Core concept
Mass balance
The amount of any substance in the body changes only by what enters, leaves, is made or is used up.
The amount of any substance in your body changes only by what comes in, what goes out, what is made and what is used up. If intake and output match, the amount stays steady. If you take in more salt than you excrete, total body sodium rises until something changes.
This simple bookkeeping explains a great deal. Blood volume falls after bleeding because output jumped; it recovers as your kidneys cut output and thirst raises intake. When a lab value is abnormal, mass balance tells you where to look: something is coming in too fast, leaving too slowly, being made or being used.
Where it shows up
Chemistry and physics for physiology
Bone tissue
- Bone and blood calcium
The endocrine system
- The pancreas and blood glucose
The cardiovascular system
Lymphatic and immune systems
- The lymphatic system
The digestive system
- Chemical digestion and absorption
Metabolism and nutrition
- Nutrients, vitamins and minerals
- Lipid and protein metabolism
- Fed and fasting states
The urinary system
- Tubular reabsorption and secretion
- Urine, clearance and kidney hormones
Fluid, electrolyte and acid–base balance
- Body fluid compartments and water balance
- Electrolyte balance
- Regulating acid–base balance