13.3 Absorption and Elimination

The digestive system's final act is absorption and elimination, where nutrients are taken in and waste is expelled. This process involves the small intestine absorbing vital nutrients and the large intestine handling water and electrolytes.

Absorption in the small intestine is key for getting nutrients into the body, while the large intestine deals with water balance and waste removal. These processes are crucial for maintaining health and energy levels in the body.

Nutrient Absorption in the Small Intestine

Anatomy and Physiology of the Small Intestine

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• The small intestine is the primary site of nutrient absorption with its large surface area enhanced by circular folds, villi, and microvilli
• The small intestine is divided into three segments: the duodenum, jejunum, and ileum, each with specific functions in nutrient absorption

Carbohydrate Absorption

• Carbohydrates are primarily absorbed as monosaccharides (glucose, fructose, and galactose) through specific transport proteins in the intestinal epithelial cells
• The brush border enzyme sucrase breaks down sucrose into glucose and fructose, while lactase breaks down lactose into glucose and galactose for absorption

Protein Absorption

• Proteins are absorbed as amino acids and small peptides after being enzymatically digested by proteases such as trypsin, chymotrypsin, and peptidases
• Amino acids are absorbed through specific transport systems, including sodium-dependent and independent carriers, while small peptides are absorbed via the peptide transporter PepT1

Fat Absorption

• Fats are absorbed as monoglycerides and free fatty acids after being emulsified by bile salts and broken down by pancreatic lipase
  • The products of fat digestion are then reassembled into triglycerides within the intestinal epithelial cells and packaged into chylomicrons for transport via the lymphatic system
• Fat-soluble vitamins (A, D, E, and K) are incorporated into chylomicrons for absorption and transport

Vitamin and Mineral Absorption

• Vitamins and minerals are absorbed through various mechanisms, including passive diffusion, facilitated diffusion, and active transport, depending on the specific nutrient
• Water-soluble nutrients (vitamin C, B vitamins) are absorbed directly into the bloodstream, while fat-soluble nutrients (vitamins A, D, E, and K) are incorporated into chylomicrons and transported via the lymphatic system
• Minerals such as calcium, iron, and zinc are absorbed through specific transport systems, often requiring carrier proteins or ion channels

Water and Electrolyte Absorption in the Large Intestine

Functions of the Large Intestine

• The large intestine, or colon, is responsible for absorbing water, electrolytes, and some nutrients from the remaining digestive material (chyme) that enters from the small intestine
• The colon absorbs approximately 90% of the water from the chyme, concentrating the remaining material into feces
• The colon also absorbs short-chain fatty acids (SCFAs) produced by the fermentation of undigested carbohydrates by gut bacteria, providing an additional energy source for the body

Electrolyte Absorption and Secretion

• Sodium and chloride ions are actively absorbed by the colonic epithelial cells, creating an osmotic gradient that drives the passive absorption of water
• Potassium and bicarbonate ions are secreted into the colonic lumen to maintain electrolyte balance
• The absorption and secretion of electrolytes are regulated by hormones such as aldosterone and vasopressin, as well as by local factors like SCFAs

Fecal Formation and Defecation

• As the material moves through the colon, it becomes more solid due to water absorption, forming feces that are stored in the rectum until elimination through defecation
• The consistency and frequency of bowel movements are influenced by factors such as diet, hydration, physical activity, and gut motility
• The defecation reflex is initiated when the rectum is distended, triggering the relaxation of the internal anal sphincter and the contraction of the external anal sphincter to allow for voluntary control of defecation

Gastrointestinal Disorders

Malabsorptive Disorders

• Celiac disease is an autoimmune disorder triggered by gluten consumption, leading to damage of the small intestinal villi and impaired nutrient absorption
• Lactose intolerance is the inability to digest lactose due to a deficiency in the enzyme lactase, causing symptoms such as bloating, diarrhea, and abdominal pain after consuming dairy products
• Short bowel syndrome occurs when a significant portion of the small intestine is removed or non-functional, resulting in decreased absorptive capacity and malnutrition
• Malabsorption syndromes, such as tropical sprue and Whipple's disease, are characterized by impaired nutrient absorption due to various factors, including infections, structural changes, or enzyme deficiencies

Inflammatory and Secretory Disorders

• Inflammatory bowel diseases (IBDs), including Crohn's disease and ulcerative colitis, are characterized by chronic inflammation of the gastrointestinal tract, leading to impaired absorption and increased secretion
• Crohn's disease can affect any part of the gastrointestinal tract, while ulcerative colitis primarily affects the colon and rectum
• Secretory diarrhea is caused by an increased secretion of fluids into the intestinal lumen, often due to bacterial toxins (cholera toxin) or certain medications, leading to dehydration and electrolyte imbalances
• Irritable bowel syndrome (IBS) is a functional gastrointestinal disorder characterized by abdominal pain, bloating, and altered bowel habits, without any identifiable structural or biochemical abnormalities

Gut Microbiota and Digestive Health

Functions of the Gut Microbiota

• The human gastrointestinal tract is home to trillions of microorganisms, collectively known as the gut microbiota, which play a crucial role in maintaining digestive health
• Gut bacteria ferment undigested carbohydrates (fiber) to produce short-chain fatty acids (SCFAs), which serve as an energy source for colonocytes and help maintain the integrity of the intestinal barrier
• The gut microbiota helps to prevent the overgrowth of potentially harmful bacteria by competing for nutrients and space, as well as by producing antimicrobial substances
• Certain gut bacteria synthesize essential vitamins, such as vitamin K and some B vitamins, contributing to the body's nutrient supply

Gut Microbiota and Immune Function

• The gut microbiota interacts with the immune system, helping to develop and maintain immune tolerance to harmless antigens while mounting appropriate responses to pathogens
  • Dysbiosis, or an imbalance in the gut microbial community, has been associated with various gastrointestinal disorders, such as inflammatory bowel diseases and irritable bowel syndrome
• The gut-associated lymphoid tissue (GALT) is the largest component of the immune system, and its development and function are heavily influenced by the gut microbiota
• Disruptions in the gut microbiota, such as those caused by antibiotics or dietary changes, can lead to immune dysregulation and increased susceptibility to infections and inflammatory conditions

Probiotics and Prebiotics

• Probiotics, which are live microorganisms that confer health benefits when consumed in adequate amounts, can help to maintain or restore a healthy gut microbiota and alleviate certain digestive symptoms
• Common probiotic strains include Lactobacillus and Bifidobacterium species, which are found in fermented foods like yogurt, kefir, and sauerkraut
• Prebiotics, which are non-digestible food components that promote the growth and activity of beneficial gut bacteria, can also support digestive health by selectively stimulating the growth of favorable microbial populations
• Prebiotics are found in foods such as garlic, onions, leeks, asparagus, and whole grains, and they include compounds like inulin, fructo-oligosaccharides (FOS), and galacto-oligosaccharides (GOS)