Digestion in Animals – Biology 11

1. Definition of Digestion

Digestion is the process of breaking down food from complex molecules into simpler substances that can be easily absorbed and used by the body.

* **Note:** Digestion is a breakdown process, not a synthesis process.

2. The Role of Digestion

• Food contains nutrients such as starch, cellulose, lipids, and proteins.
• These substances must be broken down into their respective monomers:
  • Starch → glucose
  • Cellulose → glucose (in ruminants)
  • Lipids → fatty acids and glycerol
  • Proteins → amino acids
• These monomers can then be absorbed by the body and used for various functions.

3. Forms of Digestion

a. Intracellular digestion:

• Occurs in unicellular organisms like flagellates, paramecia, and amoeba.
• Food is digested within food vacuoles.

b. Extracellular digestion:

• Occurs in multicellular animals.
• Food is digested within the digestive tract.
• The digestive tract can be simple or complex, depending on the type of food and the animal species.

4. Digestive Tract and Adaptations to Food

a. Carnivores:

• Tooth characteristics:
  • Canines: developed, used for tearing meat.
  • Incisors: sharp, used for cutting meat.
  • Molars: used for grinding food.
• Intestinal characteristics: Short intestines because food is easily digested.
• Digestion: Food is digested both mechanically (chewing, grinding) and chemically (enzymes).

b. Herbivores:

• Tooth characteristics:
  • Incisors: for nibbling, biting.
  • Molars: developed, with broad surfaces, used for grinding food.
• Intestinal characteristics: Long intestines, with a developed caecum.
• Digestion:
  • Mechanical and chemical digestion.
  • Biological digestion with the help of symbiotic microorganisms (in the rumen or caecum).

c. Some special animal species:

• Horses and rabbits:
  • Single-chambered stomach, developed caecum.
  • Food is partially digested in the small intestine.
  • The remainder is transferred to the caecum and further digested biologically by symbiotic microorganisms.
• Ruminants (cattle, sheep, goats, deer):
  • Four-chambered stomach: rumen, reticulum, omasum, abomasum.
  • Food pathway: mouth → rumen → reticulum → regurgitated for chewing → omasum → abomasum → small intestine → large intestine → anus.
  • Digestion is mainly facilitated by symbiotic microorganisms in the rumen.

5. Nutritional and Health Considerations

• Foods high in cholesterol can increase the risk of cardiovascular diseases and high blood pressure.
• A high-salt diet can also lead to high blood pressure.
• Vitamin A deficiency can lead to night blindness.
• Iron deficiency can lead to anemia.

6. Respiration in Animals

a. Modes of Respiration:

• Cutaneous respiration: earthworms, flatworms.
• Tracheal respiration: insects.
• Gill respiration: fish, shrimp, crabs.
• Lung respiration: reptiles, birds, mammals.

b. Characteristics of Lungs:

• Bird lungs:
  • Lack alveoli.
  • Composed of a system of air sacs surrounded by capillaries.
  • Highly efficient gas exchange, adapted for flight.
• Lungs of other species:
  • Have alveoli.
  • Large surface area for gas exchange.

7. Circulatory System in Animals

a. Open circulatory system:

• Found in most species of mollusks (snails, mussels, clams) and arthropods (shrimp, crabs, insects).
• Blood does not flow within blood vessels, but in body cavities.
• Blood directly contacts cells, allowing for direct exchange of substances.
• Low blood pressure, slow circulation rate.

b. Closed circulatory system:

• Blood flows within enclosed blood vessels.
• Blood flows at high pressure, enabling rapid circulation.
• More efficient transport of nutrients and oxygen.

c. Types of blood:

• Oxygenated blood: rich in oxygen.
• Deoxygenated blood: rich in carbon dioxide.

d. Blood cells:

• Red blood cells: transport oxygen.
• White blood cells: protect the body from infections.
• Platelets: help in blood clotting.

e. Single circulatory system:

• Found in fish.
• Heart has two chambers: one atrium, one ventricle.
• One circulatory loop: blood passes through the heart once.

f. Double circulatory system:

• Found in amphibians, reptiles, birds, and mammals.
• Heart has three or four chambers.
• Two circulatory loops: blood passes through the heart twice.

g. Types of hearts:

• Two-chambered heart: fish.
• Three-chambered heart: amphibians, reptiles (except crocodiles).
• Four-chambered heart: birds, mammals.

8. Blood Pressure

a. Definition:

• Blood pressure is the force of blood exerted against the walls of blood vessels.
• Blood pressure is highest in arteries and lowest in veins.

b. Factors affecting blood pressure:

• Amount of blood pumped out of the heart.
• Elasticity of blood vessel walls.
• Blood volume.

c. Blood pressure changes:

• During strenuous exercise: blood pressure increases.
• During rest: blood pressure decreases.

9. Blood Velocity

• Blood velocity is inversely proportional to the total cross-sectional area of blood vessels.
• Blood velocity is slowest in capillaries because capillaries have the largest cross-sectional area.

10. Note

• This article provides a basic understanding of digestion and circulation in animals.
• For a more comprehensive understanding of these topics, refer to textbooks and related resources.
• Combine theoretical learning with practical exercises for better knowledge retention.