Respiration Tutorial

Click on 'Model' in the menu to the left to view this at any time. The model depicts the interrelationships between the major topics of this tutorial.

Click on "Design" in the Menu on the left.

respiration model

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respiration model

Design of the Model

As you read through each section below, scroll back up to the top of the page
and locate the part of the model being described.

The flask at the upper left represents an alveolus and a respiratory passageway. The donut encircling it represents pulmonary capillaries. A pulmonary arteriole is at the top of the model carrying blood to the alveolus and air passage; a pulmonary venuole below the alveolus carries blood to the left heart (LH).

The circle at the bottom of the model represents a body cell; the encircling donut represents systemic capillaries. Blood is brought to these capillaries via systemic arterioles and blood is carried away by systemic venuoles that eventually lead to the right heart (RH) .

The oval in the center represents a red blood cell. Various chemical reactions are shown to occur inside the cell. The circles and small pentagons seen along the left side of the model represent various neural pathways that control breathing.


Throughout the model nerves are represented as both solid and dashed lines with small perpendicular lines at their ends. Solid green nerves indicate that these will cause a stimulatory response of their target; dashed red nerves indicate inhibitory responses.

Solid and dashed red and green arrows indicate non-neural relationships; solid green arrows represent direct relationships while dashed red arrows indicate inverse relationships.

The thin red and blue arrows within and around the oval RBC indicate chemical reactions.


Respiration is the exchange of oxygen and carbon dioxide across capillary walls throughout the body. Exchange of these gases with the outside environment is external respiration and takes place across the walls of pulmonary capillaries. Exchange of these gasses between the blood and cells throughout the body is internal respiration and takes place across the walls of systemic capillaries.

internal respiration

Systemic Gas Exchange / Internal Respiration

The insert shows a body cells encircled by systemic capillaries. Blood enters via systemic arterioles (left) left and leaves through systemic venuoles (right). The insert shows carbon dioxide entering the blood and oxygen leaving the blood.

Carbon Dioxide

Carbon dioxide is a waste product of metabolism and is constantly given off by cells. The incoming blood has relatively low levels of this dissolved gas (partial pressure of 40 mm Hg) so it diffuses from the cells into the blood as indicated by the arrow. When the blood leaves these cells its level of carbon dioxide has increased to 45 mm Hg.


The oxygen concentration is relatively higher in the blood (partial pressure of 104 mm Hg) leading into systemic capillaries. As a result, oxygen diffuses from the blood into the cells as shown by the arrow. Therefore, when blood enters the systemic venuoles its concentration has dropped to 40 mmHg.

external respiration

Pulmonary Gas Exchange / External Respiration

The inset shows an alveolus and air passage of the lungs. Blood enters the pulmonary capillaries (donut) via pulmonary arterioles (right) and leaves via pulmonary venuoles (left). Atmospheric oxygen within the alveolus diffuses into the blood as shown by the arrow; carbon dioxide leaves the blood (arrow) and enters the alveolus. The double-headed arrow represents the inward and outward flow of air (ventilation).

Carbon Dioxide

Carbon dioxide is at extremely low levels in the atmosphere (partial pressure of 0.3 mm Hg). Carbon dioxide entering the pulmonary capillaries has a partial pressure of 45 mm Hg that it picked up from body cells. Therefore, this gas diffuses out of the blood (arrow) and into the lungs. When the blood leaves the pulmonary capillaries its concentration has decreased to 40 mm Hg. This decrease seems small but it is only measuring the carbon dioxide dissolved in the plasma; much more carbon dioxide is in the form of bicarbonate ions and bound within red blood cells as described in the next section.


Blood entering the pulmonary capillaries has an oxygen concentration of 40 mm Hg. The concentration of atmospheric oxygen is 160 mm Hg so it diffuses from the alveoli into the blood. Oxygen in the blood leaving the lungs has increased to 104 mm Hg.

Transport of Respiratory Gases

Cell membranes are completely permeable to the passage of carbon dioxide and oxygen. On diffusing into the red blood cell these gases interact with other compounds. Once beyond the capillaries no further gain or loss of these gases can occur and the compounds involved reach an equilibrium.

