Unit 3: The digestive and respiratory systems                             1   2   3 

3.2. Pulmonar ventilation

Ventilation is the process to renew the air of lungs.

a) Accessorial structures:

They are not part of the respiratory system but they are essential to its operation. These structures are:

- Thoracic cage

It is formed by bony box made by the ribs, the sternum and the spinal column.

- Respiratory muscles

The main one is the diaphragm, but there are other muscles implied for example intercostal muscles among others. Diaphragm forms the floor of the thoracic cavity and intercostal muscles are attached to ribs.

b) Respiratory movements:

It is a reflex act (an involuntary and automatic process). Pulmonary ventilation has place in two phases:

a) Inspiration (inhalation)

It is the entry of air into lungs. It is an active process (it expends energy)

               - The diaphragm contracts and goes down and the intercostal

                  muscles contract and ribs go up and out.

               - As a result, the volume of the thoracic cavity increases and its internal

                 pressure decreases.

               - The atmospheric pressure becomes higher than air pressure

                 inside the lungs and the air flows into lungs which are filled passively.

b) Expiration (exhalation)

It is the exit of air out lungs. It is a passive process (it does not expend energy)

               - The diaphragm relaxes and goes up and the intercostal muscles

                  relax and ribs go down and in.

               - As a result, the volume of the thoracic cavity decreases and its internal

                  pressure increases.

               - The atmospheric pressure becomes lower than air pressure

                  inside the lungs and air flows out lungs which are emptied passively.

   The respiratory rhythm is the number of inspirations in a minute.

   Although it is variable according to sex, age or size, it has an average

   value about 17 inspirations/minute. Anyway it can adapt to provide

   an optimal flow of oxygen to cells in every circumstance. For example:

           - It increases during the physical exercise and exciting situations

             (nervousness, fear, danger, etc)

           - It decreases during sleep and relaxing situations

3.8.  In a determine moment, the air within the trachea of a person

         is compounded by 78 % of nitrogen, 16.4 % of oxygen, 5 % of

         carbon dioxide and 0.6 % of other gases. What do you think,

         is this air entering to lungs or exiting from them? Why?

3.9. What difference is there between ventilation and respiration?

3.10. Listen and indicate what sentences are referred to: