Chapter 3 Notes: The Human Body and Digestion

Organization of the Body

• Atoms are the smallest units of matter; atoms bond to form molecules.
• Molecules are groups of atoms bonded in specific configurations.
  • Examples include H2O and CO2.
• Carbohydrates, proteins, fats, and vitamins are usually very large molecules.
• Goals of digestion:
  • Break large molecules down into smaller molecules.
  • Absorb the smaller molecules into the cells of the body.
• Molecules are the building blocks of cells; cells are the smallest unit of life.
• Digestion products are used to build the cells of the body.

Basic structural organization (from small to large)

• Molecules → Cells → Tissues → Organs → Organ systems.
• Diagram concepts (from page figures):
  • Cells join to form tissues.
  • Tissues form organs (e.g., stomach, heart, brain).
  • Organs form organ systems (e.g., urinary system).

Cell membrane and cellular components

• Cell membrane: outer layer enclosing each cell.
  • Composed of two layers of phospholipids.
  • Long lipid tails face interior; phosphate heads line interior and exterior surfaces.
  • Cholesterol and proteins are embedded in the membrane.
• The cell membrane is selectively permeable: it controls the passage of materials into and out of the cell.
• Inside the cell:
  • Cytoplasm: the liquid within the cell.
  • Organelles: tiny structures that perform cellular functions (examples include the nucleus and mitochondria).

Organization of the Body (cont.)

• Tissues: groups of cells acting together to perform a common function.
• Four types of tissue:
  • Muscle tissue
  • Nervous tissue
  • Epithelial tissue
  • Connective tissue

Organization of the Body (cont.)

• Organs are organized tissues that perform specific functions (e.g., stomach, heart, brain).
• Organ systems are groups of organs working together for a particular function (e.g., urinary system).

Why Do We Want to Eat?

• Appetite: a desire to eat stimulated by.
  • Sight, smell, thought of food, emotions.
• Hunger: a physiologic drive to eat that occurs when the body senses it needs food.

Appetite and Hunger (cont.)

• The hypothalamus contains:
  • Feeding (hunger) center.
  • Satiety (fullness) center.
• Nerve signals from the stomach and small intestine convey food presence to the hypothalamus.
• Hormones relay messages to the hypothalamus.
• Amount and type of food influence satiety.

The Hypothalamus Triggers Hunger

• Diagram cues: Hypothalamus and pituitary gland involved in signaling.

Appetite and Hunger (cont.)

• Signals prompting eating include:
  • Nerve receptors in the stomach indicating fullness or emptiness.
  • Blood glucose levels triggering hormones insulin and glucagon.

Hormones and Satiety/Cues

• Hormones that regulate hunger and fullness:
  • Hunger-promoting: ext{Ghrelin}.
  • Satiety-promoting: ext{Cholecystokinin (CCK)}, ext{Leptin}.

Foods and Hunger/Satiety

• Foods have different effects on hunger/satiety:
  • Proteins have the highest satiety value.
  • Carbohydrates have lower satiety than fats.
  • Bulky foods provide a sense of fullness.
  • Solid foods are more filling than semisolid or liquid foods.

What Happens to the Food We Eat?

• GI tract: a long tube through which food passes; includes organs and sphincters that regulate passage between organs.
• Involves ingestion, mechanical digestion, chemical digestion, propulsion, absorption, and elimination.

Digestive System Overview

• The digestive system includes GI tract organs and accessory organs.
• Processes: ingestion, mechanical digestion, chemical digestion, propulsion, absorption, elimination.
• Key GI organs (with examples):
  • Mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, anus.
• Accessory organs:
  • Salivary glands
  • Liver (produces bile)
  • Gallbladder (stores bile)
  • Pancreas (produces digestive enzymes and bicarbonate)

Digestive System: Mouth

• Ingestion: food enters via the mouth.
• Mechanical digestion: mastication tears, shreds, and mixes food with saliva.
• Chemical digestion: saliva contains enzymes; salivary amylase begins carbohydrate breakdown.
• Salivary glands produce saliva.
• Basic anatomy: pharynx and esophagus begin propulsion via swallowing.

