Lung Volumes Affect Pulmonary Vascular Resistance

Pulmonary vascular resistance is also significantly affected by lung volume. Because pulmonary capillaries have little

Lung Volume Laparoscopic

, Measuring pulmonary wedge pressure. A

catheter is threaded through a peripheral vein in the systemic circulation, through the right heart, and into the pulmonary artery. The wedged catheter temporarily occludes blood flow in a part of the vascular bed. The wedge pressure is a measure of downstream pressure, which is pulmonary venous pressure. Pulmonary venous pressure reflects left atrial pressure.

How Measure Pulmonary Blood Pressure

Effect of cardiac output on pulmonary vas-"cular resistance. Pulmonary vascular resistance falls as cardiac output increases. Note that if pulmonary arterial pressure rises, pulmonary vascular resistance decreases.

Pulmonary Recruitment And Distension

Capillary recruitment and capillary distension. These two mechanisms are responsible for decreasing pulmonary vascular resistance when arterial pressure increases. In the normal condition, not all capillaries are perfused. Capillary recruitment (the opening up of previously closed vessels) results in the perfusion of an increased number of vessels and a drop in resistance. Capillary distension (an increase in the caliber of vessels) also results in a lower resistance and higher blood flow.

structural support, they can be easily distended or collapsed, depending on the pressure surrounding them. It is the change in transmural pressure (pressure inside the capillary minus pressure outside the capillary) that influences vessel diameter. From a functional point of view, pulmonary vessels can be classified into two types: extra-alveolar vessels (pulmonary arteries and veins) and alveolar vessels (arterioles, capillaries, and venules). The extra-alveolar vessels are subjected to pleural pressure—any change in pleural pressure affects pulmonary vascular resistance in these vessels by changing transmural pressure. Alveolar vessels, however, are subjected primarily to alveolar pressure.

At high lung volumes, the pleural pressure is more negative. Transmural pressure in the extra-alveolar vessels increases, and they become distended (Fig. 20.6A). However, alveolar diameter increases at high lung volumes, causing transmural pressure in alveolar vessels to decrease. As the alveolar vessels become compressed, pulmonary vascular resistance increases. At low lung volumes, pulmonary vascular resistance also increases, as a result of more positive pleural pressure, which compresses the extra-alveolar vessels. Since alveolar and extra-alveolar vessels can be viewed as two groups of resistance vessels connected in series, their resistances are additive at any lung volume. Pulmonary vascular resistance is lowest at functional residual capacity (FRC) and increases at both higher and lower lung volumes (Fig. 20.6B).

Since smooth muscle plays a key role in determining the caliber of extra-alveolar vessels, drugs can also cause a change in resistance. Serotonin, norepinephrine, histamine, thromboxane A2, and leukotrienes are potent vasoconstrictors, particularly at low lung volumes when the vessel walls are already compressed. Drugs that relax smooth muscle in the pulmonary circulation include adenosine, acetylcholine, prostacyclin (prostaglandin I2), and isoproterenol. The pulmonary circulation is richly innervated with sympathetic nerves but, surprisingly, pulmonary vascular resistance is virtually unaffected by autonomic nerves under normal conditions.

Low Oxygen Tension Increases Pulmonary Vascular Resistance

Although changes in pulmonary vascular resistance are accomplished mainly by passive mechanisms, resistance can be increased by low oxygen in the alveoli, alveolar hy-

Sobredistension Alveolar

Effect of lung volume on pulmonary vascular resistance. A, At high lung volumes, alveolar vessels are compressed but extra-alveolar vessels are actually distended because of the lower pleural pressure. However, at low lung volumes, the extra-alveolar vessels are compressed from the pleural pressure and alveolar vessels are distended. B, Total pulmonary vascular resistance as a function of lung volumes follows a U-shaped curve, with resistance lowest at functional residual capacity (FRC).

poxia, and low oxygen in the blood, hypoxemia. Hypoxemia causes vasodilation in systemic vessels but, in pulmonary vessels, hypoxemia or alveolar hypoxia causes vasoconstriction of small pulmonary arteries. This unique phenomenon of hypoxia-induced pulmonary vasoconstriction is accentuated by high carbon dioxide and low blood pH. The exact mechanism is not known, but hypoxia

A Regional hypoxia

A Regional hypoxia

Hypoxic VasoconstrictionPulmonary Hypoxaemia Vasoconstriction

^BGflHHIHil^ Effect of alveolar hypoxia on pulmonary ar-

^llfllllHBMVteries. Hypoxia-induced vasoconstriction is unique to vessels of the lungs and is the major mechanism regulating blood flow within normal lungs. A, With regional hypoxia, precapillary constriction diverts blood flow away from poorly ventilated regions,- there is little change in pulmonary arterial pressure. B, In generalized hypoxia, which can occur with high altitude or with certain lung diseases, precapillary constriction occurs throughout the lungs and there is a marked increase in pulmonary arterial pressure.

