ANP Study Guide: Homeostasis, Endocrine System, Digestive System, and Metabolism

Introduction to Homeostasis

Homeostasis and Homeostatic Imbalance

Homeostasis refers to the body's ability to maintain a stable internal environment despite changes in external conditions. Disruptions in homeostasis can lead to a state known as homeostatic imbalance, which may result in disease or dysfunction.

• Homeostasis: The maintenance of relatively constant internal conditions (e.g., temperature, pH, glucose levels).

• Homeostatic Imbalance: A disturbance in homeostasis, often leading to illness or disease (e.g., diabetes, dehydration).

Feedback Mechanisms

Feedback mechanisms are processes that help maintain homeostasis by detecting changes and initiating responses.

• Negative Feedback: The most common mechanism; the response reduces or shuts off the original stimulus. Example: Regulation of body temperature, blood glucose levels.

• Positive Feedback: The response enhances or exaggerates the original stimulus. Example: Blood clotting, labor contractions.

Systems Involved in Feedback

• Nervous System: Provides rapid, short-term, and targeted responses (e.g., muscle contraction, gland secretion).

• Endocrine System: Provides slower, long-lasting, and widespread responses via hormones (e.g., growth, metabolism).

The Endocrine System

Endocrine vs. Exocrine Glands

• Endocrine Glands: Ductless glands that secrete hormones directly into the bloodstream (e.g., thyroid, pituitary).

• Exocrine Glands: Glands that secrete substances through ducts to an epithelial surface (e.g., sweat, salivary glands).

Hormones and Receptors

• Hormone: A chemical messenger secreted by endocrine glands that regulates physiological activities.

• Types of Hormonal Receptors:

  • Membrane-bound receptors: Bind amino acid-based hormones (e.g., insulin).

  • Intracellular receptors: Bind steroid hormones (e.g., cortisol) and thyroid hormones.

• Hormone Effects on Cells: Can alter membrane permeability, stimulate or inhibit protein synthesis, activate or deactivate enzymes, induce secretory activity, or stimulate mitosis.

Gland Stimulation Mechanisms

• Hormonal: Hormones stimulate other endocrine glands (e.g., pituitary hormones stimulating the thyroid).

• Humoral: Changes in blood levels of ions or nutrients trigger hormone release (e.g., insulin release in response to blood glucose).

• Neural: Nerve fibers stimulate hormone release (e.g., adrenal medulla releasing epinephrine).

Hormone Interactions

• Permissiveness: One hormone cannot exert its effects without another hormone being present.

• Synergism: More than one hormone produces the same effects on a target cell, and their combined effects are amplified.

• Antagonism: One hormone opposes the action of another hormone.

Major Endocrine Glands and Hormones

Gland	Hormones Secreted	Main Effects	Control of Secretion
Hypothalamus & Neurohypophysis	Oxytocin, Antidiuretic hormone (ADH)	Uterine contraction, water retention	Neural
Adenohypophysis (Anterior Pituitary)	GH, TSH, ACTH, FSH, LH, Prolactin	Growth, metabolism, stress response, reproduction	Hormonal
Thyroid	T3, T4, Calcitonin	Metabolism, calcium regulation	Hormonal, Humoral
Parathyroid	Parathyroid hormone (PTH)	Increases blood calcium	Humoral
Adrenal Cortex	Aldosterone, Cortisol, Androgens	Electrolyte balance, stress response, sex hormones	Hormonal
Adrenal Medulla	Epinephrine, Norepinephrine	Fight-or-flight response	Neural
Pineal	Melatonin	Regulates sleep-wake cycles	Neural
Pancreas	Glucagon, Insulin	Regulates blood glucose	Humoral
Ovaries	Estrogens, Progesterone	Female reproductive functions	Hormonal
Testes	Testosterone	Male reproductive functions	Hormonal
Thymus	Thymosins	T cell development	Unknown/varied

Diabetes and Homeostatic Imbalances

• Type 1 Diabetes: Autoimmune destruction of beta cells; insulin deficiency.

• Type 2 Diabetes: Insulin resistance; often associated with obesity.

• Other Imbalances: Hyperthyroidism, hypothyroidism, Addison's disease, Cushing's syndrome.

The Digestive System

General Functions and Divisions

The digestive system breaks down food, absorbs nutrients, and eliminates waste. It is divided into the alimentary canal (GI tract) and accessory organs.

• Alimentary Canal: Mouth, pharynx, esophagus, stomach, small intestine, large intestine, anus.

• Accessory Organs: Teeth, tongue, salivary glands, liver, pancreas, gallbladder.

