The lungs are located within the chest cavity behind the ribs and make up the primary organ in the respiratory system, comprising the majority of the lower respiratory system.
Human Lung Anatomy
The lungs consist of two sets of lobes: three lobes make up the right lung and two lobes make up the left lung. The left lung is smaller in order to accommodate the heart, which is situated under the left lung and behind the rib cage. Each lung has a branch of the bronchial tree that splits off the windpipe. The bronchi branches become smaller as they extend into the lung lobes, becoming bronchioles. The bronchi carry the air into and out of the lungs.
The functional units of the lung tissue are thin walled air sacs called alveoli, which are the ends of the smallest bronchioles. Each lung contains millions of these air sacs. The lungs inflate and deflate as air is transported into and out of the air sacs. Between the alveoli is elastic connective tissue, which is important to accommodate the expansion and collapse of the tissue. The pulmonary artery and pulmonary vein transport blood between the heart and lungs for gas exchange, with a network of capillaries surrounding each alveolus.
Inside the alveoli are macrophages, specialized white blood cells, which act as the first line of defense against foreign particles and pathogens that make it past the cilia in the bronchial tree and the defenses of the upper respiratory system.
Alveolar Cells and Surfactant
There are two types of alveolar, or pulmonary, cells, which are also called pneumocytes. Type 1 alveolar cells are flat, simple squamous epithelial cells that form the thin walls of the alveoli. Type II alveolar cells, also known as septal cells, secrete a lipoprotein called surfactant. Pulmonary surfactant prevents the lungs from collapsing by decreasing surface tension within the alveoli.
When a baby is born, the first breath outside the womb inflates the lungs for the first time and surfactant keeps them from collapsing to the fetal state. If a newborn, particularly those born prematurely, has difficulty with this process it is called newborn respiratory distress syndrome. Neonatal surfactant replacement therapy can correct this issue (the current guidelines for use of this therapy are available from the American Academy of Pediatrics)
Inhalation and Exhalation – Gas Exchange
The lungs are the portal for gases between the blood and the atmosphere. Carbon dioxide (a waste gas from cellular respiration) filters out of the blood through the thin alveolar wall into the air sacs to be released into the atmosphere through the mouth and nose during exhalation. Oxygen is breathed into the air sacs during inhalation and passes through the thin alveolar wall into the blood for transport to all the tissues and cells of the body. This exchange also regulates the pH of the blood.
Breathing is regulated by the respiration center of the brain. Inhalation, also known as inspiration, is an active process that requires the respiratory muscles to open up the chest cavity, adjusting the pressure so air enters the lungs. Exhalation, also known as expiration, is a passive activity that does not require an exertion of energy under normal conditions.
The Respiratory Muscles
The respiratory muscles are skeletal muscle controlled by the nervous system with some limited voluntary control. The muscles involved in respiration are:
- Diaphragm – concave, or dome-shaped, muscle under the lungs and ribs, separating the chest cavity from the abdomen. When the diaphragm contracts, it flattens, allowing the lungs to expand and air is inhaled. When the diaphragm relaxes, air is expelled from the lungs. The diaphragm can be somewhat controlled with practice, such as the breathing exercises of singers.
- Internal intercostals – between the ribs, inside the rib cage. The interosseous internal intercostals (actually between the bony parts of the ribs) pull the rib cage inward upon contraction for forced exhalation. The intercartilaginous internal intercostals help lift the ribcage when they contract, allowing the lungs to expand for inhalation.
- External intercostals – between the ribs, on the outside of the rib cage. Like other muscles, the external intercostals shorten when they contract, pulling the rib cage up and out, aiding in inhalation.
Additional Information:
Scanlon and Sanders. Essentials of Anatomy and Physiology, 4th edition. Saunders.
Alicia Mae Prater - Alicia received her doctorate in Experimental Pathology in 2007. She has been a freelance writer and scientific editor since 2008.
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