Animal Body Systems: Digestion, Respiration, & Circulation

Animal Nutrition and Digestion

Animals have specific nutritional needs. Digested food cannot be used as it is ingested; animals need to extract nutritious substances, which are the components that must be digested. Most animals possess a digestive system, more or less complex, with a digestive tract divided into regions and various associated glands.

The Digestive System

Vertebrate Digestive Tract

The digestive tract of vertebrates is clearly differentiated into various regions, including the mouth, esophagus, stomach, and intestine. It also features several glands such as salivary glands, the liver, and the pancreas.

Variations in Stomach Structure

Avian Stomach

In birds, the stomach is divided into several cavities. The proventriculus is involved in chemical digestion, while the gizzard is muscular and helps pulverize food. The crop functions as a storage organ.

Ruminant Stomach

In ruminants, the stomach is divided into four sections:

• The rumen: where food is stored and undergoes partial digestion.
• The reticulum: receives chewed food, which is then regurgitated for re-chewing.
• The omasum: where partial absorption of nutrients takes place.
• The abomasum: where digestion is completed.

Intestine Differences

Herbivores typically have a longer intestine and a large cecum, which aids in digesting plant-based food. Carnivores, conversely, have a shorter intestine and may lack a cecum.

Gas Exchange and Respiration

Simpler animals perform gas exchange across their body surface. More complex animals utilize specialized breathing apparatuses.

Respiratory Mechanisms

Types of Respiration

Common types of respiration include:

• Cutaneous respiration (through the skin)
• Branchial respiration (gills)
• Pulmonary respiration (lungs)
• Tracheal respiration (tracheae)

Gas Exchange Process

During gas exchange, oxygen and carbon dioxide are generally exchanged between the respiratory and circulatory systems. Oxygen moves from the respiratory system to the circulatory system, while carbon dioxide moves from the circulatory system to the respiratory system.

Excretion and Waste Elimination

Excretion is the process of eliminating waste substances from the body. Simpler animals expel cellular waste by simply diffusing it into the surrounding water or body cavities. More complex animals eliminate waste through specialized organs or excretory apparatuses.

The Excretory Process

Types of Excretory Organs

Invertebrate Excretory Structures

• Nephridia: Tube-shaped structures found in annelids and mollusks.
• Green glands: Complex excretory organs typical of crustaceans.

Substance Transport and Circulation

Simpler animals transport substances by allowing water to penetrate through their body cavities. More complex animals utilize a circulatory system.

The Circulatory System

Components of Circulation

A circulatory system typically consists of:

• A circulating fluid (e.g., blood, hemolymph)
• A pump (which can be a contractile vessel or a heart)
• Vessels (such as veins, arteries, and capillaries, which conduct the fluid)

Vertebrate Circulatory Systems

Vertebrate circulatory systems can be categorized as:

• Simple Circulation

  A single circuit, typical of fish, where blood passes through the heart once per complete circuit.

• Double Circulation

  The heart is divided into 3 or 4 chambers, forming two distinct circuits: the systemic circuit (to the body) and the pulmonary circuit (to the lungs).

  • Complete Double Circulation: Oxygenated blood does not mix with deoxygenated blood (e.g., mammals, birds).
  • Incomplete Double Circulation: Oxygenated and deoxygenated blood mix to some extent (e.g., amphibians, most reptiles).