Flatworms and other flattened animals need no special organs for gas exchange
because no cell is very far from a body surface.
More complex animals use lungs, gills, or tracheal tubes.
Plants rely on special holes (stomates) in the
lower epidermis of each leaf.
Anatomy and physiology in land vertebrates:
Nostrils take air into nasal cavity, then into pharynx.
Floor of pharynx opens behind mouth into larynx (voice-box); entrance to larynx
is guarded by epiglottis.
The trachea, bronchi, and bronchioles form tree-like branchings within
each lung.
The lungs have air sacs lined with box-like alveoli.
Air exchange: In inhalation (inspiration), diaphragm contracts
and moves downward while intercostal muscles raise rib cage.
In exhalation (expiration), muscles relax, rib cage falls,
diaphragm springs upward.
Gas exchange in alveoli: Oxygen enters capillaries of lung through
thin walls; CO2 leaves capillaries and diffuses into air sac.
Gas exchange within capillary blood: In lung alveoli, oxygen
enters red blood cells, combines with hemoglobin, and is transported
as HbO2 (oxyhemoglobin); bicarbonate ions enter blood cells
and are split into water and CO2. The reverse occurs in body tissues:
oxyhemoglobin breaks down to release oxygen; CO2 and water combine to
form bicarbonate ions (HCO3–).
Gill systems: In fishes and many other aquatic animals,
thin-walled arteries run through gills with direction of blood flow usually
opposite to flow of water (counter-current exchange).
Oxygen diffuses into these arteries; CO2 diffuses into
surrounding water.
Insect tracheal systems: Air diffuses through numerous branched tubes
(tracheae). Rhythmic muscular contractions force air in and out when
flying, but air movement is passive most of the time.
Stomates in plants are openings in the lower epidermis of each leaf.
Two large kidney-shaped guard cells flank the stomate on each side.
Swelling of guard cells closes the stomate; shrinking of guard cells opens it.
Gases enter and leave by passive diffusion.
During photosynthesis (by day),
CO2 enters and oxygen diffuses out.
During respiration (especially at night), CO2 and water diffuse out; O2 enters.
Stomates usually close when water loss would be excessive.
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