Which organ is the primary site of nutrient absorption in the digestive system?

Small intestine. The small intestine is the primary site of nutrient absorption, particularly the jejunum. The three sections of the small intestine are the duodenum (where bile and pancreatic enzymes enter), the jejunum (major absorption site), and the ileum (absorbs B12 and bile salts). The large intestine primarily absorbs water and forms feces.

Which hormone LOWERS blood glucose levels?

Insulin. Insulin, produced by the beta cells of the pancreatic islets of Langerhans, lowers blood glucose by facilitating glucose uptake into cells and promoting glycogen storage in the liver. Glucagon (alpha cells) has the opposite effect — it raises blood glucose. Cortisol and epinephrine also raise blood glucose levels.

The sympathetic nervous system is often described as the "fight or flight" response. Which of the following is a sympathetic response?

Dilated pupils and increased heart rate. The sympathetic nervous system prepares the body for stressful or emergency situations ("fight or flight") by increasing heart rate, dilating pupils, diverting blood to skeletal muscles, and inhibiting digestion. The parasympathetic system does the opposite ("rest and digest") — slowing heart rate, constricting pupils, and stimulating digestion.

Digestive, Endocrine & Nervous Systems — Free Study Guide 2026

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

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)

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2.3 Digestive, Endocrine & Nervous Systems

Key Takeaways

Digestive (Gastrointestinal) System

Endocrine System

Nervous System