Digestive System in Mammals

Enzymes and Digestive Juices in the Duodenum

The duodenum is the first section of the small intestine where several digestive enzymes and juices are added to the food. These substances play a crucial role in breaking down complex molecules into absorbable units.

• Pancreatic Enzymes: The pancreas produces enzymes that digest starch (amylase), proteins (proteases), and lipids (lipases).

• Bile: Produced by the liver and stored in the gall bladder, bile is a green liquid that is released into the duodenum via the bile duct. Bile does not contain enzymes but helps emulsify fats, increasing their surface area for enzyme action.

• Function of Bile: Bile's main role is to break down large fat globules into smaller droplets, aiding lipid digestion.

Example: After a fatty meal, bile is released to help digest and absorb fats efficiently.

Absorption in the Ileum

The ileum is the final section of the small intestine, specialized for nutrient absorption. Its lining contains structures that maximize the surface area for efficient uptake of digested food.

• Villi: Small, finger-like projections (1-2 mm long) called villi line the ileum. Each villus is covered with microvilli, further increasing the surface area.

• Microvilli: Minute projections on the surface cells of villi, enhancing absorption.

• Blood Capillaries: Each villus contains capillaries that absorb most nutrients (e.g., glucose, amino acids) into the bloodstream.

• Lacteal: A central lymph vessel in each villus absorbs products of fat digestion and transports them via the lymphatic system.

Example: The structure of villi and microvilli allows for rapid absorption of nutrients after digestion.

Role of the Liver in Digestion

The liver receives nutrient-rich blood from the ileum via the hepatic portal vein and processes these nutrients.

• Hepatic Portal Vein: Connects the ileum to the liver, transporting absorbed nutrients.

• Liver Functions: The liver stores glucose as glycogen, converts glycogen back to glucose, and processes other molecules for use or storage.

Example: After a meal, excess glucose is stored in the liver as glycogen.

Large Intestine and Elimination of Waste

The large intestine is responsible for absorbing water and forming waste material for elimination.

• Colon: The first part of the large intestine, which absorbs most of the remaining water from the gut contents.

• Faeces: Semi-solid waste material composed of indigestible remains (e.g., cellulose), water, dead bacteria, and cell debris. Stored in the rectum until expelled through the anus.

Example: Water absorption in the colon prevents dehydration and forms solid faeces.

Circulatory System in Mammals

Overview of the Circulatory System

The circulatory system in mammals is a closed circuit consisting of the heart and blood vessels. It is essential for transporting materials throughout the body.

• Blood Transport: Blood carries oxygen from the lungs to tissues, carbon dioxide from tissues to the lungs, nutrients from the gut to cells, and urea from the liver to the kidneys.

• Other Functions: Blood also transports hormones, antibodies, and distributes heat.

Example: Oxygen is delivered to muscle cells during exercise, while carbon dioxide is removed.

Need for a Circulatory System

Multicellular organisms require a circulatory system to efficiently transport substances due to their size and complexity.

• Single-celled Organisms: Do not have circulatory systems; they rely on diffusion across their surface for exchange of materials.

• Diffusion Limitations: The rate of diffusion is determined by the cell's surface area to volume ratio.

Example: Amoeba can absorb oxygen directly through its cell membrane, but larger animals need a circulatory system.

Surface Area to Volume Ratio

The efficiency of diffusion in single-celled organisms is governed by their surface area to volume ratio.

• High Ratio: Small cells have a high surface area relative to their volume, allowing efficient exchange.

• Low Ratio: Larger organisms have a lower ratio, making diffusion insufficient for their needs.

Formula:

Additional info: In multicellular organisms, specialized transport systems (like blood vessels) overcome the limitations of diffusion.

Comparison Table: Single-celled vs Multicellular Organisms