Flow down gradients
Blood, air, ions, water and heat all move from high to low pressure, concentration, voltage or temperature, at a rate set by the gradient and the resistance.
Almost everything that moves in your body moves down a gradient: from high to low pressure, concentration, voltage or temperature. How fast it moves follows one rule. Flow equals the driving difference divided by the resistance. Double the difference and flow doubles; double the resistance and flow halves.
The rule is the same whether the thing moving is blood, air, water, ions or heat. Blood flows from the aorta toward the veins because pressure falls along the way, and narrowing an arteriole cuts flow sharply because resistance rises steeply as a tube gets narrower. Sodium rushes into a neuron because both its concentration and the voltage push it in. When you meet a new process, ask: what is the gradient, and what sets the resistance?
Where it shows up
Chemistry and physics for physiology
Cells
Signals, repair and control
Nervous tissue and neural signaling
- The action potential in detail
The cardiovascular system
- Blood flow through the heart and the two circuits
- Cardiac muscle cells and their action potential
- The cardiac cycle
- Blood pressure, flow and resistance
- Capillary exchange
The respiratory system
- Pulmonary ventilation
- Lung volumes and ventilation rates
- Gas exchange
- Oxygen transport
- Carbon dioxide transport
The digestive system
- Chemical digestion and absorption
The urinary system
- Glomerular filtration
- Tubular reabsorption and secretion
- Concentrating the urine
Development and inheritance
- The placenta
- Fetal development and fetal circulation