While in the systemic capillaries, the RBCs are subjected to increasing levels of carbon dioxide and decreasing levels of oxygen. The opposite is the case in the pulmonary capillaries. The reactions are the same but in reverse directions in these two locations. In the model the blue arrows represent the direction of the reactions occurring in the systemic capillaries; the red arrows represent the direction in the pulmonary capillaries.

systemic RBC

Chemical Reactions in Systemic Capillaries--Blue Arrows

The enzyme carbonic anhydrase (CA in circle) is found inside red blood cells. This enzyme catalyzes the combination of carbon dioxide (CO2) and water (H2O) into carbonic acid (H2CO3). Once formed within the RBC a small proportion of carbonic acid molecules ionize to produce hydrogen ions (H+) and bicarbonate ions (HCO3-).

The RBC membrane is also freely permeable to bicarbonate ions (HCO3-). These ions diffuse into the plasma along their concentration gradient in exchange for chloride ions (Cl-). This is called the 'chloride shift.'

The hydrogen ions (H+) in the RBC are buffered, that is, they are not allowed to diffuse into the plasma. Instead, they bind to hemoglobin (Hb) but only after they displace oxygen (O2) from that compound. Hemoglobin, when it binds to hydrogen ions, is said to be 'reduced.' It is in this form that hemoglobin also binds with carbon dioxide to form carbaminohemoglobin (HbH+CO2).

Carbon dioxide is transported in three forms. The smallest amount is in the form of dissolved carbon dioxide located in the plasma. There is somewhat more carbon dioxide in the form of carbaminohemoglobin confined to the RBC. The majority is in the form of bicarbonate ions that equilibrate across the RBC membrane.

Chemical Reactions in Pulmonary Capillaries--Red Arrows

pulmonary RBC

As oxygen (O2) enters the RBC it interacts with carbaminohemoglobin after displacing hydrogen ions (H+) from it. This also causes any bound carbon dioxide to dislodge. In normal situations hemoglobin becomes almost 100% saturated with oxygen. In this form it is called oxyhemoglobin (HbO2).

The 'freed' hydrogen ions (H+) combine with bicarbonate ions (HCO3-) to form carbonic acid (H2CO3). This reduces the bicarbonate ions within the RBC causing more to diffuse in from the plasma. This, in turn, causes a chloride shift from the RBC into the plasma.

The additional carbonic acid (H2CO3) within the RBC is converted to carbonic acid to carbon dioxide and water by carbonic anhydrase (CA in circle). The increasing carbon dioxide in the plasma diffuses out of the plasma and into the lungs.

Ventilation & Perfusion



Ventilation (breathing) moves air into and out of the lungs. As the diaphragm contracts it enlarges the thoracic cavity which, in turn, drops the pressure in this space. This causes the pliable lungs to expand into this space which, in turn, decreases the pressure within the lungs. There now exists a pressure gradient between the atmospheric air (Patm) and alveolar air (Pa) causing inward air flow (inhalation).

Alveolar pressure (Pa) is controlled by the activity of the diaphragm. When the diaphragm contracts, causing the thoracic cavity to expand, the alveolar pressure is reduced--an inverse relationship represented by the dashed red arrow. This establishes a pressure gradient where alveolar pressure is lower than atmospheric pressure (Patm) and inhalation occurs. When the diaphragm relaxes, allowing the thoracic cavity to recoil to a smaller volume, the alveolar pressure increases reversing the pressure gradient ... air flows from the alveoli to the atmosphere (exhalation) along this new pressure gradient.

Ventilation-Perfusion Coupling

Perfusion is blood flow within an organ and a unique anatomical design exists between pulmonary arterioles and air passages in the lungs. As shown in the insert, pulmonary arterioles travel parallel and adjacent to these air passages and their capillaries surround their alveoli. This close physical setting allows components in the blood to affect pulmonary structures as well as allowing components in the alveolar air to affect structures in the arterioles. This is ventilation-perfusion coupling.

