Respiratory System
Pathway of Air to the Lungs
Air travels a long path before it reaches the lungs, and each structure along the way plays a specific role in preparing that air for gas exchange.
- Nasal cavity — Lined with mucus and fine hairs (cilia) that filter, warm, and humidify incoming air. Air can also enter through the mouth, but the nasal cavity does a better job of conditioning it.
- Pharynx (throat) — A shared passageway for both air and food. The epiglottis, a flap of tissue, covers the larynx during swallowing to prevent food or liquid from entering the airway (a problem called aspiration).
- Larynx (voice box) — Contains the vocal cords, which produce sound when air passes over them.
- Trachea (windpipe) — Lined with mucus and cilia that trap particles and sweep them back up toward the pharynx. C-shaped cartilage rings keep the trachea from collapsing, maintaining an open airway.
- Bronchi — The trachea splits into two main bronchi (left and right), each leading into a lung. These continue to divide into smaller and smaller bronchioles, distributing air throughout the lungs.
- Bronchioles — The smallest branches of the airway. They end at the alveoli, tiny air sacs where gas exchange between air and blood actually happens.
- Lungs — Elastic organs surrounded by a double-layered pleural membrane that allows smooth expansion and contraction during breathing. Each lung contains millions of alveoli, providing a huge surface area for gas exchange. A substance called surfactant coats the inside of alveoli, reducing surface tension so they don't collapse with each breath.
Protective Mechanisms of the Respiratory System
The respiratory system is constantly exposed to particles, pathogens, and irritants from the outside environment. Several layers of defense keep the airways clean.
- Nasal hairs (vibrissae) filter out large particles before air moves deeper into the nasal cavity.
- Mucus lining in the nasal cavity, trachea, and bronchi traps smaller particles. This mucus also contains enzymes and antibodies that neutralize pathogens.
- Cilia throughout the respiratory tract beat in coordinated waves, pushing mucus and trapped debris upward toward the pharynx to be swallowed or expelled. This process is called the mucociliary escalator.
- Coughing and sneezing are reflexes that forcefully clear irritants and excess mucus from the airways.
- Alveolar macrophages are immune cells that patrol the alveoli, engulfing and destroying foreign particles or pathogens that make it past all the other defenses.
Gas Exchange Across Animal Groups
Not all animals exchange gases the same way. The strategy an organism uses depends largely on its body size, complexity, and environment. Here's how gas exchange works across the major animal groups:
- Single-celled organisms (amoeba, paramecium)
- Gas exchange happens through simple diffusion across the cell membrane.
- Their high surface area-to-volume ratio makes this efficient enough without any specialized structures.
- Multicellular invertebrates (flatworms, cnidarians)
- Gas exchange still occurs by diffusion, but across the body surface rather than a single cell membrane.
- These organisms tend to be thin or flat, which keeps all cells close enough to the surface for diffusion to work.
- Insects and other arthropods
- Use a tracheal system: a network of tubes (tracheae) that branch throughout the body.
- Air enters through small openings called spiracles on the body surface.
- Gases diffuse directly to and from individual cells through the tracheae, bypassing the circulatory system entirely.
- Fish
- Use gills: thin, feathery structures with a large surface area.
- Water flows over the gills in the opposite direction to blood flow. This countercurrent flow maintains a concentration gradient along the entire length of the gill, maximizing oxygen uptake.
- Amphibians
- Use cutaneous respiration, exchanging gases through their thin, moist skin. This requires staying in or near moist environments.
- Many also use buccal pumping, where muscles in the mouth and throat actively push air into simple lungs.
- Reptiles
- Have more complex lungs than amphibians, with internal folds called septae that increase surface area.
- Ventilate their lungs through rib and abdominal muscle contraction (costal breathing).
- Birds
- Have a unique system of rigid lungs combined with air sacs that act as bellows.
- Air flows unidirectionally through the lungs, meaning fresh air passes over gas exchange surfaces during both inhalation and exhalation.
- A cross-current arrangement of air and blood flow makes their gas exchange highly efficient, which supports the high metabolic demands of flight.
- Mammals
- Have highly alveolated lungs, providing an enormous surface area for gas exchange.
- Ventilation relies on negative pressure breathing: the diaphragm and intercostal muscles contract to expand the chest cavity, dropping the pressure inside the lungs below atmospheric pressure and drawing air in.
- Alveoli sit in extremely close contact with blood capillaries, keeping the diffusion distance short.
Gas Transport and Exchange
Gas exchange in the lungs depends on differences in partial pressure. Oxygen moves from the alveoli (where its partial pressure is high) into the blood (where it's lower), and carbon dioxide moves in the opposite direction.
Once in the blood, most oxygen binds to hemoglobin, a respiratory pigment in red blood cells that dramatically increases the blood's oxygen-carrying capacity. The circulatory system then delivers oxygen to tissues throughout the body and carries carbon dioxide back to the lungs to be exhaled.