LO5: Respiratory System - Pulmonary Ventilation and Control

Breathing: How We Get Air In and Out

Breathing, or pulmonary ventilation, is the process of moving air into and out of your lungs. It's a repeating two-part process:

• Inhaling (Inspiration): Taking air into your lungs.

• Exhaling (Expiration): Pushing air out of your lungs.

• Respiration: Everything that happens during inspiration and expiration

Both actions rely on your breathing muscles and a difference in air pressure between inside and outside your body. One inhale and one exhale make up a single breathing cycle.

Breathing Muscles

Your lungs don't move themselves. They depend on the muscles in your chest and belly to expand and shrink, which creates air movement. The most important muscle for breathing is the diaphragm.

Main Muscles

• Diaphragm: A large, dome-shaped muscle that separates your chest from your abdomen. It's the primary muscle used for breathing.

• Intercostal Muscles: Small muscles located between your ribs.

How You Inhale

Inhaling is an active process because your muscles contract:

• Diaphragm: Contracts, flattens, and moves downwards. This makes your chest cavity taller.

• External Intercostal Muscles: Contract, pulling your ribs up and out. This makes your chest cavity wider from front to back and side to side.

• Result: When your chest cavity gets bigger, the pressure inside your lungs drops. Air then rushes in to fill the space and balance the pressure.

How You Exhale

Normal exhaling is usually a passive process, meaning your muscles relax:

• Intercostal Muscles: Relax, allowing your ribs to move down and in.

• Diaphragm: Relaxes and bulges upwards, pushing against the bottom of your lungs.

• Result: These relaxations cause your chest to return to its original size, making your chest cavity smaller. The natural springiness of your lungs and chest wall pushes the air out.

Quick Fact

• Adults typically breathe between $12$ and $20$ times per minute.

Extra Breathing Muscles

When you breathe harder or struggle to breathe (like during intense exercise, singing loudly, or in a medical emergency), other muscles help with breathing. These are called accessory muscles.

• During a Deep Inhale:

  • Neck Muscles: Muscles in your neck (like the sternocleidomastoids and scalenes) contract to help lift your chest high.

  • Chest Muscles: Small chest muscles (like the pectoralis minor) also help lift the chest.

• During a Forced Exhale (an active process):

  • Abdominal Muscles: Muscles in your belly (like the rectus abdominis and external abdominal obliques) contract to pull your lower ribs and sternum (breastbone) down.

  • Internal Intercostals: These muscles pull other ribs downwards.

  • Result: These actions make your chest even smaller, pushing air out more quickly and forcefully.

The Body At Work: Using Extra Muscles

• People who have trouble breathing (for example, due to lung conditions like emphysema, where exhaling becomes very hard work) or are in severe breathing distress might use these extra muscles a lot. Seeing them work hard often means it's a medical emergency.

Brain Control of Breathing

Unlike your heart, which beats on its own, the muscles for breathing need constant signals from your brain to work. Automatic, unconscious breathing is managed by special areas in your brainstem, called the medulla and pons.

Medulla Oblongata: The Main Breathing Control Center

The medulla has two linked centers vital for controlling breathing:

• Inhale Center (Primary Breathing Center):

  1. Sends nerve signals to your intercostal muscles and diaphragm.

  2. These signals make your inhale muscles contract, causing you to breathe in.

  3. The signals then suddenly stop.

  4. The inhale muscles relax, and the springiness of your chest causes you to breathe out passively.

• Exhale Center:

  • This center only becomes active when you need to breathe out forcefully (like when exercising or speaking loudly).

  • It sends signals to your abdominal and other extra muscles, making them contract for an active exhale.

Pons: Adjusting Breathing Rhythm

The pons has two centers that help fine-tune the basic breathing pattern set by the medulla:

• Apneustic Center: Encourages the inhale center, making your inhales longer and deeper.

• Pneumotaxic Center: Slows down both the apneustic and inhale centers. This helps create a normal, regular breathing rhythm and, importantly, stops your lungs from overfilling with air.

The Body At Work: Thinking About Your Breathing and Why You Can't Hold Your Breath Forever

• The outer part of your brain (cerebral cortex) lets you control your breathing rate and rhythm when you want to, like when singing, blowing out a candle, or holding your breath.

• However, holding your breath means you're not getting rid of in your blood starts to go up. Carbon dioxide is a very strong signal for your breathing system.

• When $CO_2$ levels get too high, the breathing centers in your brainstem will take over from your conscious control, forcing you to start breathing again.

Changes in Breathing

Your breathing pattern isn't always the same; it can change a lot depending on different things.

• Things That Affect Breathing: Pain, emotions (like fear, anger, anxiety), exercise, and changes in your body can all affect how fast and deeply you breathe.

• Why These Changes Happen: These variations occur because your breathing control centers constantly receive updates from many sensors throughout your body, telling them about your body's changing needs.

Quick Fact

• Carbon dioxide – is the main thing that controls your breathing. This is because $CO_2$ can easily cross into your brain and directly affect the brain's breathing sensors.

Boyle’s Law: In the body (breathing)

• When you inhale, your chest cavity gets bigger (volume increases) so air pressure in your lungs goes down and air rushes in.

• When you exhale, your chest cavity gets smaller (volume decreases) so air pressure goes up and air is pushed out.

What Influences Breathing Rate and Depth