14. Human Physiology

Why Do We Breathe?

Your cells are like tiny factories. They need fuel to work.

The Energy Problem

Cells break down glucose to get energy. This process needs oxygen $(\text{O}_2)$. But there's a problem - it creates a harmful waste called carbon dioxide $(\text{CO}_2)$.

The Human Respiratory System

The Journey of Air

Let's follow the path air takes through your body:

1. Entry Point: Nostrils

Air enters through your nose. The nostrils lead to the nasal chamber.

What happens here:

• Hair filters out dust
• Mucus traps germs
• Air gets warmed
• Air becomes moist

How Do We Breathe?

The Basic Principle

Breathing works on a simple rule:

Air moves from HIGH pressure to LOW pressure

Your body creates pressure differences to move air in and out.

Two Stages of Breathing

Stage 1: Inspiration (Breathing IN)

Goal: Make pressure inside lungs LESS than outside air

Stage 2: Expiration (Breathing OUT)

Goal: Make pressure inside lungs MORE than outside air

Measuring Your Breath

Respiratory Volumes

Different activities use different amounts of air. Scientists measure these volumes using a device called a spirometer.

1. Tidal Volume (TV)

What is it: Air you breathe in OR out during normal, quiet breathing

Amount: $500 \text{ mL}$ (about half a liter)

Example: Right now, as you sit reading this, each breath moves about 500 mL of air.

Per minute: $500 \text{ mL} \times 12\text{-}16 \text{ breaths} = 6000\text{-}8000 \text{ mL/min}$