Pulmonary Circulation - Basic Definition

The Circuit (Step by Step)

1. Right ventricle contracts and ejects deoxygenated blood into the pulmonary artery
2. The pulmonary artery enters each lung at the hilum, traveling alongside the bronchi and branching with each airway generation down to the level of the respiratory bronchioles
3. Blood flows through pulmonary arterioles → capillaries (surrounding the alveoli) where:
   • CO₂ diffuses out of blood into the alveolar air (exhaled)
   • O₂ diffuses from alveolar air into the blood
4. Oxygenated blood drains into pulmonary venules → pulmonary veins, which return it to the left atrium

Key Characteristics

Unique Feature: Hypoxic Vasoconstriction

Anatomy Highlights (from Murray & Nadel's)

• Pulmonary arteries travel alongside the bronchi at every airway generation
• Pulmonary veins, by contrast, travel away from airways in the peripheral connective tissue
• Each small pulmonary artery supplies a specific volume of respiratory tissue; each vein drains portions of several zones
• Capillary volume: 60-200 mL, with a gas exchange surface area of 50-70 m²

Physiological Anatomy of the Pulmonary Circulation System

(According to Guyton and Hall - Medical Physiology)

The Lung Has TWO Circulations

1. Pulmonary Blood Vessels

• Extends only 5 cm beyond the apex of the right ventricle, then splits into right and left branches for each lung
• Its wall is only one-third the thickness of the aorta - much thinner and more delicate
• All pulmonary arteries (even small ones) have larger diameters than equivalent systemic arteries
• Because the vessels are thin and wide, the whole pulmonary arterial tree is very distensible (compliant) - compliance averages ~7 mL/mmHg, similar to the entire systemic arterial tree
• This high compliance lets the pulmonary arteries easily accommodate the stroke volume from the right ventricle without a big pressure spike

• Also short - they immediately empty blood into the left atrium

2. Bronchial Blood Vessels

• Small bronchial arteries branch off the thoracic aorta (systemic blood supply)
• They carry oxygenated blood - about 1-2% of total cardiac output
• They supply the supporting tissues: connective tissue, septa, large and small bronchi
• After nourishing these tissues, this blood drains into the pulmonary veins (not back to the right atrium)
• This means the left ventricle outputs ~1-2% more blood than the right ventricle at any given time

3. Lymphatics

• Lymph vessels run throughout the lung's supportive tissues, starting around the terminal bronchioles
• They travel to the hilum of the lung and drain mainly into the right thoracic lymph duct
• They serve two important purposes:
  1. Remove particulate matter (dust, debris) that enters the alveoli
  2. Carry away leaked plasma proteins from lung capillaries - this actively prevents pulmonary edema

Simple Summary Diagram

Right Ventricle
      ↓
Pulmonary Artery (thin walls, large diameter, very distensible)
      ↓  splits into Right & Left branches
Alveolar Capillaries → Gas Exchange (O₂ in, CO₂ out)
      ↓
Pulmonary Veins (short)
      ↓
Left Atrium → Left Ventricle → Body

Why This Anatomy Matters

Physiological Anatomy of the Pulmonary Circulation System

The Lung Has 2 Circulations

Part 1 - Pulmonary Blood Vessels

• Leaves the right ventricle, extends only 5 cm, then splits into right and left branches
• Wall is only 1/3 the thickness of the aorta
• All pulmonary arteries (even small ones) have wider diameters than equivalent systemic arteries
• Because they are thin + wide = very distensible (stretchy)
• Compliance averages ~7 mL/mmHg (same as the entire systemic arterial tree)
• This stretchiness lets them easily absorb each stroke volume without a big pressure rise

• Also very short
• Drain directly into the left atrium

Part 2 - Bronchial Blood Vessels

• Small bronchial arteries branch off the thoracic aorta
• Carry oxygenated blood = only 1-2% of cardiac output
• Supply the lung's support structures: connective tissue, septa, bronchi
• After use, this blood drains into the pulmonary veins (NOT back to the right atrium)
• Result: left ventricular output is 1-2% more than right ventricular output at all times

Part 3 - Lymphatics

• Found in all supportive tissues of the lung
• Start around terminal bronchioles → travel to the hilum → drain into the right thoracic lymph duct
• Two jobs:
  1. Remove dust and particulate matter from alveoli
  2. Drain away leaked plasma proteins from lung capillaries → prevents pulmonary edema

