Pulmonary circulation

Blood vessels that belong to the pulmonary circulation are marked in color.

The pulmonary circulation or small circulation is the part of the blood circulation that brings the blood from the heart to the lungs and back again.

The oxygen-poor blood is discharged from the right ventricle ( heart chamber ) via the pulmonary trunk ( pulmonary trunk ). This trunk divides into the right and left pulmonary arteries ( arteria pulmonalis ). These branch out into ever finer vessels in the respective lungs and finally go into the hair vessels ( capillaries ), which span the pulmonary sacs (alveoli). This is where the gas exchange takes place, ie the blood releases carbon dioxide (CO ₂ ) and absorbs oxygen (O ₂ ) (by diffusion ).

The blood, which is now rich in oxygen, flows back to the heart via merging and increasingly larger veins . There two right pulmonary veins (Venae pulmonales dextrae) and two left ones (Venae pulmonales sinistrae) flow into the left atrium (atrium cordis sinistrum) via a main vein trunk (Vena pulmonalis ).

Since the pulmonary arteries transport oxygen-poor blood, the lungs themselves are mainly supplied with oxygen-rich blood in a bronchial circuit through the bronchial arteries ; these bronchial arteries are branches of the aorta .

In the fetus, there is a connection from the pulmonary trunk to the aorta , the ductus arteriosus . There is also an opening between the right and left atrium, the foramen ovale . These two formations largely short-circuit the pulmonary circulation; the fetus's lungs are not yet ventilated.

For the history of the discovery of the pulmonary circulation beginning with Ibn an-Nafīs ( al-Quraschi ) in the 13th century, see Blood circulation # Research history .

Overview of the pulmonary circulation

Transport of "low-oxygen" blood
Transport of "oxygen-rich" blood

right atrium → tricuspid valve → right ventricle → pulmonary valve → pulmonary arteries → capillary area of the alveoli → pulmonary veins → left atrium → mitral valve → left ventricle

Pulmonary Vascular Resistance (PVR)

The pulmonary vascular resistance (the resistance of the pulmonary vessels) is defined according to the law of Hagen-Poiseuille as the ratio of the pressure difference between the pulmonary artery and the left atrium to the pulmonary blood flow. The pulmonary vascular resistance (PVR) of the pulmonary circulation in healthy people is only about 1/10 as great as the total peripheral resistance of the body's circulation (norm: 45-120 dyn x sec x cm ⁻⁵ = 0.56-0.94 mmHg x min / l). Therefore, the arterial blood pressure in the pulmonary circulation is 20/8 mmHg significantly lower than in the large circulation (120/80 mmHg).

This pulmonary vascular resistance can also be measured in R units (resistance units); this unit is also referred to as Wood unit after its first descriptor Wood. This usual Wood unit (resistance unit) is therefore 1 RU = 1 mmHg / (l / min) = 1 mmHgmin / l. These Wood units can be converted into metric resistance units ( Pa × s × m ⁻³ ) in the International System of Units (SI) by multiplying them by the factor 8 × 10 ⁶ . The Wood units are used in pediatric cardiology and are often related to the child's body surface. This division by the body surface leads to a normalization of the resistance to a standard body surface of one square meter. The unit mmHgmin / l becomes the unit mmHgmin / lm². In internal medicine, the metric SI units are usually given. The pulmonary vascular resistance increases with the severity of the pulmonary vascular changes.

Pathophysiology

A decrease in the O2 concentration in the alveoli (= hypoxia) leads to vasoconstriction of the pulmonary arteries in the corresponding lung sections and thus to an increase in the PVR. This mechanism is known as pulmonary hypoxic vasoconstriction or the Euler-Liljestrand mechanism . It is used to reduce blood flow in poorly ventilated lung areas and thus redirect the pulmonary blood flow to better ventilated lung areas. This reduces the proportion of blood that flows through the lungs but is not loaded with oxygen (oxygenated) there (reduction of the shunt volume). The neuronal regulation of the pulmonary circulation is of little importance. The PVR is therefore only subject to a slight sympathetic (= vasoconstriction) and parasympathetic regulation.

A constriction of the pulmonary circulation (e.g. due to pulmonary sclerosis, pulmonary embolism, inflammatory constriction of the pulmonary vein, destruction of the lung parenchyma, pleural obstruction or constriction of the pulmonary arterioles) can lead to pulmonary hypertension due to the pressure increase in the small circulation .

literature

Peter Lotz: Anatomy and Physiology of the Respiratory Tract. In: Jürgen Kilian, Herbert Benzer, FW Ahnefeld (ed.): Basic principles of ventilation. Springer, Berlin a. a. 1991, ISBN 3-540-53078-9 , 2nd, unchanged edition, ibid 1994, ISBN 3-540-57904-4 , pp. 3–45, here: pp. 35–43.

Individual evidence

↑ Mewis, Riessen, Spyridopoulos (ed.): Cardiology compact - Everything for ward and specialist examination . 2nd Edition. Thieme, Stuttgart, New York 2006, ISBN 3-13-130742-0 , pp. 110 .
^ Herbert Reindell , Helmut Klepzig: Diseases of the heart and the vessels. In: Ludwig Heilmeyer (ed.): Textbook of internal medicine. Springer-Verlag, Berlin / Göttingen / Heidelberg 1955; 2nd edition ibid. 1961, pp. 450-598, here: pp. 578-580 ( The hypertension in the large and small circulation ).

[Mewis-1] Mewis, Riessen, Spyridopoulos (ed.): Cardiology compact - Everything for ward and specialist examination . 2nd Edition. Thieme, Stuttgart, New York 2006, ISBN 3-13-130742-0 , pp. 110 .

[2] Herbert Reindell , Helmut Klepzig: Diseases of the heart and the vessels. In: Ludwig Heilmeyer (ed.): Textbook of internal medicine. Springer-Verlag, Berlin / Göttingen / Heidelberg 1955; 2nd edition ibid. 1961, pp. 450-598, here: pp. 578-580 ( The hypertension in the large and small circulation ).