2.1 The Respiratory System

The respiratory system

A system made up of organs and tissues that deliver oxygen to body cells and remove carbon dioxide from the body.

Main organs of the respiratory system

• Nasal cavity

• Pharynx

• Epiglottis

• Larynx

• Trachea

• Bronchi

• Bronchioles

• Lungs

• Diaphragm

• Alveoli

Nasal cavity

A hollow space within the nose and skull that is lined with hairs and mucus membrane.

Function of the nasal cavity

To warm, moisturise and filter air entering the body, before it reaches the lungs.

Cilia

Tiny hair-like structures lining the respiratory tract, including the nose, which help filter air entering the body by waving back and forth to sweep away mucus that has trapped dust and microorganisms.

Pharynx

A muscular tube extending from the base of the skull down to the oesophagus and larynx

Function of the pharynx

Acts as a shared pathway for both the respiratory and digestive systems.

Epiglottis

A small leaf-shaped flap of tissue at the top of the larynx

Functions of the epiglottis

• It stays open during breathing

• It closes when swallowing to allow food pass into the oesophagus and prevent choking

Larynx

It connects the back of the nose and the trachea, forming an air passage to the lungs.

Function of the larynx

It contains vocal cords which vibrate to produce sound, as air from the lungs passes through.

What the larynx is known as

Voice box

Trachea

A strong tube that starts from the larynx to the bronchi and is supported by C-shaped rings of cartilage to prevent it from collapsing

Function of the trachea

It acts as a main passage to carry air in and out of the lungs.

Another name for trachea

Windpipe

Bronchi

The two main airways that branch off from the bottom of the trachea into each lung and have rings of cartilage along their length to stop them from collapsing.

Function of the bronchi

Carries air into and out of the lungs

Bronchioles

Smaller airways branching off from the bronchi and delivering air to the air sacs, deep within the lungs.

Function of the bronchioles

• Deliver air directly to the microscopic air sacs

• Changes size to control how much air enters the air sacs

Singular form of bronchi

Bronchus - there is a left bronchus which leads into the left lung and vice versa

Lungs

A pair of large, spongy organs in the chest cavity that consist of millions of tiny elasticated air sacs.

Functions of the lungs

• The site where oxygen is taken into the body

• The site where carbon dioxide is removed

Pleural membranes

Two thin layers of smooth, fluid-secreting tissue (serous tissue) that enclose the lungs and line the inside of the chest cavity.

Function of the pleural membranes

It acts as a lubricant to reduce friction, while keeping the lungs securely expanded against the chest wall.

The two layers of pleural membranes

Internal and external

Internal pleural membranes

The inner layer of pleural membrane that directly covers the lungs.

External pleural membranes

The outer layer of pleural membrane that lines the chest cavity, anchoring the lungs to the rib cage to relay breathing movements.

Another name for internal pleural membranes

Visceral pleural membranes

Another name for external pleural membranes

Parietal pleural membranes

Pleural fluid

A slippery, liquid derived from the blood in the capillaries in the lungs, found in small quantities between the two layers of pleural membranes.

Functions of pleural fluid

• It serves as a lubricant to reduce friction between the pleural membranes

• It creates mechanical surface tension that keeps the lungs expanded so that they can efficiently fill with air.

Ribs

They protect vital organs such as the heart and lungs, and facilitate respiration by allowing the chest cavity to expand and contract.

Intercostal muscles

They are muscles found between the ribs that help the ribcage move during breathing.

Function of intercostal muscles

They assist the rib cage with expansion and contraction during breathing.

The two layers of intercostal muscles

Internal and external

Diaphragm

A dome-shaped muscle below the lungs that separates the chest from the abdomen and is the main muscle involved in breathing

Functions of the diaphragm

• It contracts and flattens to draw air into the lungs (inhalation)

• It relaxes and returns to a dome shape to push air out of the lungs (exhalation)

Alveoli

Tiny air sacs at the end of bronchioles, surrounded by capillaries and is the site of gas exchange.

Function of alveoli

Where oxygen diffuses into the blood and carbon dioxide diffuses out from the blood.

How many alveoli are contained in the lungs

Around 300 million alveoli per lung.

Singular form of alveoli

Alveolus

Alveolar membrane

The ultra-thin surface layer of the air sacs where gases pass between the lungs and the blood.

Structural adaptations of the alveoli

• Large surface area

• Extremely thin walls

• Surrounded by capillaries

How the large surface area of the alveoli helps it adapt to its function

It provides a massive total space for gasses to cross at the same time, allowing a much higher volume of oxygen to enter the body quickly.

How the extremely thin walls of the alveoli helps it adapt to its function

The walls are only one cell thick, creating a very short distance for oxygen and carbon dioxide to travel across easily.

How the alveoli being surrounded by capillaries helps it adapt to its function

It continuously moves blood to maintain a steep diffusion gradient, while keeping the distance as short as possible for rapid gas exchange.

Other structural adaptations of the alveoli

• Fluid lined

• Partially permeable

• Movement of external medium e.g. air

• Movement of internal medium e.g. blood

How the linings of the alveoli containing fluid helps the alveoli adapt to its function

The moist lining helps the gases dissolve easily, which maintains the shape of each air sac and helps to keep it open, so that oxygen and CO₂ can pass through.

How the movement of external medium helps alveoli adapt to its function

It maintains a diffusion gradient

How the movement of internal medium helps alveoli adapt to its function

It maintains a diffusion gradient

The process of respiratory system

1. The diaphragm contracts and flattens as the external intercostal muscles contract to lift the rib cage up and out, to increase the chest volume

2. Air is breathed in through the nose

3. It is then warmed, filtered and moistened in the nasal cavity, as the lungs expand

4. The epiglottis opens and stays open during breathing

5. The air then passes through the pharynx, larynx and travels down the trachea

6. The bronchi carry the air into the lungs and the bronchioles deliver it deeper into the lungs

7. The air reaches the alveoli where gas exchange occurs

8. Oxygen enters the bloodstream from the alveoli

9. Carbon dioxide leaves the bloodstream and enters into the alveoli

10. The air is exhaled from the lungs as the diaphragm and intercostal muscles relax.