One-Line Takeaway for Each Component

Pulmonary artery - thin, wide, stretchy → absorbs stroke volume at low pressure Pulmonary veins - short → quick return to left atrium Bronchial vessels - feed lung tissues, drain into pulmonary veins Lymphatics - clean the lung, prevent fluid buildup

Pressures in the Pulmonary Circulatory System

The Big Picture First

Pressure at Each Station

1. Right Ventricle

• Systolic: ~25 mmHg (only 1/5th of the left ventricle's ~120 mmHg)
• Diastolic: ~0-1 mmHg
• The right ventricle is a low-pressure pump - it does far less work than the left ventricle

2. Pulmonary Artery

• Systolic: 25 mmHg (matches right ventricle during contraction)
• Diastolic: 8 mmHg
• Mean: 15 mmHg
• When the pulmonary valve closes after systole, right ventricular pressure drops sharply - but pulmonary artery pressure falls slowly because blood continues flowing through the lungs

3. Pulmonary Capillaries

• Mean: ~7 mmHg
• Extremely low - this is intentional. If capillary pressure were too high, fluid would be forced into the alveoli causing pulmonary edema

4. Left Atrium & Pulmonary Veins

• Mean: ~2 mmHg (range: 1-5 mmHg)
• The lowest pressure in the whole circuit - blood drains easily into the left atrium

All Pressures at a Glance

The Guyton Diagrams

How Left Atrial Pressure is Measured (Wedge Pressure)

• A catheter is passed through a peripheral vein → right atrium → right ventricle → pulmonary artery → wedged into a small pulmonary artery branch
• When wedged, blood flow stops and the catheter "sees" pressure from the pulmonary capillaries onward
• Wedge pressure reads ~5 mmHg - only 2-3 mmHg higher than true left atrial pressure
• This technique is used clinically to detect left heart failure (rising left atrial pressure shows up as rising wedge pressure)

Why Low Pressure Matters

• Pulmonary capillary pressure (~7 mmHg) stays below plasma oncotic pressure (~28 mmHg)
• This means fluid is held inside the capillaries and does NOT leak into alveoli
• If left atrial pressure rises (e.g., left heart failure) → backs up into pulmonary capillaries → pressure exceeds oncotic pressure → pulmonary edema

One-Line Memory Aid

RV = 25/1 → PA = 25/8 (mean 15) → Capillaries = 7 → Left Atrium = 2 Pressure keeps dropping as blood flows toward the left heart.

Blood Volume of the Lungs

Normal Blood Volume

• Total lung blood volume = ~450 mL = about 9% of total body blood volume
• How is it distributed?

Lungs Act as a Blood Reservoir

• Blowing hard (e.g., playing a trumpet) → builds up high pressure in lungs → pushes ~250 mL of blood OUT of pulmonary vessels into systemic circulation
• Hemorrhage (blood loss) → lungs automatically shift blood into systemic vessels to help compensate

What Happens in Heart Disease?

• Blood cannot drain properly from the lungs into the left heart
• Blood dams up in the pulmonary circulation
• Pulmonary blood volume can increase by up to 100%
• This raises pulmonary vascular pressure → leads to pulmonary edema

Systemic circulation volume is 9x larger than pulmonary volume. So a small shift of blood into the lungs causes a big rise in pulmonary pressure, but only a mild drop in systemic pressure.

One-Line Summary

The lungs hold ~450 mL of blood (9% of total), act as a flexible reservoir that can shift blood in or out as needed - but when the left heart fails, blood backs up into the lungs and causes dangerous pressure rises.

Blood Flow Through the Lungs and Its Distribution

Basic Rule

Pulmonary blood flow = Cardiac output

How Blood is Distributed to the Right Areas

Hypoxic Pulmonary Vasoconstriction

1. Low O₂ → inhibits K⁺ channels in pulmonary smooth muscle → cell membrane depolarizes → Ca²⁺ channels open → Ca²⁺ floods in → vasoconstriction
2. Low O₂ → triggers release of vasoconstrictors (endothelin, reactive oxygen species)
3. Low O₂ → reduces nitric oxide (a vasodilator) from lung tissue

Effect of Gravity - The 3 Zones of Pulmonary Blood Flow

• Zone 2 = apex (intermittent flow)
• Zone 3 = all lower areas (continuous flow)
• Zone 1 = does NOT normally exist (would require dangerously low blood pressure or artificial ventilation with very high airway pressure)

What Happens During Exercise?

Simple Summary

One-Line Takeaway

The lungs send blood where O₂ is good (hypoxic vasoconstriction) and gravity pulls most blood to the base - creating three flow zones from top to bottom.