4.3 Nervous, Digestive & Other Body Systems
Key Takeaways
- The nervous system divides into the CNS (brain + spinal cord) and PNS (cranial and spinal nerves)
- A neuron has dendrites (receive), a cell body (process), and an axon (transmit); the myelin sheath speeds conduction
- The autonomic PNS splits into sympathetic (fight-or-flight) and parasympathetic (rest-and-digest)
- Digestive path: mouth → esophagus → stomach → small intestine → large intestine → rectum/anus
- Most nutrient absorption is in the small intestine; most water reabsorption is in the large intestine
- Endocrine glands use hormones; pituitary = master gland, pancreas controls glucose via insulin/glucagon
- Kidneys filter ~180 L of blood daily yet excrete only 1–2 L of urine; the nephron is the functional unit
The Nervous System
The nervous system is the body's fast communication network. Its two great divisions are the central nervous system (CNS) — brain and spinal cord, the processing center — and the peripheral nervous system (PNS) — the cranial and spinal nerves that carry signals to and from the CNS.
PNS branches:
- Somatic (voluntary): controls skeletal muscle and conscious movement.
- Autonomic (involuntary): controls organs, glands, and smooth muscle, and splits further:
- Sympathetic — “fight or flight”: ↑ heart rate, dilates pupils, shunts blood to muscles, releases epinephrine.
- Parasympathetic — “rest and digest”: ↓ heart rate, stimulates digestion, constricts pupils.
The Neuron
| Part | Job |
|---|---|
| Dendrites | Receive incoming signals |
| Cell body (soma) | Holds nucleus; integrates signals |
| Axon | Carries the impulse away from the cell body |
| Myelin sheath | Insulates the axon; speeds conduction |
| Synapse | Gap where neurotransmitters cross to the next cell |
Trap: signals travel into the dendrite and out through the single axon. A neuron has many dendrites but only one axon.
Brain Regions
| Region | Controls |
|---|---|
| Cerebrum | Thought, memory, language, voluntary movement |
| Cerebellum | Coordination, balance, fine motor control |
| Brain stem | Vital functions: breathing, heart rate, blood pressure |
| Hypothalamus | Homeostasis: temperature, hunger, thirst, hormone control |
| Thalamus | Relay station routing sensory input |
The Digestive System
Digestion is both mechanical (chewing, churning) and chemical (enzymes). Track the path and where each job happens.
| Organ | Function | Digestion Type |
|---|---|---|
| Mouth | Chewing; salivary amylase begins starch breakdown | Mechanical + chemical |
| Esophagus | Peristalsis moves the bolus | Mechanical |
| Stomach | HCl and pepsin break down protein | Mechanical + chemical |
| Small intestine | Bile + pancreatic enzymes; most nutrient absorption | Chemical + absorption |
| Large intestine | Absorbs water and electrolytes; forms feces | Absorption |
| Rectum/anus | Stores and eliminates feces | Elimination |
The small intestine has three parts: duodenum (receives chyme, bile, pancreatic juice), jejunum (bulk of nutrient absorption), and ileum (absorbs B₁₂ and bile salts). Accessory organs: the liver makes bile (emulsifies fat, not an enzyme), the gallbladder stores it, and the pancreas supplies enzymes and bicarbonate — plus insulin as an endocrine role. Trap: the small intestine, not the stomach, is the main absorption site.
The Endocrine System
The endocrine system signals slowly using hormones carried in the blood (versus the fast electrical nervous system).
| Gland | Key Hormones | Role |
|---|---|---|
| Pituitary (master gland) | GH, TSH, ACTH, ADH | Controls other glands; water balance |
| Thyroid | T3, T4, calcitonin | Sets metabolic rate; lowers blood calcium |
| Parathyroid | PTH | Raises blood calcium |
| Adrenal | Cortisol, aldosterone, epinephrine | Stress response, blood pressure |
| Pancreas | Insulin, glucagon | Blood-glucose control |
Glucose control: insulin (beta cells) lowers blood glucose by driving it into cells; glucagon (alpha cells) raises it by breaking down liver glycogen. Their balance is a textbook example of homeostasis and is disrupted in diabetes. Trap: the pituitary is the master gland, but the hypothalamus (nervous system) directs it.
The Urinary System
| Structure | Function |
|---|---|
| Kidneys (2) | Filter blood; make urine; balance fluid, electrolytes, pH |
| Ureters (2) | Carry urine to the bladder |
| Bladder | Stores urine (~500 mL capacity) |
| Urethra | Carries urine out of the body |
The nephron is the functional unit; each kidney holds about 1 million of them. The kidneys process roughly 180 L of blood per day but excrete only 1–2 L of urine because about 99% is reabsorbed. They also secrete erythropoietin (EPO), which signals red-marrow to make red blood cells, and renin to help regulate blood pressure.
Worked Example: An Endocrine Feedback Loop
Worked example: You eat a meal and blood glucose rises. The pancreatic beta cells release insulin, which moves glucose into cells and back to a normal level. Hours later, between meals, glucose falls, so alpha cells release glucagon, which breaks down liver glycogen to raise glucose again. This push-pull is negative feedback — the body senses a deviation and corrects it. NEX questions love to ask which hormone you would expect after a meal (insulin) versus during fasting (glucagon).
Common Traps Across These Systems
- Insulin versus glucagon direction: insulin lowers glucose, glucagon raises it. Reversing them is the most-missed endocrine item.
- Absorption site mix-up: the small intestine absorbs most nutrients; the large intestine reabsorbs water. The stomach mainly digests protein and absorbs very little.
- Bile is not an enzyme: the liver's bile emulsifies fat physically; pancreatic lipase is the enzyme that chemically digests it.
- Dendrite versus axon: dendrites receive, the axon transmits. A neuron has many dendrites but only one axon.
- Master gland chain: the pituitary is the master gland, but the hypothalamus (part of the brain) directs the pituitary — so the true top of the chain is neural, not endocrine.
How These Systems Maintain Homeostasis
The through-line of every system in this chapter is homeostasis — keeping internal conditions stable. The nervous and endocrine systems are the body's two control networks: the nervous system corrects fast and briefly (a nerve impulse acts in milliseconds), while the endocrine system corrects slowly but for longer (hormones circulate for minutes to hours). The kidneys quietly tune fluid, electrolytes, and pH around the clock, and the digestive system supplies the glucose and nutrients that fuel all of it.
A single NEX item can require you to see, for instance, that the kidney's EPO links the urinary system to the bone-marrow hematopoiesis from 4.1 and the red blood cells from 4.2 — the systems are one integrated whole, and the highest-scoring answers reflect that.
Which division of the autonomic nervous system drives the "fight or flight" response?
Where does MOST nutrient absorption occur in the digestive system?
Which hormone LOWERS blood glucose levels?
What is the functional unit of the kidney?
Arrange the organs of the digestive tract in the correct order food travels through them.
Arrange the items in the correct order
Match each endocrine gland to its primary function.
Match each item on the left with the correct item on the right
Which part of the brain controls basic life functions such as breathing and heart rate?
The liver produces _____, which is stored in the gallbladder and emulsifies (breaks up) fats.
Type your answer below
Which of the following are functions of the kidneys? (Select all that apply)
Select all that apply
Neurons transmit electrical impulses away from the cell body along a long fiber called the: