Animal Respiration and Digestion
Digestive System Evolution
Primitive organisms with extracellular digestion, like microphages, had a simple digestive system. Food particles entered a single opening, serving as both mouth and anus, where digestion and absorption occurred. Nematodes evolved two openings: a mouth for ingestion and an anus for egestion. Macrophages developed specialized structures like jaws or teeth for food intake and muscles for peristaltic movement along the digestive tract. Further developments increased digestive efficiency:
- Elongation and folding of the digestive tube, especially the gut.
- Distinct areas with specific functions and sphincters.
- Increased absorptive surface due to mucosal folds.
Respiratory System
Respiration encompasses two processes:
- Internal (cellular) respiration: Energy release, often requiring oxygen.
- External respiration: Gas exchange between the organism and its environment.
Tracheal Breathing
Insects and other terrestrial arthropods use tracheal breathing. Gas exchange occurs through a network of tubes called tracheae. These tubes originate as invaginations of the integument, with air entering through small body surface openings called spiracles or stigmata. Tracheae decrease in size as they branch, with the finest branches (tracheoles) contacting cells directly for gas exchange via diffusion. A circulatory system for gas transport is unnecessary. Ventilation occurs through coordinated opening and closing of spiracles via muscle contractions.
Hindgut
The hindgut, also known as the large intestine, has a larger diameter than the midgut and is separated from it by the ileocecal valve.
Diffusion Mechanism
Diffusion is a physical phenomenon caused by molecular agitation, allowing a dissolved substance to pass through a membrane separating two solutions. The diffusion rate is proportional to the concentration difference across the membrane and the surface area of separation.
Incorporation of Gaseous Nutrients
Animals require oxygen for cellular energy production. They exchange gases with their environment, absorbing oxygen and releasing carbon dioxide produced during cellular respiration. Gas exchange varies depending on the animal’s size, habitat, and complexity. Simple animals in aquatic environments exchange gases directly with the surrounding water through diffusion, lacking specialized structures. Animals in humid or aquatic environments use cutaneous respiration, absorbing oxygen through their skin. This requires thin, permeable, and constantly moist skin, with the internal environment close to the external surface. These animals have a large surface area relative to their size and low metabolic activity. More complex animals have specialized structures to enhance diffusion:
- Large respiratory surface for gas exchange.
- Thin walls bathed in fluid for dissolving and diffusing gases.
- Ventilation to maintain a concentration gradient.
- Muscle-driven ventilation mechanisms.
- Transport systems (blood, hemoglobin) for oxygen distribution and carbon dioxide collection.