the lung alveoli


In by Raphikammer

Alve o le [von * alveoli -], pulmonary alveoli , gas-filled end chamber of the lungs , site of diffusion-based gas exchange ( alveolar air , blood gases ) with the blood(release of carbon dioxide [CO 2 ] to the respiratory air and absorption of oxygen [O 2 ] from the respiratory air [ respiratory gas transport ]). The typical alveolus of man has a diameter of approx. 0.2 mm. It is surrounded by a dense blood-perfused capillary network. The inner surface of the alveolus is lined with a surfactant layer (surfactant layer, surfactant factor ). This film, a secretion of the pneumocytes, consisting of characteristic proteins, phospholipids and carbohydrates, lowers the alveolar surface tension ( interfaces , capillarity ) between the lung epithelium and the gas-filled interior, preventing the alveoli from collapsing on exhalation and sticking together the epithelia. Venous blood reaches the capillaries of the alveolus from the right half of the heart ( heart) arising pulmonary artery (arteria pulmonalis) and their branches.

The guest exchange in the lungs takes place via the pulmonary circulation in the approximately 300 million alveoli.

The oxygen volume fraction in the breathing air is 20.9 percent and the carbon dioxide content is 0.038 percent. The vast majority – 78.1 percent – is nitrogen, which is not usable for breathing. Humans need to breathe about 26 liters of air to extract one liter of oxygen.

Carbon dioxide-rich and oxygen-poor blood that comes from the cells of the body is pumped from the right ventricle to the lungs. Similar to the ever-finer air conduction system, the blood vessels leading to the lungs continue to ramify. Around the alveoli, a network of the finest blood vessels is formed, the so-called capillary network. Due to the strong branching of the blood vessels in the lungs, the blood becomes slower and the walls of the blood vessels become thinner and thinner.

The wall of the alveoli – the alveolar-capillary membrane – is also very thin (about one micrometer). The respiratory gases, oxygen (O 2)and carbon dioxide (CO 2), can therefore easily pass from one side to the other side of the air sac (diffusion).

Arrived at the alveoli, the carbon dioxide migrates from the blood into the air in the lungs and oxygen is taken from the breath into the blood. The now oxygen-rich blood is pumped back to the heart and distributed from the left ventricle in the rest of the body.

The area formed jointly by alveoli and capillaries is called the respiratory surface. In humans, this is about 100 to 140 square meters in size.

In physical rest, humans need 0.3 liters of oxygen per minute and exhale about 0.25 liters of carbon dioxide per minute. In order to achieve this, he transports about seven liters of air per minute through his lungs.

If the blood pressure in the pulmonary circulation, also known as the small circulation, is permanently increased, this is called pulmonary hypertension.

The typical partial pressures for oxygen are 5.3 kPa (= 40 mmHg) and carbon dioxide 6.1 kPa (= 46 mmHg). By breathing on the other hand (ventilation of the lungs) may be in the alveolar gas partial pressures of 13.3 kPa (= 100 mmHg, 14 vol.%) O 2 and 5.3 kPa (= 40 mmHg, 5.6 vol.%) CO 2 maintained. The difference of the partial pressure differences is the cause of the gas exchange by diffusion (Fick’s law). During capillary passage, blood enters into close contact with the alveolar gas. As diffusion resistances must be overcome: the alveolar epithelium, the connective tissue, the capillary endothelium, the blood plasma and the erythrocyte membrane ( erythrocytes ). The mentioned partial structures of the alveolar-capillary membrane are not thicker than 0.5-1 μm. The capillary passage takes about 0.3 s.

This contact time is sufficient to virtually equalize the gas partial pressures in the blood with those of the alveolar gas. The oxygen-enriched blood leaves the capillaries via the vascular system of the pulmonary veins and then reaches the left half of the heart, which pumps it back into the body. The perfusion the lung is about the same as the heartbeat volume (5 l / min in a healthy adult). Normally, less than 2% of the blood reaches the left ventricle bypassing the alveoli.

