Digestive System: Key Functions and Processes

Location and Function:
The pancreas is an accessory organ that plays a critical role in the digestive process by secreting enzymes and bicarbonate.
Exocrine function: secrete digestive enzymes into the duodenum; Endocrine function: secrete hormones into the bloodstream (e.g., insulin).
Enzymes Secreted by the Pancreas:
Lipases: Break down lipids (fats).
Carbohydrate-Digesting Enzymes: Include pancreatic amylase, which breaks down carbohydrates.
Proteases: Include trypsin, chymotrypsin, and carboxypeptidase; break down proteins into amino acids.
Role of Bicarbonate:
Pancreatic juice contains bicarbonate ions that neutralize acidic chyme from the stomach, allowing digestive enzymes to function properly in an alkaline environment.

Liver Functions:
Secretes bile, contains bile salts, cholesterol, and bilirubin.
Bile salts: derived from cholesterol and act as detergents to emulsify fats during digestion.
Bile is stored in the gallbladder and released into the duodenum when fatty chyme enters.
Bilirubin:
A breakdown product of hemoglobin, necessary for the excretion of metabolites through feces.

Emulsification:
Bile salts emulsify large fat globules into smaller micelles, allowing lipases to break them down into absorbable units like fatty acids and monoglycerides.
Absorption Process:
Water-Soluble Nutrients (amino acids, monosaccharides, and some nucleotides) are absorbed by enterocytes through sodium-dependent cotransport mechanisms.
Lipid Absorption:
- Fatty acids and monoglycerides absorbed via simple diffusion;
- Reassembled into triglycerides in the endoplasmic reticulum, packaged into chylomicrons (with a protein layer) for transport.

Hepatic Portal System:
Water-soluble nutrients enter the bloodstream via the hepatic portal vein, which directs blood to the liver for processing.
Lymphatic System:
Chylomicrons bypass the bloodstream initially, entering the lymphatic system before entering the venous circulation.

Structure:
Composed of sections (cecum, colon, rectum) and the appendix.
Functions:
Reabsorption of water and ions, housing beneficial microbes (such as E. coli) that help metabolize nutrients and produce vitamins (e.g., Vitamin K, some B vitamins).
Fermentation of undigested materials, producing gases (CO2, methane).
Importance of Gut Microbiome:
Contributes to immune function, metabolic reactions, and overall health.

Chemical Digestion: Nutrients are broken down from polymers (e.g., carbohydrates, proteins) to monomers (amino acids, glucose, fatty acids) for absorption.
Metabolic Processes:
Catabolic reactions: Breakdown nutrients to generate energy (ATP).
Anabolic reactions: Use energy to synthesize macromolecules (proteins, glycogen, triglycerides).
Energy Handling: Nutrient absorption adapts based on body’s metabolic status (deficient vs. surplus), influencing whether nutrients are used for immediate energy or stored for later use.