Homeostasis and body fluids

Why this matters

Every fluid a dentist handles is a tonicity decision: isotonic saline in the socket rather than sterile water, the sodium chloride that makes an anaesthetic cartridge isotonic, the hypotonic saliva that buffers a sugar challenge, the sweet solution that fires an exposed dentinal tubule. The dentist is also often the clinician who makes a patient unable to drink for days, or who prescribes carbamazepine to an older person whose plasma sodium then falls.

The big picture

Cells never meet the outside world; they meet extracellular fluid, and physiology's whole job is to hold that fluid nearly constant. Nearly — because every regulated variable oscillates, and no feedback loop abolishes a disturbance, it only shrinks it. Water then distributes itself by osmosis, so where a fluid ends up is decided not by its label but by which of its solutes cannot cross the membrane in front of them.

The 15 cards in this lesson

1. 01The internal environment
2. 02Sensor, integrator, effector — and gain
3. 03Negative versus positive feedback
4. 04Where the 42 litres sit
5. 05Total body water and what moves it
6. 06Inside versus outside: the ionic split
7. 07Measuring a compartment you cannot see
8. 08Osmolarity, osmolality and the calculated value
9. 09Tonicity and effective osmoles
10. 10A cell in three solutions
11. 11Saliva: a hypotonic secretion
12. 12Isotonic cartridges and thirsty tubules
13. 13Evidence: salt-water rinsing in vitro
14. 14The five errors students make here
15. 15Clinical alert — in the lesson

You have got it when

You can say where the body's ~42 L sits and why that split is a convention rather than a measurement. You can convert a bag label into a tonicity and predict which compartment the fluid reaches. Given saliva at 82 mOsm/kg against plasma at 291 mOsm/kg, you can name the water-impermeable duct that makes it so.