Respiratory System

Respiratory Anatomy

All the cells of a human, even those deep in the center of the body, need a continuous supply of oxygen and must be able to eliminate carbon dioxide. The human skin has adapted to conserve water and is unsuitable for gaseous exchange.4 In addition, humans are too large for effective diffusion of gaseous exchange, as it would take too long. The respiratory system encompasses the organs of breathing, which includes the nose, pharynx (mouth), larynx (throat), trachea (windpipe), bronchi and bronchioles (airways), lungs and diaphragm. The sinuses, which are lined by the same type of cells that line the upper airways, are included in the respiratory system. The respiratory system is the principal site of gas exchange in the body. It is here that oxygen is absorbed into to your bloodstream and carbon dioxide is removed.

Air moving through the respiratory system also provides you with the ability to vocalize, such as speaking or singing. The gas exchange process is initiated when air is inhaled or drawn into the body through the nose or the mouth, which wet and warm the air so it won’t irritate your lungs. Sinuses are hollow spaces in the bones of your head above and below your eyes that are connected to your nose by small openings. Sinuses help regulate the temperature and humidity of inhaled air. The nose is the preferred entrance for outside air into the respiratory system. The hairs lining the nose’s wall are part of the air-cleaning system. Cilia (tiny mucous-covered hairs) in the airways entrap foreign particles and germs to filter the air that we breathe. We then cough or sneeze the particles out of the body. The trachea branches into two bronchi, tubes that lead to the lungs. The right lung is divided into three lobes, or sections. Each lobe is like a balloon filled with sponge-like tissue. Air moves in and out through one opening, a branch of the bronchial tube. The left lung is divided into two lobes. The pleura are the two membranes, actually one continuous one folded on itself, that surround each lobe of the lungs and separate your lungs from your chest wall. The bronchial tubes are lined with cilia that move like waves. This motion carries mucus (sticky phlegm or liquid) upward and out into your throat, where it is either coughed up or swallowed. Mucus catches and holds much of the dust, germs, and other unwanted matter that has invaded your lungs. We get rid of this matter when we cough, sneeze, clear our throat or swallow.

By staff (2014)


As the bronchial tubes pass through the lungs, they divide into smaller air passages called bronchioles. The bronchioles end in tiny balloon-like air sacs called alveoli. The body has over 300 million alveoli. The alveoli are surrounded by a mesh of tiny blood vessels called capillaries. Here, oxygen from the inhaled air passes through the alveoli walls and into the blood. The alveoli have adaptations which have increased their efficiency for gas exchange. The alveoli walls are very thin, it has a moist inner surface, a huge combined surface area and a rich blood supply. Once in the lungs, oxygen is moved into the bloodstream. Blood passes through the capillaries, entering through the pulmonary artery and leaving via the pulmonary vein. Blood carries the oxygen through the body to where it is needed. Red blood cells collect carbon dioxide from the body’s cells and transports it back to the lungs. An exchange of oxygen and carbon dioxide takes place in the alveoli, small structures within the lungs. The carbon dioxide, a waste gas, is exhaled and the cycle begins again with the next breath. The diaphragm is a dome-shaped muscle below the lungs that controls breathing. The diaphragm flattens out and pulls forward, drawing air into the lungs for inhalation. During exhalation the diaphragm expands to force air out of the lungs. Lung is elastic, it recoils as soon as you relax after breath intake. If not for the rib cage, the lung would collapse even further. Surface tension causes the lung to collapse. Surfactants produced in the alveoli decreases surface tension, and helps alveoli to stay open.

By domdomegg (Own work) [CC BY 4.0 (], via Wikimedia Commons

Henry’s Law

Henry’s law states that the amount of a gas that dissolves in a liquid is directly proportional to the partial pressure of that gas. An easy way to visualise Henry’s law is given by carbonated soft drinks. Before the bottle is opened, almost pure carbon dioxide gas is above the liquid at a pressure slightly higher than atmospheric pressure. There is also dissolved carbon dioxide in the drink itself. After opening the bottle, some of this gas escapes (noted by a hissing sound). At this point, the pressure above the liquid is now lower which causes some of the dissolved carbon dioxide to come out of solution as bubbles. If a glass of the soft drink is left in the open, the concentration of carbon dioxide in solution will come into equilibrium with the carbon dioxide in the air and the drink will go flat. Henry’s law is applicable in the respiratory system by predicting how gasses will dissolve in the alveoli and bloodstream during gas exchange. The amount of oxygen that dissolves into the bloodstream is directly proportional to the partial pressure of oxygen in alveolar air. The partial pressure of oxygen is greater in alveolar air than in deoxygenated blood, so oxygen has a high tendency to dissolve into deoxygenated blood. Conversely the opposite is true for carbon dioxide, which has a greater partial pressure in deoxygenated blood than in the alveolar air, so it will diffuse out of the solution and back into gaseous form.

