Absorption, Fecal Formation, and Defecation

Absorption

  • physiological process of moving digested nutrients, water, and electrolytes from the digestive tract into the blood stream or lymphatic system

    • primarily occurs within the small intestine

      • nutrients transported across intestinal epithelium into the blood

      • a few things are absorbed within the large intestine (e.g. water, minerals, B and K vitamins, and fatty acids from bacterial fermentation)

  • polymers are not absorbable

    • must be broken into monomers (digestion)

  • influenced by surface area, concentration gradients, and specialized transporters

    • surface area increased by villi, microvilli, and plicae circulares

  • involves both diffusion and active transport

Carb Absorption

  • proteins break down carbs for absorption

    • monomers to ECF → capillaries → liver → SI

  • monosaccharides immeadiately absorbed

  • oligosaccharies must be digested first

    • many digested by amylases, then by dextrinase

    • maltose by maltase

    • sucrose by sucrase

    • lactose by lactase

  • monosacchrides enter intestinal epithelial cells via facilitated diffusion or active transport

    • sodium-glucose linked transporters (SGLTs)

Protein Absorption

  • proteases and peptidases break proteins into amino acids, dipeptides, and tripeptides

    • e.g. pepsin (stomach), trypsin, and carboxypeptidase

  • absorbed via transport systems

    • dipeptides and tripeptides absorbed via brush border peptide systems, then encounter cytoplasmic peptidase

    • free amino acids absorbed via brush border amino acid transport system

    • basolateral transport systems move amino acids and small peptides into the bloodstream

Nucleic Acid Absorption

  • pancreas makes zymogens for ribonuclease and deoxyribonuclease to break down RNA and DNA into nucleotides

  • SI produces nucleosidase and phosphatase to break nucleotides down into pentose sugars, nitrogenous bases, and phosphates

  • absorbed by epithelial cells

Lipid Absorption

  • 10-15% digested before entering the duodenum

    • lingual lipase and gastric lipase

  • pancreatic lipase does most of the work

  • lipases emulsify lipids

    • triglycerids into 2 free fatty acids and monoglyceride

  • short-chain fatty acids are directly absorbed

  • larger lipids encounter bile acids and lecithin to be formed into micelles (can moce between microvilli)

  • epithelial cells reincorporate free fatty acids and monoacylglycerides into triglycerides

  • triglycerides mis with phospholipids and cholesterol to create chylomicrons

    • in the bloodstream, triglycerides of chlomicrons egt broken down by enzyme lipoprotein lipase into free fatty acids and glycerol

      • used for energy or sent for storage in adipose

  • remainder of chylomicrons sent to the liver

    • makes lipoproteins for transporting cholsterol

      • low-density lipoproteins (LDL)

        • move cholsterol from the liver to the arteries

        • too much can cause the formation of plaques

      • high-density lipoproteins (HDL)

        • move cholsterol out of the arteries back to the liver

        • help prevent plaques formation

        • converted to bile salts in the liver

Vitamin Absorption

  • absorbed in different regions of the SI

  • absorbed in different ways

    • Na+ co-transported with carbs and amino acids

    • Cl- exchanged for bicarbonate

    • K+ absorbed by simple diffusion

    • Fe absorption regulated by hepcidin in liver

    • others absorbed via diffusion and active transprt

  • gut disorders can increase risk of mineral deficiences if absorption is comprimised

Mineral Absorption

  • iron - primarily in duodenum

  • zinc - primarily in jejunum

  • magnesium - duodenum, jejunum, and ileum

  • calcium - duodenum and jejunum

  • selenium - primarily duodenum

Role of LI

  • SI absorbs most of nutrients and up to 90% of water before material reaches the LI

  • vital for absorption of water and electrolytes

    • actively absorbs Na+ via sodium channels

    • K+ absorbed/secreted dependent on lumen concentrations (thanks to Na+ gradients)

    • Cl- exchanged for HCO3-

  • helps to produce and absorb vitamins

    • vitamin K and some B vitamins (e.g. biotin and folate)

    • gut bacteria produced vitamins via fermentation

  • forms and propels feces

    • receices ~500mL of indeigestible residue

      • reduces residue to 150mL of feces

Gut Bacteria

  • home to 39 trillion bacterial cells (~500 species)

    • most are obligate anaerobes (e.g. bacteroides)

    • digest things that cells cannot and synthesize vitamins

  • help protects from pathogens:

    • compete for space and nutrients

    • supporting immune sysytem develpoment

    • secrete anti-microbial chemicals and induce IgA produciton

  • help with metabolic functions:

    • secrete short-chain fatty acids for intestinal epithelial cells

    • modulate fat deposition through glucose metabolism

    • matabolize dietary carcinogens

    • metabolize bilirubin into urobilinogen

      • half reabsorbed into bloodstream → excreted by kidneys

        • oxidized into urobilin (yellow color of urine)

      • other half reduced to stercobilin (brown color of feces)

Flatus and Odor

  • humans produce ~500mL of flatus/day

    • mostly swallowed air, but also from bacteria

    • on average, humans pass gas 20 times/day

      • this is often done unknowningly

  • odor from hydrogen sulfide, indole, and skatole

    • fiber-rich foods contain sulfur

    • composition of gut flora impacts smells

      • antibiotics can disrupt gut flora and encourage growth of harmful bacteria such as Clostridiodes difficile (“C. diff”)

      • gut bacteria convert trytophan into indole and skatole

        • help the bacteria communicate with one another

Indole

  • indole can reduce bacterial aggregation and motility, suppress toxin production, and influence host cell invasion

    • gets reaborbed in the LI and processed in the liver

    • can have both positive and negative effects on the brain and other organs

Skatole

  • skatole os formed by tryptophan decompisition and is the main source of fecal odor

  • has been linked to various pathologies and is used as a biomarker for microbiome disruption

    • tied to pulmonary toxicity

    • may be carcinogenic

  • inhibits the growth of Gram-negative bacteria by increasing endogenous oxidative stress, which damamges bacterial cell membranes

Feces

  • 75% water and 25% solids

    • typically, 84-93% of solids are organic

      • 25-54% bacterial biomass

      • 2-25% protein/nitrogenous matter

      • 25% carboohydrates/undigested plant matter

      • 2-15% lipids

    • solids also include mucus, salts, digestive secretions, and sloughed cells (mostly epithelial from the LI)

  • water and electrolytes reabsorbed from feces

  • takes about 26-48 hours to make residue into feces

    • most time spent in the transverse colon

Defecation

  • sensory neurons send pressure, mucosal irritation, and abdominal information to brainstem

    • medial (barrington’s). nucleus controls autonomic pathway to the distal colon and rectum

      • project to the intermediolateral column (IML) in the spinal defecation center (lumbosacral region → L6S1)

    • lateral cell group (LCG) controls external anal sphincter

      • project to onuf’s nucleus (ON) in the spinal defecation center to neurons that supply the external anal sphincter

  • modulated by the frontal lobe, hypothalamus, and basal ganglia

Healthy Stools

  • 1997 - bristol stool chart developed for clinical assessment of feces

    • 7 types defined by fecel morphology

      • range from severe constipation to severe diarrhea

  • healthy defection:

    • 1-3 times per day

    • typically ~30 minutes after a meal

    • should happen easily, without pain

    • should happen without a minute of starting and should completely empty each time