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Pulmonary circulation

Pulmonary circulation is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Pulmonary circulation rather than just read about it. In short: The pulmonary circulation is a division of the circulatory system in all vertebrates. The circuit begins with deoxygenated blood returned from the body to the right atrium of the heart where it is pumped out from the right ventricle to the lungs.

Pulmonary circulation — main illustration
Pulmonary circulation — illustration

Key takeaways

  • Pulmonary circulation belongs to science; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Pulmonary circulation to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Pulmonary circulation from memory before moving on to harder problems.

Reference excerpt

The pulmonary circulation is a division of the circulatory system in all vertebrates. The circuit begins with deoxygenated blood returned from the body to the right atrium of the heart where it is pumped out from the right ventricle to the lungs. In the lungs the blood is oxygenated and returned to the left atrium to complete the circuit. The other division of the circulatory system is the systemic circulation that begins upon the oxygenated blood reaching the left atrium from the pulmonary circulation. From the atrium the oxygenated blood enters the left ventricle where it is pumped out to the rest of the body, then returning as deoxygenated blood back to the pulmonary circulation. A separate circulatory circuit known as the bronchial circulation supplies oxygenated blood to the tissues of the lung that do not directly participate in gas exchange.

Anatomy

The pulmonary arteries have both an internal and external elastic membrane, whereas pulmonary veins have a single (outer) elastic layer.

Arteries

From the right ventricle, blood is pumped through the semilunar pulmonary valve into the left and right main pulmonary artery (one for each lung), which branch into smaller pulmonary arteries that spread throughout the lungs.

Veins

Oxygenated blood leaves the lungs through pulmonary veins, which return it to the left part of the heart, completing the pulmonary cycle.

Physiology

Two pulmonary circulations The lung actually possesses a high-flow, low-pressure circulation which passes deoxygenated blood from the right heart through the capillaries surrounding the alveoli to be oxygenated, and a low-flow, high-pressure (just slightly lower than systemic arterial pressure) circulation which supplies oxygenated blood to other structures of the lung (airways, supporting tissues, and the vasa vasorum) via the bronchial arteries. This oxygenated blood supplied by the bronchial arteries amounts to 1–2% of left heart output, and is drained into the pulmonary venous system and returned to the left atrium. Pulmonary arterial pressure normally measures about 25 mmHg during systole, about 8 mmHg during diastole, for a mean arterial pressure of 15 mmHg.

Capacity and compliance Compared to the systemic circulation, the pulmonary circulation has less than 1/8th the capacitance and contains ~1/10th the blood volume. The pulmonary arteries and veins are short vessels. To accommodate the right ventricular stroke volume, the pulmonary arterial system has very high compliance; this is achieved by all pulmonary arteries possessing much larger diameters compared to systemic counterparts, as well as thin and distensible walls. Pulmonary blood flow is essentially equal to cardiac output. Pulmonary vessels typically function as distensible conduits that distend at higher intraluminal pressures and narrow with lower pressures.

Blood flow, blood pressure and vascular resistance The (alveolar) pulmonary circulation operates as a very low pressure and resistance system. Indeed, blood pressure in the circulation is normally just sufficient to maintain blood flow in all parts of the lungs. Nevertheless, the pulmonary circulation can accommodate significantly increased flow during periods of increased demand. Abnormally high blood pressures in the pulmonary circulation (e.g. in left-sided heart failure) cause excess fluid to transude from the blood vessels into the alveoli and accumulate here, causing pulmonary edema and impairing gas exchange. Pulmonary capillary pressure is estimated to normally stand at about 7mmHg (compared to about 17 mmHg in capillaries of the systemic circulation), though it has not been measured directly. During very heavy demand (e.g. during strenuous exercise), pulmonary blood flow may be increased to 4-fold to 7-fold of normal. The additional blood flow is accommodated by increasing the number of open capillaries (up to 3-fold greater), distending capillaries (up to 2-fold greater flow), and increasing pulmonary blood pressure; the former two mechanisms accommodate the additional blood flow by reducing vascular resistance and can normally accommodate all the additional required blood flow even at peak demand with little additional increase of pulmonary blood pressure. Blood normally passes through a pulmonary capillary in about 0.8 s, but the time spent traversing the capillaries is as little as 0.3 s during maximal blood flow.

Autoregulation of alveolar blood flow Lung tissue is capable of reducing perfusion of poorly ventilated alveoli to redirect blood flow to better ventilated ones. Reduced O2 concentration in an alveolus causes adjacent blood vessels to constrict; vascular resistance may increase more than 5-fold with very low alveolar O2 levels. Hypoxic vasoconstriction of alveolar blood vessels is thought to be mediated by increased action of vasoconstrictors (e.g. endothelin, and reactive oxygen species), decreased release of vasodilators (e.g. nitric oxide), and closing of oxygen-sensitive K+ channels in vascular smooth muscle (directly causing depolarisation and consequent constriction of muscle).

Hydrostatic effects and pulmonary blood flow The hydrostatic pressure of blood within blood vessels exhibits a gradient across the lung (as do blood vessels across any axis of the body). In an upright person, the lung normally measures 30 cm top-to-bottom for a hydrostatic pressure gradient of 23 mmHg, of which 15 mmHg is superior to the level of the heart. Consequently, in a standing person at rest, there is 5 times more blood flow at the bottom of the lung than at the top. The hydrostatic pressure gradient can lead to three different blood flow scenarios in different parts of the lung:

… excerpt ends here. Continue reading the full article.

Illustrations

Pulmonary circulation illustration
Pulmonary circulation: 3D rendering of a high resolution computed tomography of the thorax. The anterior thoracic wall, the airways and the pulmonary vessels anterior to the root of the lung have been digitally removed in order to visualize the different levels of the pulmonary circulation.
3D rendering of a high resolution computed tomography of the thorax. The anterior thoracic wall, the airways and the pulmonary vessels anterior to the root of the lung have been digitally removed in order to visualize the different levels of the pulmonary circulation.
Pulmonary circulation: Image showing main pulmonary artery coursing ventrally to the aortic root and trachea. The right pulmonary artery passes dorsally to the ascending aorta, while the left pulmonary artery passes ventrally to the descending aorta.
Image showing main pulmonary artery coursing ventrally to the aortic root and trachea. The right pulmonary artery passes dorsally to the ascending aorta, while the left pulmonary artery passes ventrally to the descending aorta.
Pulmonary circulation: The opening page of one of Ibn al-Nafis's medical works
The opening page of one of Ibn al-Nafis's medical works

Worked examples

Example 1 — a first encounter with Pulmonary circulation

Start with the simplest possible case. Write down what Pulmonary circulation claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Pulmonary circulation before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Pulmonary circulation ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Pulmonary circulation

In research
Pulmonary circulation appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Pulmonary circulation in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Pulmonary circulation is common in secondary-school and first-year university syllabi. It links to neighbouring topics Angiology, Respiratory physiology, Underwater diving physiology, so understanding it makes those chapters shorter.
In everyday life
Look for Pulmonary circulation outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Pulmonary circulation in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Pulmonary circulation means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Pulmonary circulation out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Pulmonary circulation in simple terms?

The pulmonary circulation is a division of the circulatory system in all vertebrates. The circuit begins with deoxygenated blood returned from the body to the right atrium of the heart where it is pumped out from the right ventricle to the lungs.

Why does Pulmonary circulation matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Pulmonary circulation?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Pulmonary circulation.

Tags

  • Angiology
  • Respiratory physiology
  • Underwater diving physiology

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