C.4.1. Breathing

A. Introduction:

1.
Before we study what is going on inside the lungs, it is important to understand the mechanisms and structures that make the air go in and out of the lungs (= breathing).

2.
Breathing consists of two phases:
a. inspiration (= air goes in)
b. expiration (= air goes out)

3.
In order to breathe, two parts of the chest must work:
a. the diaphragm
b. the rib cage

B. The Diaphragm:

1.
The diaphragm is the muscle that divides the chest cavity from the abdominal cavity (=the belly). At rest, the diaphragm has a curved shape that extends into the chest cavity.

2.
When the diaphragm contracts, it becomes more flat. This increases the volume inside the chest (inspiration). Note that this will also decrease the volume in the abdomen.

Animated?

3.
When the diaphragm relaxes, it returns to its original shape and thereby squeezes the lungs: expiration.

4.
For normal breathing, the diaphragm is the most important inspiratory muscle.

C. The Rib Cage: top?

1.
The ribs, together with the spine at the back (=dorsal) and the sternum in front (=ventral), really form a cage around the vital organs such as the lungs and the heart (this serves as a protection for potential injuries from outside).

2.
In order to breathe, the ribs go up and down as shown in the diagram. Notice that during the inspiration, the ribs go up and become almost horizontal, thereby pushing the sternum in front and increasing the distance between the spine and the sternum. When you expire, the opposite happens.

Rib cage in inspiration and expiration mode

Animated?

3.
Moving the ribs to increase or decrease the chest volume is more complicated then contracting the diaphragm.

4.
As shown in the diagram below, the muscles that are located between the ribs perform these movements. These muscles are called the intercostal muscles.

5.
There are two types of intercostal muscles:
a. the external intercostals, used for inspiration
b. the internal intercostals used for expiration

6.
Because the fibers of these two muscles run in the opposite direction, contractions of the two muscles will have an opposite effect. The external will lift the ribs whereas the internal will pull the ribs downwards.

D. Respiration at rest and during exercise: top?

1.
At rest, breathing is rather shallow (i.e. not deep) and hardly noticeable. Inspiration is caused mainly by contracting the diaphragm and, second, by the contraction of the external intercostal muscles.

2.
At rest, expiration is passive and occurs when the diaphragm relaxes. The ribs will sag by their own weight and do not require contraction of the internal intercostals.

3.
When more, deeper and faster breathing is required, such as during physical exercise or during a disease, then the accessory muscles help the inspiration and the expiration.

4.
During forced respiration, more muscles are used for forced inspiration and other muscles are used for forced expiration.

5.
These are the accessory muscles involved in forced Inspiration:
   a. sternocleidomastoid muscles
   b. scalenes muscles
   c. pectoralis minor

(these are all muscles that will pull the rib cage upwards).

6.
These are the accessory muscles involved in forced Expiration:
   a. abdominal wall muscles such as the oblique and transversal muscle
   b. internal intercostals
   c. latissimus dorsi
   d. quadratum

(these are all muscles that will pull the rib cage downwards).

Diaphragm in action during inspiration and expiration

Breathing with your ribcage

The trachea + lungs:

A cross-section of the Trachea:

The Bronchial Tree:

Function of the interior and external intercostal muscles: