Animal Nutrition and Metabolic Functions

Animal Feeding and Digestion

Animals are heterotrophs. They need to capture food to transform it into nutrients and utilize the energy and nutrients they contain.

The food enters the mouth and travels along the digestive tract: mouth, stomach, small intestine, large intestine, and anus.

The Process of Digestion

Digestion is the process by which the food ingested by a living being is transformed into simple molecules or nutrients that can reach the cells and be utilized. This occurs in two ways:

• Intracellular digestion: Occurs within the cell.
• Extracellular digestion: Occurs outside the cell.

Intracellular Digestion

This occurs in single-celled creatures such as protozoa. These protists use phagocytosis to trap food particles and form digestive vacuoles where they are digested. Nutrients then pass into the cytoplasm, and undigested materials are expelled outside.

Extracellular Digestion

This takes place outside the cells. Food passes through the digestive tube and comes into contact with digestive juices that turn food into simple biomolecules. It is achieved through mechanical processes, such as smashing food, and chemical processes. This type of digestion occurs in both invertebrates and vertebrates.

The Circulatory System

The circulatory system, consisting of arteries, veins, and capillaries, transports nutrients, oxygen, carbon dioxide, and other essential substances.

Open and Closed Circulation

• Open circulation: The heart and arteries pump fluid into open spaces. This is common in mollusks and arthropods.
• Closed circulation: The heart and blood vessels keep the blood contained. This is found in annelids and vertebrates.

Cellular Respiration and Energy

How do cells obtain energy? Cellular respiration is the process by which nutrients are degraded in the presence of oxygen, producing carbon dioxide as a byproduct.

Four Types of Animal Respiration

Cutaneous respiration occurs through the skin of certain areas or the oral cavity. It consists of gas exchange directly through the surface of the body.

Branchial respiration is realized through gills. In these organs, gas exchange occurs via a counter-current exchange system: blood in the gills circulates in the opposite direction of the water, allowing for the maximum extraction of oxygen by diffusion.

Lung respiration involves the use of lungs and the diaphragm, which expands to inhale and contracts to exhale. You can feel this movement by placing your hands on your stomach; as you inhale and exhale repeatedly, you can feel the diaphragm expanding and contracting.

Tracheal respiration is carried out on the surface of the body, where there are pores (spiracles) that communicate with the outside through tubes. these lead oxygen directly to the cells. This is typical of insects and other arthropods.

The Excretion Process

Cellular activity produces residues such as carbon dioxide. The process of deleting or removing these wastes from the body is called excretion.