11.9 Pressures in the Body

Pressures in the Human Body

Pressure differences drive nearly every major process in the human body, from circulating blood to inflating the lungs to protecting the brain. In physics terms, these are applications of fluid statics: the same principles of pressure, density, and depth that apply to water in a tank also apply to fluids inside you.

This section covers how pressure functions in the body, how blood pressure is measured and interpreted, and how pressure varies across several organ systems.

Functions of Pressure in the Human Body

Pressure gradients are what move fluids and gases where they need to go. Without a pressure difference between two points, there's no flow.

• Driving circulation and gas exchange. The heart creates a pressure difference that pushes blood through arteries, capillaries, and veins. In the lungs, a pressure difference between the air outside and the air inside the lungs is what draws each breath in and pushes it back out.
• Maintaining cell structure. Hydrostatic pressure inside and outside cells keeps them at the right volume. If the balance between intracellular and extracellular fluid pressure shifts too far, cells can swell or shrink. This ties directly to osmotic balance.
• Sensory detection and feedback. Your body monitors its own pressures using specialized receptors. Baroreceptors in blood vessel walls detect changes in blood pressure and trigger adjustments to heart rate. Mechanoreceptors in skin and joints respond to touch and body position. These feedback loops are how the body maintains homeostasis.

Systolic vs. Diastolic Blood Pressure

Blood pressure is the force blood exerts against the walls of blood vessels. It's reported as two numbers because pressure in your arteries isn't constant; it pulses with each heartbeat.

• Systolic blood pressure (SBP) is the peak pressure, measured when the heart contracts (systole). Normal range: 90–120 mmHg.
• Diastolic blood pressure (DBP) is the minimum pressure, measured when the heart relaxes between beats (diastole). Normal range: 60–80 mmHg.

A reading of "120 over 80" means an SBP of 120 mmHg and a DBP of 80 mmHg. The unit mmHg (millimeters of mercury) comes from the traditional measurement device, where blood pressure supports a column of mercury to a certain height.

How blood pressure is measured:

1. A cuff (sphygmomanometer) is inflated around the upper arm until it compresses the brachial artery enough to stop blood flow.
2. The cuff is slowly deflated. A stethoscope placed over the artery picks up Korotkoff sounds, the turbulent flow noises that appear when blood starts squeezing past the cuff.
3. The pressure at which sounds first appear is the systolic value. The pressure at which sounds disappear is the diastolic value.
4. Automatic monitors use a different approach, detecting oscillations in cuff pressure rather than listening for sounds.

Why it matters for health:

Keeping blood pressure in the normal range ensures organs receive adequate blood flow. Hypertension (chronically high blood pressure) damages vessel walls over time and increases the risk of atherosclerosis, stroke, aneurysms, and kidney damage. Hypotension (abnormally low blood pressure) can cause dizziness, fainting, and in severe cases, shock from inadequate organ perfusion.

Pressure Variations Across Body Systems

The same physics applies everywhere in the body, but normal pressure ranges differ by location.

Eyes

Intraocular pressure (IOP) is the fluid pressure inside the eye, maintained by the balance between production and drainage of aqueous humor. Normal IOP ranges from 10–21 mmHg. Chronically elevated IOP can damage the optic nerve, leading to glaucoma and progressive vision loss.

Lungs

Breathing depends on changing the pressure inside the lungs relative to atmospheric pressure.

• During inspiration, the diaphragm contracts and the chest cavity expands. This lowers intrapulmonary pressure below atmospheric pressure, so air flows in.
• During expiration, the diaphragm relaxes and the chest cavity shrinks. Intrapulmonary pressure rises above atmospheric pressure, pushing air out.

This is sometimes called negative pressure breathing because the lungs fill by creating a slight vacuum, not by forcing air in. Efficient gas exchange ($O_2$ in, $CO_2$ out) requires that ventilation and blood perfusion in the lungs stay well matched.

Spinal Column

Cerebrospinal fluid (CSF) surrounds the brain and spinal cord, cushioning them against impact. Normal CSF pressure ranges from 5–15 mmHg, typically measured via lumbar puncture. Elevated CSF pressure (intracranial hypertension) can cause headaches, vision problems, and neurological damage. Conditions like hydrocephalus involve a buildup of CSF that raises this pressure.

Bladder

As the bladder fills with urine, intravesical pressure gradually increases. At capacity, normal bladder pressure is roughly 10–20 $\text{cmH}_2\text{O}$. Stretch receptors in the bladder wall detect this rising pressure and trigger the micturition reflex, which signals the urge to urinate.

Skeletal Structure

Weight-bearing bones and joints experience compressive forces from gravity and muscle contractions. Wolff's law states that bone remodels in response to the mechanical loads placed on it: areas under more stress grow denser, while unloaded areas weaken. Uneven pressure distribution across joints can contribute to osteoarthritis and stress fractures over time.

Fluid Dynamics in the Body

Blood flow through vessels follows the same fluid dynamics principles you've studied for pipes and tubes.

• Poiseuille's law describes flow rate through a vessel: $Q = \frac{\pi r^4 \Delta P}{8 \eta L}$, where $r$ is the vessel radius, $\Delta P$ is the pressure difference, $\eta$ is the blood viscosity, and $L$ is the vessel length. The $r^4$ dependence is the key takeaway: even a small change in vessel radius has a huge effect on flow rate. A vessel that narrows by half allows only $\frac{1}{16}$ the original flow.
• Bernoulli's principle relates fluid speed and pressure. Where blood speeds up (e.g., through a constriction), pressure on the vessel wall drops. Where blood slows down, pressure rises.
• Laminar vs. turbulent flow. In most healthy vessels, blood flows in smooth, parallel layers (laminar flow). At branch points, sharp bends, or narrowed regions, flow can become turbulent. Turbulent flow is less efficient and can be detected as a heart murmur or bruit with a stethoscope.
• Viscosity is blood's internal resistance to flow. Blood is more viscous than water because of red blood cells and plasma proteins. Higher viscosity means greater resistance and requires more pressure to maintain the same flow rate.