The Nervous System and Breathing

Breathing is regulated by the autonomic nervous system, specifically the medulla oblongata. The nerve cells in this centre automatically send signals to the diaphragm and intercostal muscles, making them contract at regular intervals. Another section of the brain involved in regulating breathing is the pons which has two control centres, the Apneustic centre which helps stimulate inspiration and the Pneumotaxic centre which inhibits the Apneustic centre, allowing expiration. Between these respiratory centres a regular rhythm is established so that the average person breaths 15 times per minute.

Breathing can be affected by the pH level of the blood. Chemoreceptors send signals to the medulla and this determines the depth and rate of breathing. At low pH levels the medulla becomes stimulated and this results in an increase in breathing rate. When pH levels are high there is a decrease in breathing rate due to a lack of stimulation.

Most of the time breathing is automatic, however other parts of the brain can override the medulla and pons to make breathing a conscious effort, such as when swimmers hold their breath or when breathing is controlled while meditating.

References:

http://people.hofstra.edu/sina_y_rabbany/engg81/breathingcoordination.html

www.cdli.ca/~dpower/resp/control.htm

The conditions necessary for effective gaseous exchange

In order for effective gaseous exchange to take place certain functions need to be fulfilled. During inspiration a number of things happen. The diaphragm contracts along with intercostal muscles to make room in the thoracic cavity for the lungs to expand. Air can then be drawn down the respiratory tract into the lungs. Upper respiratory infections such as the common cold or flu can inhibit inspiration by narrowing the respiratory tract.

Air that has passed through the upper respiratory tract has been humidified. This so that when it reaches the lungs it is easily dissolved in water before being diffused in to the blood stream.


Within the lungs themselves the conditions have to be in a constant state of balance. Gas exchange happens by diffusion, high concentrations of carbon dioxide in the blood diffuse into the alveoli where concentrations are lower. Oxygen concentrations are high in the alveoli and low in the blood.

Another important factor for gas exchange is pulmonary surfactant. This is produced by cells within the alveoli and consists of a mixture of lipids and proteins that reduces surface tension of the thin liquid film that lines each alveolus. (Sherwood 2007). The tension of this liquid is such that without the surfactant each alveolus would collapse in on itself.

In order for effective gaseous to take place effectively the entire respiratory tract needs to be functioning correctly and be free from infection. Any number of infections can cause this delicate system to function below optimum capacity.

References:

Sherwood L (2007), 'Human Physiology', From Cells to Systems
Thomson, Brooks/Cole

The Structure of the Respiratory System.

The respiratory tract begins at the nose. During inspiration air is drawn into the nasal cavity where it is warmed and humidified. The nasal cavity contains microscopic hairs known as cilia, there are also cells that produce mucus. Together these serve to trap dust, bacteria and other foreign particles that are in the air we breath and prevent them from reaching the lungs. The nasal cavity also contains a large number of capillaries in order to warm the air before it passes to the pharynx http://www.cdli.ca/~dpower/resp/struct~1.htm

From the nasal cavity air moves into the pharynx (throat) and then into the larynx. The larynx contains the vocal chords, and it is the act of air flow (expiration) across these that cause them to vibrate, enabling humans to make sounds for speech. The entrance to the larynx is blocked by the epiglottis, this prevents food and liquid from entering and diverts it down the oesophagus http://www.cdli.ca/~dpower/resp/struct~1.htm

The nasal cavity, pharynx and larynx are known as the upper respiratory tract. Their main function is to warm and humidify the air, as well as removing dust and bacteria from the air before it moves into the lower respiratory tract.

http://www.encognitive.com/node/1128

The larynx leads directly into the trachea. This is a tube approximately 12cm in length and 2.5cm wide. The trachea is kept open with 16 rings of cartilage that form a 'c' shape. Like the nasal cavity the trachea is lined with cilia which move mucus back to the pharynx http://www.bbc.co.uk/dna/h2g2/A27019505.

At the centre of the chest the trachea splits into the left and right primary bronchi. The right bronchus is wider than the left and is often where inhaled foreign objects end up. These two primary bronchi divide into secondary bronchi which divide further into tertiary bronchi, with eight on the left and ten on the right http://www.bbc.co.uk/dna/h2g2/A27019505.

These tertiary bronchi then split into a network of bronchioles each ending in an alveoli. This network is what makes up the lungs. Each lung is split into lobes with two on the left and three on the right. The reason the left only has two is because the heart sits on that side of the body.

Alveoli are tiny air sacs at the end of bronchioles, it is here that gaseous exchange takes place. Alveoli are only one cell thick, this is so that air can diffuse quickly into the blood stream. There are thought to be around 600 million alveoli in an adult human giving a surface area of 100 square meters. This provides a hugh area for gaseous exchange.http://www.bbc.co.uk/dna/h2g2/A27019505