Respiration Tutorial

Double-click on 'Model' in the menu to the left to view it at any time. The model depicts the interrelationships between the major topics of this tutorial. Study the topics in the order shown in the Menu.
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.

respiration model

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 inset 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.


Because cells are constantly using up oxygen the concentration of this gas is relatively higher in the blood (partial pressure of 104 mm Hg). 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

Blood entering the pulmonary capillaries has a partial pressure of 45 mm Hg that it picked up from body cells. Carbon dioxide is at extremely low levels in the atmosphere (partial pressure of 0.3 mm Hg) and therefore in the alveoli of the lungs. 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 concentration of carbonic acid (H2CO3) within the RBC does not increase because of the presence of carbonic anhydrase (CA in circle). This enzyme converts carbonic acid to carbon dioxide and water. 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).

The direction (arrow is double-headed) of the pressure gradient alternates; when alveolar pressure is greater than atmospheric pressure exhalation occurs, when alveolar pressure is lower then inhalation occurs. Alveolar pressure (Pa) is controlled by the activity of the diaphragm. When the diaphragm contracts the alveolar pressure is reduced--an inverse relationship represented by the dashed red arrow.

The respiratory bronchioles (neck of the flask) contain smooth muscle tissue that narrows the diameter of these air passages as it contracts-- bronchoconstriction. As these muscles relax, bronchodilation (BCL) occurs ... air flows through the air passages with less resistance when these vessels are wider. It is the nearby concentration of carbon dioxide (CO2) in pulmonary arterioles that controls these smooth muscles (solid green arrow). This direct relationship insures that alveoli with good blood flow will also be well ventilated; this is called ventilation-perfusion coupling.


Perfusion is blood flow. A unique anatomical design exists between pulmonary arterioles and air passages. As shown in the inset, pulmonary arterioles travel parallel and adjacent to these air passages and their capillaries surround their alveoli. In well ventilated alveoli, oxygen diffuses toward the arterioles causing vasodilation (VDL) ... another example of ventilation-perfusion coupling .

Involuntary Neural Controls

There are three respiratory centers in the brainstem: dorsal respiratory group, pneumotaxic center (a.k.a., pontine respiratory group) and the ventral respiratory group .

Dorsal Respiratory Group

neural control

The medullary dorsal respiratory group (DRG), also called the 'inspiratory center', sends stimulatory signals to the diaphragm by way of the phrenic nerve (solid green line labeled P). The phrenic nerve (solid green line, P) carries stimulatory impulses to the diaphragm which begin slowly then gradually increase. If isolated from other inputs, inhalation will last for about 2 seconds then pause for 3 seconds before the next inspiration. This breathing rate varies depending on the depth of breathing, as sensed by stretch receptors in the lungs, and carbon dioxide levels in the blood, as sensed by peripheral chemoreceptors .

Pneumotaxic Center & Stretch Receptors

Contraction of the diaphragm expands the lungs dropping the pressure in the alveoli (Pa) below atmospheric pressure (Patm) causing inhalation. This inverse relationship is represented by the dashed red arrow. The decreased alveolar pressure activates surrounding stretch receptors (SR hexagon) ... another inverse relationship (dashed red arrow). Activated stretch receptors send stimulatory (solid green line ) signals to the pneumotaxic center (PC) via the vagus nerve (V). The PC inhibits (dashed red line) the DRG that stops stimulating the diaphragm and allows it to relax. The DRG also receives direct inhibitory signals (dashed red line) from the stretch receptors.

Peripheral Chemoreceptors

The primary mechanism that controls the rate and depth of breathing is the carbon dioxide level in the blood. Increased metabolic activity elevates CO2 levels and lowers O2 levels. Chemoreceptors in the carotid and aortic bodies (CAB hexagon) are activated (green arrows) by increased CO2 and H+ ... direct relationships ... stimulating the DRG via the glossopharyngeal nerve (GP).

These receptors are also sensitive to decreasing levels of oxygen (O2) but only when the level is very low. This inverse relationship is shown by the dashed red arrow. The reverse is also true, that is, high oxygen levels can inhibit respiration.

The DRG is also affected by the chemistry of the cerebrospinal fluid (to the left of the DRG) . Even though hydrogen ions cannot cross the blood-brain barrier carbon dioxide can and, within the cerebrospinal fluid, it reacts with water to liberate hydrogen ions. These direct relationships are represented by the solid green arrows between carbon dioxide and hydrogen ions and the DRG.

Ventral Respiratory Group

An overdrive mechanism, active only when greater ventilation is required, consists of two parts; one stimulates expiratory muscles and the other stimulates inspiratory muscles other than the diaphragm. If one part is active is inhibits the other as shown by the pair of dashed red lines. Some theories hold that the ventral respiratory group (VRG) is activated by 'spill over' from the DRG when it's activity is increased. This is indicated by the solid green line representing stimulatory nerve fibers.