Bodily circuits in Physics

Pressure difference: Often minuscule, involves conversion between different units.
Unit systems:
- Pascals (Pa): System International (SI) unit.
- Millimeters of mercury (mmHg): Common in health sciences, referred to as "torr".
- Centimeters of water (cmH2O): Sometimes used in physiology.

Blood pressure commonly expressed in mmHg:
- Systolic pressure: Highest pressure during the heartbeat (e.g., reading of 120/80 mmHg).
- Diastolic pressure: Lowest pressure when the heart is at rest.
Blood pressure measured with a sphygmomanometer; can involve a valve gauge pressure reading in PSI (pounds per square inch).

Circulatory flow: Blood moves from heart to arteries, then to arterioles, then capillaries, and returns via venules and veins.
- Blood flow: Governed by pressure differential created by the heart's pumping action.
- Continuity equation: Flow rate remains constant; changes in speed based on cross-sectional area.

Flow rate (Q) is given by: Q = A imes v where:
- A = cross-sectional area
- v = velocity
Example: Blood flow speed in the aorta is 30 cm/s, and can be converted, for instance, to show that 100 mL of blood circulates in various systems based on measurements.

Hydraulic resistance (R): Resistance to blood flow through vessels, outlined in: R = \frac{\Delta P}{Q} where:
- \Delta P = pressure gradient and Q = flow rate.
Significant influence from vessel length, radius, and viscosity of blood.
Poiseuille's Law: Governs how resistance changes with the radius of the tube, emphasizing that a small change in radius affects resistance significantly due to the fourth power relation: R = \frac{8 \eta L}{\pi r^4} where:
- η = viscosity of the fluid
- L = length of the vessel
- r = radius of the vessel.

Typical viscosity of blood is approximately 3.5 centipoise (cP).
Measurement conversions:
- 1 cP = 0.001 Pas.
- Variations of viscosity can occur in different bodily conditions.

Blood flow throughput measured in liters per minute; typical flow is about 5.6-6.0 L/min for an adult.
Example: A person’s body can rapidly circulate blood when necessary (e.g., during digestion) by greatly increasing blood flow rates to specific organs.
Radius increase required to increase blood flow from 1 L/min to 5 L/min calculated using resistance principles which shows substantial radius increase requirements (by factor of approx 1.5).

Pressures in various body sections:
- Main arteries can exhibit pressures higher than other veins.
- Blood pressures are actively measured in clinics to determine health status and can vary based on body position, time of day, and other factors.

When arteries or veins are damaged significantly, understanding blood pressure and flow rates can provide insight into life-threatening situations – such as severe bleeding.
Importance of tourniquets in emergencies to control bleeding and elevation of limb pressure to maintain circulation until help arrives.

Gases exchange in the lungs occurs through methods influenced by surface tension and pressure ratios within the alveolar sacs.
Role of surface tension:
- Relationship between surface tension and pressure in spheres:
  P = \frac{4T}{r}
  where T is the surface tension and r is the radius of the bubble.

Understanding pressure, flow, and resistance in cardiovascular health is crucial for evaluating physiological performance and diagnosing issues in health sciences.
Vials, measuring devices, and blood pressure cuffs provide key tools to gauge pressure readings necessary for health assessment.