^BGflHHIHil^ Effect of alveolar hypoxia on pulmonary ar-

^llfllllHBMVteries. Hypoxia-induced vasoconstriction is unique to vessels of the lungs and is the major mechanism regulating blood flow within normal lungs. A, With regional hypoxia, precapillary constriction diverts blood flow away from poorly ventilated regions,- there is little change in pulmonary arterial pressure. B, In generalized hypoxia, which can occur with high altitude or with certain lung diseases, precapillary constriction occurs throughout the lungs and there is a marked increase in pulmonary arterial pressure.

can directly act on pulmonary vascular smooth muscle cells, independent of any agonist or neurotransmitter released by hypoxia.

Two types of alveolar hypoxia are encountered in the lungs, with different implications for pulmonary vascular resistance. In regional hypoxia, pulmonary vasoconstriction is localized to a specific region of the lungs and diverts blood away from a poorly ventilated region (e.g., caused by bronchial obstruction), minimizing effects on gas exchange (Fig. 20.7A). Regional hypoxia has little effect on pulmonary arterial pressure, and when alveolar hypoxia no longer exists, the vessels dilate and blood flow is restored. Generalized hypoxia causes vasoconstriction throughout both lungs, leading to a significant rise in resistance and pulmonary artery pressure (Fig 20.7B). Generalized hy-poxia occurs when the partial pressure of alveolar oxygen (Pao2) is decreased with high altitude or with the chronic hypoxia seen in certain types of respiratory diseases (e.g., asthma, emphysema, and cystic fibrosis). Generalized hypoxia can lead to pulmonary hypertension (high pulmonary arterial pressure), which leads to pathophysiologi cal changes (hypertrophy and proliferation of smooth muscle cells, narrowing of arterial lumens, and a change in contractile function). Pulmonary hypertension causes a substantial increase in workload on the right heart, often leading to right heart hypertrophy (see Clinical Focus Box 20.2). Generalized hypoxia plays an important nonpatho-physiological role before birth. In the fetus, pulmonary vascular resistance is extremely high as a result of generalized hypoxia—less than 15% of the cardiac output goes to the lungs, and the remainder is diverted to the left side of the heart via the foramen ovale and to the aorta via the ductus arteriosus. When alveoli are oxygenated on the newborn's first breath, pulmonary vascular smooth muscle relaxes, the vessels dilate, and vascular resistance falls dramatically. The foramen ovale and ductus arteriosus close and pulmonary blood flow increases enormously.

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

Get My Free Ebook


Responses

  • Andreas
    How to measure pulmonary blood pressure?
    5 years ago
  • Diamond
    What drug acts on venour and arterial smooth muscle causing vasodilation?
    5 years ago
  • haben nebay
    What affects pulmonary vascular resistance?
    5 years ago
  • theodoric
    What is pulmonaryvascularity lung volumes decreased?
    5 years ago
  • prima
    Why low lung volumes increse vascular resistance?
    5 years ago
  • Karen
    How lung volume affect pulmonary?
    5 years ago
  • Ofelia
    Why hypoxia in alveoli cause vasoconstriction?
    5 years ago
  • Eilidh
    How changes in pleural pressure cause change in alveolar volume?
    5 years ago
  • clio
    When does pressure in the pulmonary artery decreased?
    5 years ago
  • melanie
    What conditions decrease pulmonary vascular resistance?
    5 years ago
  • Marina
    What causes high lung volume?
    5 years ago
  • Hamid
    How to measure pulmonary arterial pressure?
    5 years ago
  • MIRELLA
    How vascular resistance affect blood circulation?
    4 years ago
  • Sini
    How will increase in lung volumes affect extra alveolar vessels?
    8 months ago
  • kai
    How will increase in lung volume affect extra alveolar vessels?
    8 months ago
  • maria
    How will increase in lung volume affect extra alveolar vessels,alveolar capillaries and pvr?
    7 months ago
  • t
    How will increased lung volume affect extra alvelor vessels?
    7 months ago
  • Martin
    How will increased lung volume affect extra alveolar vessels?
    4 months ago
  • gruffo
    How will increse in lung volume affect extra alveoli vessels?
    4 months ago
  • lelio
    How will increase in lung volume affect pulmonary vascular resistance?
    4 months ago

Post a comment