What Is the Relationship Between Pulmonary and Systemic Circulation?

Also known as a small cycle. Blood returned to the heart from the systemic circulation flows into the right ventricle from the right atrium. When the ventricle contracts, blood enters the pulmonary artery from the right ventricle, branches through it to reach the pulmonary capillaries, and performs gas exchange here, and the venous blood becomes arterial blood. Pulmonary veins return into the left atrium and then into the left ventricle. Pulmonary arteries are short and thick, about the fourth thoracic spine, divided into left and right pulmonary arteries. The left pulmonary artery is shorter, reaching the left hilum and divided into the upper and lower lobe of the left lung; the right pulmonary artery is longer, reaching the right hilar into the upper, middle, and lower lobe of the lung. Pulmonary veins have no valves, two left and right, called the left and right superior pulmonary veins and inferior pulmonary veins, respectively. Injected into upper left posterior.

Also known as a small cycle. Blood returned to the heart from the systemic circulation flows into the right ventricle from the right atrium. When the ventricle contracts, blood enters the pulmonary artery from the right ventricle, branches through it to reach the pulmonary capillaries, and performs gas exchange here, and the venous blood becomes arterial blood. Pulmonary veins return into the left atrium and then into the left ventricle. Pulmonary arteries are short and thick, about the fourth thoracic spine, divided into left and right pulmonary arteries. The left pulmonary artery is shorter, reaching the left hilum and divided into the upper and lower lobe of the left lung; the right pulmonary artery is longer, reaching the right hilar into the upper, middle, and lower lobe of the lung. Pulmonary veins have no valves, two left and right, called the left and right superior pulmonary veins and inferior pulmonary veins, respectively. Injected into upper left posterior.

Chinese name

Pulmonary circulation

Foreign name

pulmonary circulation

Regulation of blood flow

Neuromodulation

blood circulation

Systemic and pulmonary circulation

Lung blood volume

About 450ml, 9% of total body blood

Plasma osmotic pressure

3.3kPa (25mmHg)

Capillaries

About 0.9kPa (7mmHg)

Pulmonary circulation characteristics

1. The pulmonary circulation path is shorter than the systemic circulation, the pulmonary artery wall is thin, the elastic fibers are less, and it is easy to expand; the pulmonary vessels have many branches and short branches, the diameter is large, the peripheral resistance is small, and the pulmonary artery pressure is only 1/6 of the aortic pressure. The blood pressure of the pulmonary capillaries is also very low, about 0.93 kPa. Although the pulmonary capillary pressure is lower than the plasma colloid osmotic pressure, small molecule plasma proteins can penetrate the pulmonary capillaries, the colloid osmotic pressure outside the tube is high, and tissue fluid is still formed in the alveoli under normal circumstances. However, due to the rapid return of lymph in the lungs, pulmonary edema does not form. The pressure of the lung tissue fluid is negative, which is beneficial to absorb the fluid in the alveoli, so that there is no fluid accumulation in the alveoli. In certain pathological conditions, such as pulmonary congestion due to left ventricular failure, pulmonary venous pressure rises, and the capillary pressure in the pulmonary circulation increases with fluid volume accumulating in the alveoli or the interstitial spaces of the lungs, forming pulmonary edema.

2. Lung tissue and pulmonary blood vessels have great dilatability, so the blood volume of the lung is large. Therefore, the pulmonary circulation vessels can be regarded as a blood bank of the systemic circulation. When the body loses blood, the pulmonary circulation can transfer part of the blood to the systemic circulation, which plays a compensatory role.

3. Although the regulation of pulmonary circulation is dominated by the sympathetic and vagus nerves, the physiological significance of its regulation is unclear. Among the humoral factors, pulmonary blood vessels contract when blood oxygen decreases, and blood vessels dilate when blood oxygen increases. When the alveolar ventilation of a part of the lung is insufficient, and the partial pressure of oxygen is reduced, the blood vessels in this part contract and the blood flow decreases, so that more blood flows through the well-ventilated alveoli for effective exchange.

Relationship between changes in pulmonary circulation and pulmonary hypertension

(I) Pulmonary Artery Changes and Pulmonary Hypertension:

The pulmonary artery is the main vascular segment of the pulmonary blood vessels. It starts from the right heart and descends along the bronchus in steps, and is connected to the front of the alveolar capillaries. The pulmonary artery is different from the body artery in terms of structure and function. It has the characteristics of thin tube wall, large cross-sectional area, small number of smooth muscles, low activity, and high compliance. Therefore, in terms of hemodynamics, it has the characteristics of high irrigation and low resistance. Therefore, even if the right ventricular ejection volume exceeds 2.5 times normal, the pulmonary arterial pressure does not change. However, when the structure of the pulmonary artery is changed, the characteristics of high perfusion and low resistance will be damaged or disappeared, resulting in pulmonary circulation disorders. Changes in pulmonary artery structure are closely related to pulmonary hypertension.

(B) changes in pulmonary veins and pulmonary hypertension:

Pulmonary vein is another important vascular segment of the pulmonary circulation. It starts from the back of the pulmonary capillaries and ends up in the left atrium. The normal pulmonary vein pressure is very low, about 0.27kPa (2mmHg) on average, and its blood volume accounts for about 40% of the total pulmonary blood volume % To 50%. It can be seen that the pulmonary vein has the characteristics of low pressure and high blood volume. Because the pulmonary vein is directly connected to the left atrium, all diseases of the left heart, such as myocardial infarction, mitral stenosis, advanced hypertension, coronary heart disease, cardiomyopathy, and Left atrial thrombus, etc., can cause changes in the hemodynamic properties of pulmonary veins and the deformation of vascular wall structures, and even involve pulmonary arteries or capillaries.