Digestion: The Mouth (cont.)

• Components involved in chewing and swallowing include:
  • Bolus of food, teeth, tongue, soft palate, epiglottis, upper esophageal sphincter.
  • Movement through esophagus via peristalsis; epiglottis covers trachea during swallowing; esophagus and trachea roles visualized in diagrams.

Digestion: The Mouth (cont.)

• Summary: chewing initiates mechanical digestion; salivary amylase begins chemical breakdown of carbohydrates; minimal digestion occurs in the mouth.

Digestion: The Esophagus and Peristalsis

• Esophagus propels food to the stomach via peristalsis (waves of muscular contractions).
• No digestion occurs in the esophagus.
• The gastroesophageal sphincter separates the esophagus from the stomach.

Digestion: Peristalsis

• Peristalsis: successive waves of contracting muscles move food toward the stomach.
• Esophagus transitions content to the stomach; example figure shows peristaltic movement.

Digestion: Stomach

• The stomach mixes, digests, and stores food.
• Digestive processes in the stomach include:
  • Extensive mechanical digestion to mix food with gastric juice.
  • Chemical digestion of proteins and fats.

Digestion: Stomach (cont.)

• Gastric juice contents:
  • ext{HCl} (hydrochloric acid) to denature proteins and activate pepsin.
  • Intrinsic factor, a protein critical for absorption of ext{vitamin B}_{12}.
  • Pepsin: enzyme to digest protein.
  • Gastric lipase: enzyme to digest fat.
• Chyme: semisolid product of mechanical and chemical digestion in the stomach.

Digestion: Stomach (cont.)

• Anatomy references:
  • Gastroesophageal sphincter, pyloric sphincter.
  • Stomach with mechanical digestion and gastric juice secretion.
  • Pepsin initiates protein digestion; gastric lipase initiates lipid digestion.
  • Small amounts of water, minerals, drugs, and alcohol absorbed in the stomach.

Hydrochloric Acid on the pH Scale

• pH scale: ranges from 0 (very acidic) to 14 (very basic).
• pH 7 is neutral.
• Common substances:
  • Battery acid (very acidic) near pH 0.
  • Pure water around pH 7.
  • Gastric juice is acidic; HCl is a strong acid.
• Visual: scale from 0 to 14 showing relative acidity/basicity.

Digestion: Small Intestine

• From the stomach, chyme is slowly released through the pyloric sphincter into the small intestine.
• Chemical digestion continues here with pancreatic enzymes and bile.

Digestion: Large Intestine

• Undigested residues move to the large intestine via the ileocecal valve.
• In the large intestine:
  • Very little digestion occurs.
  • Water and some nutrients are absorbed.
  • Material is stored 12–24 hours prior to elimination.
  • Bacteria digest remaining food particles.
  • Propulsion concentrates waste into feces and propels toward the rectum.

Elimination

• Path: large intestine → rectum → anal canal → anus.
• Feces excreted; water and chemicals absorbed along the way.

Digestive Tract Accessory Organs

• Surrounding organs:
  • Salivary glands
  • Liver: produces bile to emulsify fats.
  • Pancreas: produces digestive enzymes and bicarbonate; released into the small intestine via the pancreatic duct.
  • Gallbladder: stores bile.

Absorption

• Absorption: process of taking molecules across a cell membrane into body cells.
• A small amount occurs in the stomach; most occurs in the three sections of the small intestine:
  • Duodenum
  • Jejunum
  • Ileum

Absorption (cont.)

• The small intestine length is about 20\,\text{ft} and its wall has three features that increase surface area:
  • Circular folds
  • Villi (finger-like projections)
  • Microvilli (brush border)
• Each villus contains capillaries and a lacteal for transporting absorbed nutrients.
• The brush border is formed by microvilli at the end of enterocytes.
• The small intestine increases surface area by more than a factor of 600.

Absorption (cont.)

• Nutrient pathways:
  • Water-soluble nutrients (carbohydrates, proteins, minerals, some vitamins) enter the portal vein and are transported to the liver.
  • Fat-soluble nutrients (lipids and some vitamins) enter lymphatic vessels and are transported directly to the bloodstream.