Differential Pressure

Differential pressure refers to the difference between air pressure inside the lungs (intrapulmonary pressure) and the air pressure outside the lungs (intrapleural pressure). When intrapulmonary pressure is the same as atmospheric pressure, the lung is open to the outside which results in it having the same pressure as outside. When intrapleural pressure is less than atmospheric pressure, it sucks on the lungs and prevents the lungs from collapsing. During breath intake, intrapleural pressure decreases even further, causing the lung to expand.

Respiratory Role in Thermoregulation

The body has a desire to carry out homeostasis, which refers to keeping the body at an internal balance and ensuring that the body is healthy and within the right range of functioning properly. This results in cooling down the body when it is too hot, in addition to heating up the body when it is too cold. In order to maximize gas exchange, there is a tremendous surface area over which the alveoli and capillaries interact. Because the entire respiratory tract is highly vascular, it can also be used for thermoregulation, or the regulation of body temperature. One of the purposes of the nose and nasal cavity is to warm and moisten the inspired air. However, it also functions to retrieve moisture and heat from expired air. The high water content of the mucus film acts to humidify inhaled air. Underneath the nasal epithelium there is a rich supply of capillaries and thin walled veins that serve to warm the incoming air. If the air is especially cold, this plexus (network) of veins and capillaries fills with blood, and the air-heating process is intensified. The conchea was previously cooled by incoming cold inspired air and it can be used to retrieve moisture and heat from expired air. As there is warm air leaving, it precipitates moisture on the conchea which also acts to extract heat from the humid air flowing over them. When the body becomes too hot, due to the surrounding environment area being over heated, the capillaries that are found at the surface of the skin open up and widen to ensure that the blood flow is flowing towards the skin’s surface in order to let off heat which will then eventually cool down the body, to the right temperature that is needed. Water within the blood is released as sweat in order to cool our body down also – this process is called vasodilation as the capillaries dilate (expand) which gives off the heat to return us cooler. On the other hand, when the body is too cool the capillaries near the surface of the skin close up and get narrower to trap the heat to push the blood flow away from the skin’s surface in order to trap the heat and keep us warm. This process is called vasoconstriction as the capillaries constrict when lying close to the skin in order to allow the body’s organs/muscles to function properly and to keep warm if they happen to be too cold to function. Thermal panting is also a next method to remove heat from the body as heated air within the body is expelled in expired air. This results from a build up of body heat, usually from an increase in the environmental temperature or from additional activity.

Acid-Base Regulation

An acid–base imbalance can occur if the blood pH shifts out of its normal range of 7.35 to 7.45. An excess of acid in the blood is called acidemia and an excess of base is called alkalemia. Several buffering agents that reversibly bind hydrogen ions and impede any change in pH exist. The bicarbonate buffering system is one such system, as carbon dioxide (CO2) can be shifted through carbonic acid (H2CO3) to hydrogen ions and bicarbonate (HCO3−) as shown below:

Bicarbonate Buffer System

Acid–base imbalances in the blood’s pH that overcome the buffer system can be altered by changes in breathing to expel more CO2 and raise pH back to normal. This alters the concentration of carbon dioxide in the blood, shifting the above reaction according to Le Chatelier’s principle, which in turn alters the pH. For instance, if the blood pH drops too low (acidemia), the body will compensate by increasing breathing thereby expelling CO2, and shifting the above reaction to the left such that fewer hydrogen ions are free; thus the pH will rise back to normal. For alkalemia, the opposite occurs.

Respiratory Control

Involuntary respiration is any form of respiratory control that is not under direct, conscious control. Breathing is required to sustain life, so involuntary respiration allows it to happen when voluntary respiration is not possible, such as during sleep. Involuntary respiration also has metabolic functions that work even when a person is conscious. The respiratory centers contain chemoreceptors that detect pH levels in the blood and send signals to the respiratory centers of the brain to adjust the ventilation rate to change blood pH by increasing or decreasing the removal of carbon dioxide. The medulla oblongata is the primary respiratory control center. Its main function is to send signals to the muscles that control respiration to cause breathing to occur. The pons is the other respiratory center and is located underneath the medulla. Its main function is to control the rate or speed of involuntary respiration. The apneustic center of the pons sends signals for inspiration for long and deep breaths. It controls the intensity of breathing and is inhibited by the stretch receptors of the pulmonary muscles at maximum depth of inspiration, or by signals from the pnuemotaxic center. It increases tidal volume. The pnuemotaxic center of the pons sends signals to inhibit inspiration that allows it to finely control the respiratory rate. Its signals limit the activity of the phrenic nerve and inhibits the signals of the apneustic center. It decreases tidal volume.


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