Mechanisms of Nutrient Transport and Waste Egestion in Animals

Nutrient Transport and the Circulatory System

Once nutrients have been absorbed, the circulatory system is responsible for transporting them to the rest of the body's cells. In animals, the circulatory system varies from poorly developed to highly complex. In more developed animals, the system is composed of transport liquids, vessels, and a heart.

Transport Liquids (Connective Tissue)

The transport liquid is a connective tissue that circulates through the circulatory system. It is formed by water, minerals, proteins, cells, and various pigments, primarily responsible for transporting gases and nutrients. Types include:

• Hydrolymph (Echinoderms)

  Its composition is very similar to that of seawater. It transports nutrients and waste substances.

• Hemolymph (Invertebrates)

  This is the primary transport fluid in many invertebrates.

• Blood (Vertebrates and Annelids)

  A complex mixture consisting of blood plasma and three types of cells: red cells (erythrocytes), white cells (leukocytes), and platelets. Blood defends the organism against infections and injuries and distributes heat throughout the body.

• Lymph (Vertebrates)

  Present in the lymphatic circulatory system of vertebrates. Its composition is similar to that of blood, but it lacks erythrocytes (red cells) and platelets, and carries a high concentration of leukocytes (white cells).

Vascular System Components

Blood Vessels

These tubes form the vascular system through which the transport liquid circulates. Types include:

1. Arteries: Vessels through which blood leaves the heart. They have thick, hard, and elastic walls to withstand the high pressure generated by the heart.
2. Veins: Vessels through which blood returns to the heart. They typically have a higher caliber than arteries and thinner walls.
3. Capillaries: The site where the exchange of gases and nutrients occurs between the circulatory system and body cells. Their walls are formed by a single layer of cells.

The Heart

The heart is responsible for propelling transport liquids through movements of contraction (systole) and dilation (diastole). Hearts are classified by structure:

• Tubular Heart: A simple tube structure.
• Accessory Hearts: Responsible for boosting circulation in specific areas.
• Loculated Hearts: Possess chambers (locules). They typically have two types of chambers:
  • Atria: Where blood enters.
  • Ventricles: Where blood exits.

Egestion: Elimination of Digestive Waste

Egestion is the process of eliminating digestive waste, which forms stool (feces) in the large intestine. In most vertebrates, the expulsion of stool occurs through defecation, which manifests in different ways depending on the animal group.

• Amphibians, Reptiles, and Birds

  Waste passes into the cloaca (a widening of the intestine) where the excretory and reproductive ducts also terminate.

• Mammals

  Feces pass directly to the outside through the anus.

• Specific Avian Egestion

  In some types of birds, egestion takes place in two ways: by defecation and through the mouth via the regurgitation of a compact ball, known as a pellet, which contains undigested remains.