomerulus (a knotted capillary bed) forces water, glucose, amino acids, ions, and urea across the porous fenestrated capillary wall into Bowman's capsule, producing glomerular filtrate. Blood cells and large proteins are too large to pass and remain in the blood.
- Tubular Reabsorption: As filtrate moves along the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule, epithelial cells actively transport valuable nutrients—nearly 100% of glucose and amino acids, along with roughly 99% of filtered water and sodium—back into peritubular capillaries.
- Tubular Secretion: Capillaries actively pump excess potassium ions, hydrogen ions, ammonium, and drug metabolites directly into the renal tubule for excretion. The remaining concentrated fluid—now termed urine—empties into collecting ducts.
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| THE NEPHRON ARCHITECTURE |
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| Glomerulus & Bowman's Capsule (Filtration) |
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| Proximal Convoluted Tubule (Active Reabsorption of Glucose & Salts) |
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| Loop of Henle (Concentrates filtrate via Medullary Osmotic Gradient) |
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| Distal Tubule & Collecting Duct (Hormonal Fine-Tuning: ADH & Aldost.) |
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Exercises and Solutions
Exercise 1: Tracing the Physiological Fate of Macromolecules. A student consumes a meal consisting of broiled chicken breast (rich in protein) and steamed white rice (rich in starch). Describe the step-by-step chemical and mechanical digestive events that must occur before the amino acids from the chicken and the glucose molecules from the rice can enter systemic blood circulation.
Solution 1: In the oral cavity, teeth masticate both the chicken and rice, increasing their physical surface area. Salivary amylase begins chemically hydrolyzing the cooked rice starches into maltose disaccharides, while the chicken protein undergoes only mechanical mastication. The swallowed bolus travels down the esophagus via peristaltic contractions into the stomach. In the stomach, mechanical churning turns the meal into liquid chyme, while hydrochloric acid denatures the chicken proteins and activates pepsin, which cleaves the long protein chains into shorter polypeptides; salivary amylase is inactivated by the gastric acid. The acidic chyme enters the duodenum of the small intestine, where the pancreas secretes alkaline bicarbonate to neutralize acid, alongside pancreatic amylase, trypsin, and chymotrypsin. Pancreatic amylase finishes breaking down remaining rice starches into maltose, which brush-border maltase splits into individual glucose monomers. Concurrently, pancreatic proteases and brush-border peptidases break the chicken polypeptides down into individual amino acids. Finally, along the jejunum and ileum, both glucose molecules and amino acids are actively transported across the microvilli brush border of enterocytes via sodium-dependent cotransporters, diffusing across the basolateral membrane into villus capillaries to travel through the hepatic portal vein to the liver.
Exercise 2: Evaluating the Endocrine Control of Water Reabsorption by the Nephron. A hiker treks through an arid desert environment for five hours without drinking water, losing significant fluid volume through sweat. Explain how the hypothalamus, the posterior pituitary gland, and the nephrons of the kidneys respond to this dehydration state to conserve body water.
Solution 2: Dehydration caused by sweating decreases blood volume and increases blood solute concentration (osmolarity). Osmoreceptors located within the hypothalamus detect this heightened osmolarity and stimulate the posterior pituitary gland to secrete antidiuretic hormone (ADH, also known as vasopressin) into the bloodstream. ADH circulates to the kidneys and binds to receptors on principal cells lining the distal convoluted tubules and collecting ducts of nephrons. This hormonal binding triggers intracellular vesicles to fuse with the apical plasma membrane, inserting aquaporin-2 water channels into the membrane. Because the surrounding renal medulla possesses a high osmotic salt gradient established by the loops of Henle, water rapidly exits the collecting duct filtrate through the aquaporin channels and re-enters the blood circulation via peritubular capillaries. As a direct physiological result, the kidneys conserve water by producing a very small volume of dark, highly concentrated urine, helping stabilize blood pressure and serum osmolarity until the individual can drink water.