Summary
Highlights
The human respiratory system is responsible for the exchange of gases, specifically taking in oxygen and expelling carbon dioxide. This process is distinct from cellular respiration, which uses oxygen to produce ATP, but the respiratory system provides the necessary oxygen for this cellular function.
Air enters through the nasal cavity, where it's warmed, humidified, and filtered by mucus and hairs. It then passes through the pharynx, larynx (voice box), and trachea. The trachea is supported by cartilage rings and branches into primary bronchi, leading to the lungs. The lungs contain lobes (three on the right, two on the left, with a cardiac notch for the heart).
Inside the lungs, the primary bronchi divide into secondary bronchi, tertiary bronchi, and smaller bronchioles, resembling an upside-down tree. These lead to terminal bronchioles, respiratory bronchioles, and finally alveolar ducts, which are surrounded by alveolar sacs containing alveoli. Alveoli, with their thin walls and large surface area, are where gas exchange occurs, facilitated by direct contact with capillaries of the circulatory system. Red blood cells pick up oxygen and release carbon dioxide to be exhaled.
The respiratory system works closely with other body systems. The skeletal system's ribs protect the lungs, and the muscular system's intercostal muscles, diaphragm, and abdominal muscles help expand and contract the thoracic cavity for breathing. The nervous system regulates involuntary breathing by monitoring blood pH, which is affected by carbon dioxide levels. An increase in carbon dioxide makes the blood more acidic, signaling the brain to adjust breathing rate and depth to maintain pH balance (homeostasis).
Gas exchange isn't exclusive to humans; earthworms use their skin, fish use gills, and insects have tracheal systems with spiracles. Understanding the respiratory system is crucial for treating respiratory illnesses. Professions like pulmonologists and respiratory therapists focus on conditions like asthma, emphysema, and issues in premature babies whose lungs may lack sufficient surfactant.
Alveoli require surfactant, produced by type 2 alveolar cells, to prevent collapse due to the surface tension of water. Surfactant, containing phospholipids and proteins, reduces surface tension, making it easier for alveoli to inflate. Premature babies often lack sufficient surfactant, leading to respiratory distress syndrome (RDS). Artificial surfactants are now used to treat these infants, saving many lives.