Transport by Blood and Lymph

• Lymphatic capillaries transport absorbed fats via the lymphatic system; eventually entering the bloodstream.
• The portal vein delivers water-soluble nutrients to the liver.
• Blood and lymphatic circulation permits nutrients to reach the heart and systemic circulation.

Absorption (cont.)

• Mechanisms of absorption across the mucosal membrane into cells:
  • Passive diffusion
  • Facilitated diffusion
  • Active transport
  • Endocytosis

Absorption (cont.)

• Illustrations of absorption mechanisms show:
  • (a) Passive diffusion: nutrient moves down a concentration gradient without energy.
  • (b) Facilitated diffusion: carrier proteins assist movement down gradient without energy.
  • (c) Active transport: carrier proteins move nutrients against gradient using energy (ATP).
  • (d) Endocytosis: cells engulf nutrients.

The Role of the Neuromuscular System

• Two components regulate GI tract activities:
  • Muscles of the GI tract (both voluntary and involuntary) mix and move food.
  • Nerves control contractions and secretions, including:
  • Enteric nervous system (ENS)
  • Other branches of the autonomic nervous system (ANS)
  • Central nervous system (CNS)

GI Muscles

• Types of motility:
  • Peristalsis: coordinated contractions moving food along the tract.
  • Segmentation: rhythmic contractions that mix chyme with digestive juices.

GI Tract Disorders

• The stomach lining is protected from acid, but other GI regions are more vulnerable.
• Heartburn is caused by hydrochloric acid in the esophagus.
• GERD (gastroesophageal reflux disease): chronic condition with painful, persistent heartburn as a primary symptom.

Heartburn

• Diagrammatic description: diaphragm, esophagus, stomach, and gastric juice reflux causing burning sensation.

GI Tract Disorders (cont.)

• Peptic ulcers are regions eroded by HCl and pepsin; Helicobacter pylori contributes to gastric and duodenal ulcers.

Peptic Ulcer

• Visual representation of ulceration due to gastric juice activity.

GI Tract Disorders (cont.)

• Vomiting is often associated with GI infections (e.g., norovirus).
• Cyclic vomiting syndrome (CVS): chronic condition with severe nausea and vomiting lasting hours or days.

GI Tract Disorders (cont.)

• Diarrhea: causes include food intolerances, GI infections, stress, bowel disorders; can cause severe dehydration; risk is higher for children and the elderly.
• Constipation: defined as no stool passage for two or more days.

Signs and Symptoms of Dehydration

• Adults: thirst, light-headedness, less frequent urination, dark-colored urine, fatigue, dry skin.
• Children: dry mouth and tongue, no tears when crying, no wet diapers for 3+ hours, high fever, sunken abdomen/eyes/cheeks, irritability/listlessness, skin recoil delayed (poor skin tounce).
• Data adapted from: Diarrhea, National Digestive Diseases Information Clearinghouse (NIDDK).

GI Tract Disorders (cont.)

• Irritable bowel syndrome (IBS): a disorder that interferes with normal colon function.
  • Symptoms: abdominal cramps, bloating, diarrhea or constipation.
  • IBS more common in women than in men.

GI Tract Disorders (cont.)

• Cancer can develop anywhere in the GI tract; most common forms include:
  • Oral cancer
  • Pancreatic cancer
  • Colorectal cancer

In Depth: Disorders Related to Foods

• Food intolerance: a particular food causes unpleasant symptoms (gas, pain, diarrhea); immune system not involved.
• Food allergy: hypersensitivity reaction of the immune system to a food component.

In Depth: Disorders Related to Foods (cont.)

• Celiac disease: autoimmune and genetic; complete intolerance to gluten (a protein found in wheat, rye, barley, and triticale);
  • Can damage the small intestine, leading to poor nutrient absorption.
  • Requires a diet lacking wheat, rye, barley, and triticale.

Non-Celiac Gluten Sensitivity

• Some individuals have negative GI reactions to gluten but do not have Celiac Disease.
• Symptoms: bloating, abdominal pain, diarrhea, possible joint pain.
• Symptoms improve on a gluten-free diet.