Animal Digestive and Respiratory Systems
Stomach
The stomach is a dilated section of the digestive tract that stores food temporarily before it moves to the intestine in a more broken-down state.
Stomach Variations in Vertebrates
- Fish, Amphibians, and Reptiles: Spindle-shaped stomach.
- Birds: Divided into a muscular gizzard, specializing in food trituration, and a glandular section.
- Mammals: Highly folded stomach structure.
- Ruminant Mammals: Complex stomach divided into four chambers: rumen, reticulum, omasum, and abomasum. Unchewed grass is stored in the rumen, then passes to the reticulum where small portions are regurgitated for chewing (rumination). This mass is swallowed a second time, returns to the stomach, and ferments. Next, it moves to the omasum for partial water absorption. Finally, it enters the abomasum, where gastric juices are secreted and true digestion occurs.
Chemical Digestion in the Stomach
Chemical digestion begins in the vertebrate stomach through the action of gastric juice secreted by glands lining its walls. Gastric juice consists of:
- Mucin: Protects the stomach lining.
- Pepsinogen: Inactive precursor of pepsin, an enzyme that hydrolyzes proteins into polypeptides.
- Hydrochloric Acid: Maintains acidic pH for enzyme activity and bacteria destruction.
- Renin: Present in lactating mammals; coagulates milk proteins.
Food in the stomach forms a slurry called chyme, which then passes to the small intestine.
Small Intestine
Two vital processes occur in the small intestine: complete chemical digestion and nutrient absorption.
Digestive Secretions
In all vertebrates, the small intestine receives secretions from attached glands:
- Bile: Produced by the liver and stored in the gallbladder until released into the duodenum; emulsifies fats.
- Pancreatic Juice: Produced by the pancreas.
- Intestinal Juice: Secreted by glands in the intestinal mucosa.
The enzymes in these juices vary among animals, but their overall digestive function is similar.
Absorption
Digested nutrients must cross the intestinal wall to enter the bloodstream for transport to the body’s cells. This process is called absorption.
Most organic nutrients, like glucose and amino acids, are absorbed through active transport, requiring significant energy. Other substances pass into capillaries via simple diffusion or facilitated diffusion.
Gill Respiration
Gills are projections of the body surface that have developed through evagination, increasing the respiratory surface area for oxygen uptake from water.
Types of Gills
- External Gills:
- Advantage: No ventilation system required; movement of the gills renews the water.
- Disadvantages:
- Easily damaged.
- Impede movement.
- Highly visible to predators.
- Internal Gills:
- Located within a protective cavity, addressing the disadvantages of external gills.
- Require ventilation systems. Examples:
- Bivalves: Cilia create water flow.
- Cephalopod Molluscs: Water circulates through siphons.
- Crustaceans: Appendages create water currents within gill chambers.
- Fish:
- Cartilaginous Fish: Water enters through spiracles and exits through gill slits.
- Bony Fish: Water enters the mouth, passes over highly vascularized lamellae on gill arches within a cavity covered by the operculum, and exits.