trachea, more popularly known as the windpipe.
Roughly 6 inches long in adults, it’s a tube connected to the larynx in front of the
esophagus that’s made up of C-shaped rings of hyaline cartilage and fibrous connective
tissue that strengthen it and keep it open. Like the larynx, the trachea’s lined with
mucous membrane covered in cilia. Just above the heart, the trachea splits into two
bronchi divided by a sharp ridge called the carina, with each leading to a lung. But
they’re not identical: The right primary bronchus is shorter and wider than the left primary
bronchus. Each primary bronchus divides into secondary bronchi with a branch
going to each lobe of the lung; the right side gets three secondary bronchi while the
left gets only two. Once inside a designated lobe, the bronchus divides again into tertiary
bronchi. The right lung has ten such branches: three in the superior (or upper)
lobe, two in the middle lobe, and five in the inferior (or lower) lobe. The left lung has
only four tertiary bronchi: two in the upper lobe and two in the lower lobe.
Each tertiary bronchi subdivides one more time into smaller tubes called bronchioles
which lack the supporting cartilage of the larger structures. Each
bronchiole ends in an elongated sac called the atrium (also known as an alveolar duct
or alveolar sac). Alveoli (or air cells) surround the atria, as do small capillaries that
pick up oxygen for delivery elsewhere in the body and dump off carbon dioxide
fetched from elsewhere. Overall, there are 23 branches in the respiratory system, with
a combined surface area (counting the alveoli) the size of a tennis court!
Knowing that the bronchi aren’t evenly distributed, you may have guessed that the
lungs aren’t identical either. You’re right. They’re both spongy and porous because of
the air in the sacs, but the right lung is larger, wider, and shorter than the left lung and
has three lobes. The left lung divides into only two lobes and is both narrower and
longer to make room for the heart because two-thirds of that organ lies to the left of
the body’s midline. Each lobe is made up of many lobules, each with a bronchiole
ending in an atrium inside.
Covering each lung is a thin serous membrane called the visceral pleura that folds back
on itself to form a second outer layer, the parietal pleura, with a pleural cavity between
the two layers. These two layers secrete a watery fluid into the cavity to lubricate the
surfaces that rub against each other as you breathe. When the pleural membrane
becomes inflamed in a condition called pleurisy, a sticky discharge roughens the
pleura, causing painful irritation. An accompanying bacterial infection means that pus
accumulates in the pleural cavity in a condition known as empyema.
Blood comes to the lungs through two sources: the pulmonary arteries and the
bronchial arteries. The pulmonary trunk comes from the right ventricle of the heart
and then branches into the two pulmonary arteries carrying venous blood (the only
arteries that contain blood loaded with carbon dioxide from various parts of the body)
to the lungs. That blood goes through capillaries in the lungs where the carbon dioxide
leaves the blood and enters the alveoli to be expelled during exhalation; oxygen leaves
the alveoli through the capillaries to enter the bloodstream. After that, oxygenated
arterial blood returns to the left atrium through the pulmonary veins (the only veins
that contain oxygenated blood), completing the cycle. Bronchial arteries branch off
the thoracic aorta of the heart, supplying the lung tissue with nutrients and oxygen.
"What is inside the lungs"
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