Physiology of Water Absorption PDF

Salt and water balance in the intestine Dr. Gavin Bewick  Learning objectives After studying the topics of this lecture, students should be able to: Recall the structure of the colon with particular reference to specialised features for absorption Describ...

Salt and water balance in the intestine Dr. Gavin Bewick  Learning objectives After studying the topics of this lecture, students should be able to: Recall the structure of the colon with particular reference to specialised features for absorption Describe the various functions of the colon, specifically: – Water and electrolyte absorption – Formation of stool – Fermentation Understand the importance of standing gradient osmosis and how different parts of the gut maintain this gradient. Describe the pathology of common colonic disorders including. – Congenital diarrhoea, bacterial diarrhoea, osmotic diarrhoea.  Outline Structure of the colon – Epithelium – Crypts Functions of the colon – Water and electrolyte absorption – Some vitamins eg Vitamin K – Formation of hard stool suitable for defecation – Fermentation of undigested carbohydrates Diarrheal diseases  The Large Intestine (colon) Transverse colon Ascending colon Descending colon ileocecal valve Sigmoid colon rectum  Structure of the colon Epithelium is flat with no villi Contains crypts extending deep into the epithelium Crypts can act as capillaries to suck water out of feces Many mucus secreting goblet cells throughout the epithelium ease passage of feces Colon epithelium is flat with deep crypts Water absorption in the crypts causes capillary suction pressure Suction helps remove water from feces Capillary action causes suction Na and Water absorption Fluid intake and secretion in the GI  Stool characteristics Normal human stool 65-85% water – Diarrhea >85 %water 40-50 % of solid matter is bacteria – Over 300 different species mainly anaerobic pH is 5-6 acidic due SCFA production helps keep down pathogenic strains Sloughed enterocytes, bile salts, variable amounts of electolytes  Water and electrolytes 99% of the H2O presented to the GI tract is absorbed. The absorption of water is powered by the absorption of ions. The greatest amount of water is absorbed in the small intestine, esp the jejunum. Many ions slowly absorbed by passive diffusion. (Calcium and iron are incompletely absorbed, and this absorption is regulated)  Water absorption Follows Na and other solute absorption by osmosis Water transport can be: – Paracellular via tight junctions between cells – Transcellular across cell membranes via specialised water channels called aquaporins (AQP 1-8)  Standing gradient osmosis. Driven by Na+ Transport of Na+ from lumen into enterocyte- complex and varies between species. Becomes more efficient as travel down intestine: Counter-transport in exchange for H+ (small intestine) Co-transport with amino acids, monosaccharides (small intestine) Co-transport with Cl- (small intestine) Restricted movement through ion channels (colon) Intestinal Sodium absorption Na+ Na+ aa Na+ Gluc Na+ Cl- Na+ H+ Small intestine Small intestine Colon Basement Membrane Blood  Absorption of other ions Cl- co-transported with Na+ (ileum), exchanged with HCO3- (colon) into enterocytes. Both secondary active transport. K+ diffuses in via paracellular pathways in small intestine, leaks out between cells in colon. Passive transport. Intestinal Lumen Na+ Na+ aa H2O Na+ Gluc Na+ Cl- Na+ Cl- H+ HCO3- Cl- Proximal Bowel Jejunum Colon Basement Membrane Blood  Standing gradient osmosis. Driven by Na+ Transport of Na+ from lumen into enterocyte- complex and varies between species. Becomes more efficient as travel down intestine: Counter-transport in exchange for H+ (proximal bowel) Co-transport with amino acids, monosaccharides (jejunum) Co-transport with Cl- (ileum) Restricted movement through ion channels (colon) What happens to this intracellular sodium? Active transport of Na+ into the lateral intercellular spaces by Na+K+ATPase transport in the lateral plasma membrane Intestinal Lumen K+ Na+ Na+ aa Na+ Gluc Na+ Cl- Na+ Cl- H+ HCO3- Cl- [Na+] Na+ K+ [Na+] Basement Membrane Blood  Standing gradient osmosis. Cl- and HCO3- transported into the intercellular spaces due to electrical potential created by the Na+ transport. High conc of ions in the intercellular spaces causes the fluid there to be hypertonic. Intestinal Lumen K+ Na+ Na+ aa Na+ Gluc Na+ Cl- H2O Na + Cl- H+ HCO3- Cl- [Na+] Na+ K+ [Na ] + [HCO3-] [Cl-] Basement Membrane Blood  Standing gradient osmosis Osmotic flow of water from the gut lumen via adjacent cells, tight junctions into the intercellular space. Water distends the intercellular channels and causes increased hydrostatic pressure. Ions and water move across the basement membrane of the epithelium and are carried away by the capillaries. Intestinal Lumen K+ Na+ Na+ aa H2O Na+ Gluc Na+ Cl- H2O Na + Cl- H+ HCO3- Cl- [Na+] Na+ K+ H2O [Na ] + [HCO3-] [Cl-] Basement Membrane Blood 1. Sodium/hydrogen exchanger 2. Sodium dependent secondary active transport processes generate hypertonic compartment Sodium/potassium ATPase 3.Water moves by osmosis 4.Cell swells which increases pressure 5.Water moves out to blood H+ Na+ Aquaporin due to pressure X Aquaporin Na+ K+ Na+ Tight junction 6. Na/K ATpase maintains Sodium gradient 7.Promotes further paracellular water absorption via tight junctions  Colonic Disorders Diarrhea – Congenital diarrhoea – Bacterial infection of gut eg cholera, C.difficile – Osmotic diarrhoea Constipation – Slow movement of faeces allow too much water absorption so very hard faeces Irritable bowel syndrome – Combination of the above causing abdominal discomfort cause unknown  Colon is a high resistance epithelia Normal stool Isotonic 10-300 mosmoles 300 mosmoles Osmotic Diarrhea >350 mosmoles Tight junction Osmotic gradient reversed in osmotic diarrhea Osmotic gradient  Congenital Diarrhea Deficiency of normal ion transport system – CONGENITAL CHLORIDE DIARRHEA - Cl-/HCO3- exchange lacking in ileum/colon HCO3- Cl- H 2O Na+ X X Na+ Cl-  Bacterial induced diarrhea Enhanced secretion of electrolytes and water - serious diarrheal diseases – e.g. CHOLERA – intracellular cAMP controls Cl- channel in luminal membrane (CFTR) Cl- Cholera H+ Na+ H2O Toxin Na+ Na+ cAMP  Osmotic Diarrhea Failure to absorb nonelectrolytes (various syndromes) Use of Laxatives or broad spectrum antibiotics Hypermotility of intestine - various causes - rapid flow of intestinal contents past the absorptive epithelium Essential nutrients, salts and water are lost Sugars/amino acids/fats Cl- Na+ H2 O Na+ Cl-  Large intestine flora All mammals have symbiotic relationships with their gut microbial community (microbiome) Stomach and small intestine have few bacteria. Large intestine contains many, essential to normal function. Meet your Microbiome Where did it come from? What's the point of the Microbiome  Summary Structure Osmotic water movements – Cell swelling – Increased hydrostatic pressure Aquaporins Role of bacteria – Conversion of undigested CHO into SCFA Various conditions leading to diarrhea