Six Essential Activities of Digestion

1. Ingestion: Taking food into the mouth.

2. Propulsion: Moving food through the GI tract (includes swallowing and peristalsis).

3. Mechanical Breakdown: Chewing, mixing, and segmentation.

4. Digestion: Enzymatic breakdown of food.

5. Absorption: Transport of nutrients into blood or lymph.

6. Defecation: Elimination of indigestible substances.

Peritoneum and Related Terms

• Peritoneum: A serous membrane lining the abdominal cavity and covering abdominal organs.

• Visceral Peritoneum: Covers organs.

• Parietal Peritoneum: Lines the abdominal wall.

• Mesentery: Double layer of peritoneum that supports organs.

Layers of the GI Tract

Layer	Location	Function
Mucosa	Innermost	Secretion, absorption, protection
Submucosa	External to mucosa	Blood vessels, nerves, lymphatics
Muscularis Externa	External to submucosa	Segmentation, peristalsis
Serosa	Outermost	Protective outer layer

Mouth

• Buccal Cavity: Oral cavity; entry to the digestive tract.

• Palate: Divided into hard and soft palate.

• Tongue: Assists in mixing food and swallowing.

• Salivary Glands: Parotid, submandibular, sublingual; secrete saliva.

• Teeth: Incisors, canines, premolars, molars; mechanical breakdown of food.

• Saliva Contents: Water, electrolytes, salivary amylase, mucin, lysozyme, IgA.

• Mastication: Chewing; mechanical breakdown of food.

Pharynx and Esophagus

• Deglutition: Swallowing; involves voluntary and involuntary phases.

• Peristalsis: Wave-like muscle contractions that move food through the GI tract.

Stomach

• Muscularis: Has an extra oblique muscle layer for churning food.

• Mucosa: Contains gastric pits and glands.

• Gastric Gland Cells: Parietal (HCl), chief (pepsinogen), mucous, enteroendocrine (gastrin).

• Chemical Digestion: Proteins begin digestion in the stomach.

Liver

• Functions: Metabolism, detoxification, storage, bile production.

• Bile: Produced by hepatocytes; contains bile salts, bilirubin, cholesterol.

Pancreas

• Exocrine Function: Acinar cells produce digestive enzymes.

• Pancreatic Juice: Contains enzymes (amylase, lipase, proteases) and bicarbonate.

• Enzymes: Amylase (carbohydrates), lipase (fats), trypsin/chymotrypsin (proteins).

Small Intestine

• Modifications for Absorption: Circular folds, villi, microvilli increase surface area.

• Cells in Villi and Crypts: Enterocytes (absorption), goblet cells (mucus), enteroendocrine cells (hormones), Paneth cells (antimicrobial).

Large Intestine

• Subdivisions: Cecum, colon (ascending, transverse, descending, sigmoid), rectum, anus.

• Digestive Processes: Absorption of water and electrolytes, formation and elimination of feces, bacterial fermentation.

Metabolism

Categories of Nutrients

• Carbohydrates

• Lipids

• Proteins

• Vitamins

• Minerals

• Water

Anabolism and Catabolism

• Anabolism: Synthesis of complex molecules from simpler ones (requires energy).

• Catabolism: Breakdown of complex molecules into simpler ones (releases energy).

Enzymes

• Definition: Biological catalysts that speed up chemical reactions without being consumed.

• Importance: Lower activation energy, regulate metabolic pathways.

Adenosine Triphosphate (ATP)

• ATP: The primary energy carrier in cells.

• Structure: Adenine, ribose, and three phosphate groups.

• Equation: $\mathrm{ATP} \rightarrow \mathrm{ADP} + \mathrm{P_i} + \text{energy}$

ATP Production from Glucose

• Glycolysis: Produces 2 ATP per glucose molecule.

• Total ATP (Glycolysis + Krebs Cycle + Electron Transport): 30-32 ATP per glucose molecule.

Oxygen and Carbon Dioxide in Metabolism

• Oxygen: Required as the final electron acceptor in the electron transport chain.

• Carbon Dioxide: Produced during the Krebs cycle as a waste product of cellular respiration.

Nutrient Absorption States

• Absorptive State: Period during and shortly after eating; nutrients are absorbed and stored.

• Postabsorptive State: Period when the GI tract is empty; energy is supplied by the breakdown of body reserves.

Factors Influencing Metabolic Rate

• Age

• Sex

• Body temperature

• Thyroid hormone levels

• Physical activity

Example: During exercise, metabolic rate increases due to increased energy demand and heat production.