2.3 Digestive, Endocrine & Nervous Systems
Key Takeaways
- The digestive tract pathway is: mouth → esophagus → stomach → small intestine (duodenum, jejunum, ileum) → large intestine → rectum → anus
- Accessory digestive organs include the liver (bile production, detoxification), gallbladder (bile storage), and pancreas (enzymes and insulin)
- The endocrine system uses hormones to regulate metabolism, growth, and reproduction; the pituitary gland is called the "master gland"
- Insulin (from pancreatic beta cells) lowers blood glucose; glucagon (from alpha cells) raises blood glucose — imbalance causes diabetes mellitus
- The nervous system is divided into CNS (brain + spinal cord) and PNS (cranial + spinal nerves); autonomic division controls involuntary functions
- The sympathetic nervous system triggers "fight or flight" responses; the parasympathetic system promotes "rest and digest" functions
Last updated: March 2026
Digestive, Endocrine & Nervous Systems
Digestive (Gastrointestinal) System
GI Tract Pathway: Mouth → pharynx → esophagus → stomach → small intestine → large intestine → rectum → anus
| Organ | Function | Key Details |
|---|---|---|
| Mouth | Mechanical digestion (chewing), chemical digestion begins | Salivary amylase breaks down starches |
| Esophagus | Transports food to stomach | Peristalsis (wave-like contractions); lower esophageal sphincter prevents reflux |
| Stomach | Chemical digestion; protein breakdown | HCl acid, pepsin; churning mixes food into chyme; pH 1.5-3.5 |
| Small intestine | Major site of digestion and absorption | Duodenum (bile + enzymes), jejunum (primary absorption), ileum (B12, bile salts) |
| Large intestine | Water absorption, feces formation | Ascending → transverse → descending → sigmoid colon |
| Rectum/anus | Storage and elimination | Voluntary and involuntary sphincters |
Accessory Organs:
| Organ | Function |
|---|---|
| Liver | Produces bile (emulsifies fat); detoxifies blood; stores glycogen; produces clotting factors; metabolizes drugs |
| Gallbladder | Stores and concentrates bile; releases bile into duodenum when fat is present |
| Pancreas | Produces digestive enzymes (lipase, amylase, trypsin); produces insulin and glucagon (endocrine function) |
Key Digestive Enzymes:
| Enzyme | Source | Substrate | Product |
|---|---|---|---|
| Salivary amylase | Salivary glands | Starch | Maltose |
| Pepsin | Stomach | Protein | Peptides |
| Pancreatic lipase | Pancreas | Fat | Fatty acids + glycerol |
| Trypsin | Pancreas | Protein | Amino acids |
| Bile | Liver (stored in gallbladder) | Fat globules | Emulsified fat (smaller droplets) |
Common GI Conditions:
- GERD — Gastroesophageal reflux disease; stomach acid flows back into esophagus
- Peptic ulcer disease — Erosion of stomach or duodenal lining
- Crohn's disease — Inflammatory bowel disease affecting any part of GI tract
- Ulcerative colitis — Inflammatory bowel disease affecting the colon and rectum
- Diverticulitis — Inflammation of small pouches (diverticula) in the colon wall
- Celiac disease — Autoimmune reaction to gluten damaging the small intestine
Endocrine System
The endocrine system produces hormones that regulate metabolism, growth, reproduction, and homeostasis through chemical signaling.
Major Endocrine Glands and Hormones:
| Gland | Hormones | Functions |
|---|---|---|
| Pituitary (anterior) | Growth hormone (GH), TSH, ACTH, FSH, LH, prolactin | "Master gland" — regulates other endocrine glands |
| Pituitary (posterior) | ADH, oxytocin | Water balance; uterine contractions; milk ejection |
| Thyroid | T3, T4, calcitonin | Metabolism regulation; calcium lowering |
| Parathyroid | Parathyroid hormone (PTH) | Raises blood calcium levels |
| Adrenal cortex | Cortisol, aldosterone, androgens | Stress response; sodium/water retention; sex hormones |
| Adrenal medulla | Epinephrine (adrenaline), norepinephrine | Fight-or-flight response |
| Pancreas (islets) | Insulin (beta cells), glucagon (alpha cells) | Blood glucose regulation |
| Ovaries | Estrogen, progesterone | Female sexual development; menstrual cycle |
| Testes | Testosterone | Male sexual development; sperm production |
| Pineal | Melatonin | Sleep-wake cycle regulation |
| Thymus | Thymosin | T-cell maturation (immune function) |
Glucose Regulation:
- Insulin (beta cells) — Lowers blood glucose by facilitating cellular uptake and glycogen storage
- Glucagon (alpha cells) — Raises blood glucose by stimulating glycogen breakdown in the liver
- Type 1 Diabetes — Autoimmune destruction of beta cells; requires insulin injections
- Type 2 Diabetes — Insulin resistance; treated with oral medications and/or insulin
Common Endocrine Conditions:
| Condition | Description | Key Features |
|---|---|---|
| Hypothyroidism | Underactive thyroid | Fatigue, weight gain, cold intolerance, slow heart rate |
| Hyperthyroidism | Overactive thyroid | Weight loss, heat intolerance, rapid heart rate, tremors |
| Diabetes mellitus | Impaired glucose regulation | Polyuria, polydipsia, polyphagia (the "3 Ps") |
| Cushing syndrome | Excess cortisol | Moon face, buffalo hump, weight gain, hypertension |
| Addison disease | Adrenal insufficiency | Fatigue, weight loss, hypotension, skin darkening |
Nervous System
Divisions:
| Division | Components | Function |
|---|---|---|
| Central Nervous System (CNS) | Brain + spinal cord | Processing, integration, coordination |
| Peripheral Nervous System (PNS) | 12 cranial nerves + 31 spinal nerves | Sensory input, motor output |
| Somatic NS | Voluntary motor nerves | Controls skeletal muscles (conscious movement) |
| Autonomic NS | Involuntary motor nerves | Controls organs, glands, smooth muscle |
| Sympathetic | "Fight or flight" | Increases heart rate, dilates pupils, diverts blood to muscles |
| Parasympathetic | "Rest and digest" | Decreases heart rate, constricts pupils, stimulates digestion |
Neuron Structure:
- Dendrites — Receive incoming signals
- Cell body — Contains the nucleus; processes information
- Axon — Transmits signal away from the cell body
- Myelin sheath — Insulates the axon; speeds signal transmission
- Synapse — Gap between neurons where neurotransmitters are released
- Neurotransmitters — Chemical messengers (acetylcholine, serotonin, dopamine, norepinephrine)
Common Nervous System Conditions:
- Alzheimer disease — Progressive dementia; memory loss and cognitive decline
- Parkinson disease — Dopamine deficiency; tremor, rigidity, bradykinesia
- Multiple sclerosis — Demyelination of CNS nerves; weakness, numbness, vision problems
- Epilepsy — Seizure disorder due to abnormal electrical activity in the brain
- Stroke (CVA) — Interrupted blood flow to the brain; can be ischemic or hemorrhagic
- Meningitis — Infection/inflammation of the meninges (membranes covering brain and spinal cord)
Test Your Knowledge
Which organ is the primary site of nutrient absorption in the digestive system?
A
B
C
D
Test Your Knowledge
Which hormone LOWERS blood glucose levels?
A
B
C
D
Test Your Knowledge
The sympathetic nervous system is often described as the "fight or flight" response. Which of the following is a sympathetic response?
A
B
C
D