The total oxygen intake of an adult at rest is about 300 ml / min and increases under load. The efficiency of alveolar gas exchange depends on: 1) the perfusion of the lung with blood, 2) the alveolar surface, 3) the diffusion path, and 4) the alveolar ventilation. Physiological regulatory mechanisms ( control ) set these variables so that the oxygen loading of the blood remains constant regardless of the load ( homeostasis ). All four parameters can be negatively affected by pathological processes in the lungs. respiratory system, Drilling effect , bronchi ; Respiratory system I . 2) Teeth , thecodont . 3) For belemnites a conical cavity in the front end of the rostrum, in which Phragmocone and Velamen triplex are stuck.



The respiratory organ of the human, the lung, sits in the thorax and is formed of two lungs. Due to the position of the heart on the left side of the body, the left lung is slightly smaller than the right one.

Each lung is divided into furrows by furrows. On the right side, there are three, on the left two lung lobes per lung. The individual lung lobes can be subdivided into functional areas, the so-called lung segments. Below the lungs sit the diaphragm, which separates the chest cavity from the abdomen.

Around both lungs is the so-called lung pleura (pleura visceral), a protective, thin skin. Together with the pleura (pleura parietalis) that sits opposite the inside of the thorax and the diaphragm, the lung pelt makes sure that the breathing works. The fluid-filled space between the pleura and lung is called the pleural cavity or pleural space.
For more on the function of breathing, see the chapter “Respiratory mechanics”.

The general structure of the lungs is reminiscent of an inverted tree. Its trunk is formed by the trachea, which divides into the two main bronchi, which in turn enter the two lungs. The main bronchi, in turn, continue to bifurcate to bronchi and bronchioles, small branches that end in the alveoli.

Structure of the bronchi

Bronchi and bronchioles form a tube system in the lungs, which serves as a guidance system for the air. The trachea first divides into two main bronchi, which enter the two lungs. In the further course of the tube system, the main bronchi continue to bifurcate to bronchi and bronchioles, ever decreasing branches, which eventually end in the alveoli.

In contrast to the alveoli, there is no gas exchange in the tube system of the bronchi and bronchioles.

Bronchi are larger in diameter than bronchioles

This because their walls are reinforced by cartilage clasps.

Around the tubes of bronchi and bronchioles pull muscle strands of smooth muscle. These are controlled by parts of our autonomic nervous system, the sympathetic and the parasympathetic. During active phases, such as during sports, the sympathetic nerve ensures that the smooth muscles relax and the bronchi allow as much air as possible.

The parasympathetic nervous system, on the other hand, is responsible for allowing the body to enter a resting phase and to recover. It stimulates the contraction, that is, the contraction of the smooth muscles, thereby narrowing the diameter of the bronchi. Normally, this should help support breathing. But it can also lead to cramping of these muscles, such as in an asthma attack.

The inner walls of the bronchi and bronchioles are lined with a mucous-producing skin. Learn more about the function of mucus in the lungs.

Structure of the alveoli (alveoli)

The bronchioles end in humans in about 300 million small alveoli, where the gas exchange takes place. On the inner wall of the alveoli, there is a liquid film which tends to reduce its surface area. This surface tension is reduced by surface-active substances – so-called surfactants, especially lecithin derivatives.

The wall of the alveoli – the alveolar-capillary membrane – is very thin (about one micrometer) and provides little resistance to the oxygen (O 2 ) and carbon dioxide (CO 2) breathing gases, allowing the gases to pass easily from one side to the other (Diffusion).


Approximately 300 million alveoli, whose respiratory surface is approximately 100 to 140 square meters, provide for the oxygen supply of our body.

On the side of the alveoli, which is remote from the respiratory air, a network of finest blood vessels, a so-called capillary network, is deposited. The area formed by the alveoli and capillaries together is called the respiratory surface because only here does the gas exchange takes place in the lungs. In humans, about 300 million alveoli form a respiratory surface of about 100 to 140 square meters. So the body can be optimally supplied with oxygen. More in the chapter “Gas exchange”.

Between the alveoli, there is connective tissue. If its cells proliferate excessively, pulmonary fibrosis develops.