Human Respiratory System Function: How Breathing Works

The main human respiratory system function is gas exchange: taking oxygen from the air into the blood, so cells can release energy in respiration, and removing carbon dioxide, the waste gas that respiration makes. Every breath brings fresh air down to about 480 million tiny air sacs in the lungs, where oxygen diffuses into the blood and carbon dioxide diffuses out. Without it, the brain is damaged after only about four minutes.

What are the parts of the respiratory system?

The respiratory system is divided into two parts, and air passes through them in order.

1. Upper respiratory tract: the nose and nasal cavity, sinuses, pharynx (throat) and larynx (voice box). These filter, warm and moisten the air. Hairs and sticky mucus trap dust and microbes.
2. Lower respiratory tract: the trachea (windpipe), the two bronchi, thousands of smaller bronchioles and, at their ends, the alveoli. The trachea is held open by C-shaped rings of cartilage so it doesn't collapse when you breathe in.

Part	What it does
Trachea	Carries air to the chest; lined with cilia that sweep mucus up and away from the lungs
Bronchi	Two branches, one into each lung
Bronchioles	Narrow tubes that branch about 23 times, spreading air through the lung
Alveoli	Air sacs where gas exchange happens
Diaphragm	Sheet of muscle under the lungs that drives breathing
Intercostal muscles	Muscles between the ribs that move the ribcage

How does gas exchange in lungs work?

Gas exchange in lungs works by diffusion: oxygen moves from the air in the alveoli, where its concentration is high, into the blood in the capillaries, where it is low. Carbon dioxide moves the other way, from the blood, where it is concentrated, into the alveoli, to be breathed out. No energy is used; each gas simply moves down its concentration gradient.

Alveoli function: why they are so good at it

Alveoli are microscopic air sacs, each about 0.2 mm across, wrapped in a net of capillaries. Their job is to make diffusion as fast as possible, and they have every feature that speeds it up:

• A huge surface area: together the alveoli cover about 70 m², roughly the floor of a small flat.
• A very thin barrier: the alveolus wall and the capillary wall are each one cell thick, so gases cross only about 0.5 µm.
• A moist lining: oxygen dissolves in the moisture before it diffuses through.
• A steep concentration gradient, kept up all the time: breathing keeps bringing fresh air in, and the blood keeps carrying oxygen away.

The path oxygen takes is: inhaled air → alveoli → across the alveolus and capillary walls → into red blood cells → round the body to the tissues.

What are the breathing mechanics?

The breathing mechanics are simple: breathing (ventilation) works by changing the volume of the chest, which changes the pressure inside the lungs. Air always flows from higher pressure to lower pressure.

	Breathing in (inhalation)	Breathing out (exhalation)
Diaphragm	Contracts and flattens (moves down)	Relaxes and domes upwards
External intercostal muscles	Contract: ribs move up and out	Relax: ribs move down and in
Volume of the chest	Increases	Decreases
Pressure in the lungs	Falls below air pressure	Rises above air pressure
Air flows	In	Out

At rest, breathing in is active (it uses muscle contraction) and breathing out is mostly passive: the muscles relax and the stretchy lungs spring back. During hard exercise, the internal intercostal and abdominal muscles also contract to push air out faster.

Worked example: how much air do you breathe?

Minute ventilation is the volume of air breathed in each minute:

minute ventilation = tidal volume × breathing rate

At rest, an adult's tidal volume (the volume of one normal breath) is about 0.5 litres, at about 12 breaths per minute:

minute ventilation = 0.5 × 12 = 6 litres per minute

During hard exercise both rise, for example 3 litres per breath at 40 breaths per minute = 120 litres per minute, twenty times more.

How is oxygen transported in the blood?

Oxygen transport is the job of haemoglobin, an iron-containing protein packed into red blood cells. In the lungs, where oxygen is plentiful, each haemoglobin molecule picks up four oxygen molecules to form oxyhaemoglobin. In the tissues, where oxygen is scarce, it releases them. About 98% of the oxygen in blood travels this way; only 2% dissolves in the plasma.

Carbon dioxide travels back mostly in the plasma, as hydrogencarbonate ions, and is released in the lungs. Blood leaving the lungs is about 98% saturated with oxygen, which is why a fingertip pulse oximeter normally reads 95 to 100%. You can follow blood from the lungs to the heart and round the body in the Heart & Circulation lab, and read how the blood itself works in how blood works.

Why do you breathe faster when you exercise?

You breathe faster during exercise because your muscles respire faster, using more oxygen and producing more carbon dioxide. Sensors in the brain stem and in the arteries detect the rising carbon dioxide (which makes the blood slightly more acidic), and the brain stem sends more frequent signals to the diaphragm and intercostal muscles. Breathing rate and depth both increase, and the heart beats faster to carry the extra oxygen. That breathing rhythm is set in the medulla, which you can find in the 3D brain lab.

Common mistakes

• Mixing up breathing and respiration. Breathing moves air in and out of the lungs; respiration is the chemical reaction in cells that releases energy from glucose.
• Saying the diaphragm moves up when you breathe in. It contracts and moves down, making the chest bigger.
• Saying the lungs suck air in. The lungs have no muscle; the diaphragm and ribs change the chest volume and pressure, and air flows in.
• Saying gas exchange uses energy. It happens by diffusion, which is passive.
• Forgetting that oxygen travels bound to haemoglobin, not mainly dissolved in plasma.

Frequently asked questions

What is the main function of the alveoli?

The alveoli are the site of gas exchange. Oxygen diffuses from the air inside them into the blood in the surrounding capillaries, and carbon dioxide diffuses from the blood into the alveoli to be breathed out. Their huge total surface area of about 70 m², walls one cell thick, moist lining and rich blood supply make this diffusion very fast.

How does haemoglobin transport oxygen?

Haemoglobin is an iron-rich protein in red blood cells. Each molecule can bind up to four oxygen molecules, forming oxyhaemoglobin, where oxygen is plentiful in the lungs. Where oxygen is scarce, in respiring tissues, the bond breaks and the oxygen is released. Each red blood cell carries about 270 million haemoglobin molecules.

Why is breathing out usually passive?

At rest, breathing out needs no muscle contraction. When the diaphragm and external intercostal muscles relax, the elastic lungs and chest wall recoil to their resting size. The chest volume falls, the pressure inside rises above air pressure, and air flows out. Only during heavy exercise, coughing or blowing do muscles actively push air out.

How does smoking damage the respiratory system?

Tar in cigarette smoke paralyses and destroys the cilia, so mucus and trapped dirt build up and cause coughing and infections. Over years, the walls between alveoli break down (emphysema), reducing the surface area for gas exchange, so less oxygen reaches the blood. Carbon monoxide in smoke also binds to haemoglobin in place of oxygen.

Try it yourself

Open the Heart & Circulation lab, choose Whole body and switch on Lungs and Follow one blood cell. Watch the cell turn from blue to red as it passes through the lungs and picks up oxygen, then press Running and see how the heart speeds up to carry more oxygen to the muscles.