The respiratory bronchioles (neck of the flask) contain smooth muscle tissue that controls the diameter of these air passages. If these muscles relax, bronchodilation (BCL) occurs ... air flows through the air passages with less resistance when these airways are wider. Bronchoconstriction is the opposite; narrower airways present more resistance to air flow through them. It is the nearby concentration of carbon dioxide (CO2) in pulmonary arterioles that controls these smooth muscles. When carbon dioxide concentration in the blood increases the airway smooth muscles relax and the size of the bronchioles increases (BDL). This direct relationship ... increased blood carbon dioxide and increased bronchodilation ... is represented by the (solid green arrow).

This coupling also exists in the reverse direction. That is, a component of alveolar air effects a circulatory structure. The insert shows that the alveolar oxygen concentration affects the size of pulmonary arterioles. The direct relationship (solid green arrow) between increased alveolar oxygen and increased vasodilation of the arteriole insures good blood flow (perfusion) surrounding alveoli with good oxygen levels.


Respiratory Center

The brainstem contains three groupings of neurons responsible for the depth and rate of ventilation. The dorsal respiratory group sets an inherent basic rhythm of diaphragmatic ventilation. The pneumotaxic center can modify this basic rhythm. The ventral respiratory group comes into play when there is need for more intense ventilation by activating accessory muscles such as those between the ribs.

Dorsal Respiratory Group (DRG)

The insert shows two stimulatory (green solid lines) neurons leaving the DRG; the phrenic nerve (P) targeting the diaphragm and a neural connection within the medulla targeting the nearby ventral respiratory group (VRG). Also shown are two inhibitory (dashed red lines) neurons leading toward the DRG; one is from the pneumotaxic center (PC) and the other comes from stretch receptors (SR) in the walls of the bronchioles. Additionally, there are two stimulatory inputs; hydrogen ions (H+) from interstitial fluid and neural stimulation originating from chemically-sensitive carotid and aortic bodies (CAB) in the walls of arterial blood vessels.

The dorsal respiratory group is located in the medulla of the brainstem. It targets the diaphragm via the phrenic nerve (P) causing contraction and inhalation. Impulses to the diaphragm start of at a slow rate and gradually increase (an inspiratory ramp); after 2 seconds all impulses stop. With no stimulation the diaphragm relaxes resulting in passive expiration usually lasting for 3 seconds before the cycle repeats. At this intrinsic rate there would be 12 breaths per minute. Of course this subconscious rate can be consciously overridden.

Pneumotaxic Center (PC)

This center prevents overexpansion of the lungs. Inhalation causes bronchodilation (BDL) that stretches the walls of these airways and activates stretch receptors (SR). When so activated, the usually 'silent' pneumotaxic center sends inhibitory (dashed red line) impulses to the dorsal respiratory group (DRG) that decreases the length of time spent in inspiration. This causes a shallower but more rapid breathing rate. Direct inhibition from the stretch receptors to the pneumotaxic center has the same effect.

Ventral Respiratory Group (VRG)

With increasing physical activity we need to increase the rate of ventilation. Under these conditions the carbon dioxide (CO2) concentration increases, the oxygen (O2) concentration decreases and the hydrogen ion (H+) concentration increases. The insert shows these conditions within the systemic arteriole. In the walls of the carotid arteries and the aorta there are chemoreceptors called carotid and aortic bodies (CAB). When stimulated by these changes in the blood the bodies send stimulatory (green line) impulses to the DRG via the glossopharyngeal nerve (GP). The result is increased depth and rate of breathing. The insert also shows a stimulatory nerve (green line) from the DRG to the VRG; this connection insures that inhalation impulses from the VRG and timed to occur with impulses going to the diaphragm.

Brainstem Chemoreceptors (not illustrated)

The DRG will respond indirectly to high concentrations of carbon dioxide (CO2). In the blood, some carbon dioxide is converted to carbonic acid that dissociates into bicarbonate and hydrogen ions. Capillaries in the brain are not permeable to hydrogen ions but do let carbon dioxide through the blood-brain barrier very easily. When the carbon dioxide enters the cerebrospinal fluid or the interstitial fluid it will then dissociate to release hydrogen ions. The insert shows the hydrogen ions stimulating the DRG to increase ventilation.