Digestive & Excretion Exam Preparation Notes

incisors canines

premolars

molars

mouth cavity

pharynx salivary glands oesophagus

tongue

diaphragm

liver stomach duodenum gall bladder

pancreas

pyloric sphincter transverse colon

ascending colon

descending colon

caecum appendix

rectum

The Mouth • Ingestion of food • Chewing breaks food into smaller particles (mechanical digestion) • Saliva, made by 3 salivary glands, contains mucus and a digestive enzyme, salivary amylase (begins the digestion of starch) The teeth • 4 incisors: for biting or cutting • 2 canines: for tearing • 4 premolars and 6 molars: for crushing and grinding food - After chewing, the tongue pushes the bolus (food lump) into the pharynx

The Oesophagus • Has a double layer of muscle – circular muscle (fibres arranged in a circle) and longitudinal muscle (fibres arranged along the length) • Waves of constriction pushes bolus by the circular muscle (peristalsis) – lubricated by mucus from inner lining

The Stomach • Churns food by waves of contraction (mechanical digestion) by its oblique, circular and longitudinal layers of muscles – to form chyme (liquid food) • Mucosa (lining of stomach) secretes gastric juice (secreted by gastric glands) • Gastric juice contains mucus, HCl and pepsin (breaks down proteins to polypeptides) • Stores food as it is eaten. • Do not absorb nutrients (but does absorb some drugs) • The end has a pyloric sphincter that restrict movements of stomach contents • Mucus protects stomach wall

gastric pit connective tissue

cells that secrete mucus cells that secret pepsinogen cells that secret HCl

gastric gland

Small Intestine – Digestion • Churn food by muscular contractions (mechanical digestion) • Bile salts (produced in liver, stored in gall bladder) emulsify lipids – increasing surface area for lipases to act on • Pancreatic juice (produced by pancreas) contains pancreatic amylase (breaks starch → disaccharides), pancreatic protease [trypsin] (breaks proteins + polypeptides → peptides), pancreatic lipases (breaks lipids → fatty acids + glycerol) and nucleases (digest DNA + RNA) and neutralise stomach materials with carbonate • Intestinal juice (produced by glands in lining) complete digestion; contains amylases/dissacharadases i.e. sucrase, maltase and lactase (break disaccharides → simple sugars), peptidases (breaks peptides → amino acids) and lipases (breaks lipids → fatty acids + glycerol)

Small Intestine – Absorption • Products of digestion + minerals + vitamins + water are absorbed through the intestinal wall • The mucosa (inner lining) has folds, finger-like projections called villi (with even smaller projections called microvilli) • Lacteal – a lymph capillary in the villus, surrounded by blood capillaries • Absorption is enhanced by movements of the wall – bringing villi close to contents • Some absorption occurs through simple diffusion (higher concentration of nutrients inside than in cells in lining) • Some occurs through active transport • Simple sugars, amino acids, water and water-soluble vitamins → blood capillaries → liver (by hepatic portal vein) or body cells • Fatty acids + glycerol recombine (form fats) in villi, and fatsoluble vitamins → lacteals → lymph system → blood (through veins in upper body)

Carbohydrates • Provides energy for body cells; first source of energy • Monosaccharides: simple sugars/ single-unit sugars (e.g. glucose, fructose ad galactose) • Disaccharides: 2 simple sugars joined (e.g. sucrose, maltose and lactose) • Polysaccharides: large numbers of simple sugars joined (e.g. glycogen, cellulose and starch) Lipids • Second source of energy; energy reserves in body • Each lipid molecule consists of 1 molecule of glycerol + 1-3 fatty acids molecules g fatty acid • Triglyceride: glycerol + 3 fatty acids (found in body) l Proteins • Third source of energy; can be converted to glucose • Made up of amino acids • Dipeptide: 2 amino acids joined by a peptide bond • Polypeptide: 10+ amino acids joined

y c e r o l

fatty acid

fatty acid

Large Intestine - Elimination • Absorbs remaining water • Bacteria in large intestine break down much of remaining organic compounds (some bacteria produce vitamins, that are then absorbed into the body) • Minerals are also absorbed • Faeces: contain water, undigested food material, bacteria, bile pigments (from the breakdown of haemoglobin from red blood cells) and remains of cells (that have broken away from internal lining of alimentary canal)

Excretory Organs • Lungs: excretion of CO2 from cellular respiration • Sweat glands: secretes sweat (containing by-products of metabolism i.e. urea, salts and lactic acid) • Alimentary canal: passes out bile pigments • Kidneys: principal excretory organs; maintain constant concentration of materials in body fluids Nephrons • Found in kidneys (microscopic); remove wastes from blood, regulate blood composition • Renal corpuscle = glomerulus + glomerular capsule (Bowman’s capsule) • Efferent arteriole breaks into a second capillary network of peritubular capillaries (leading to the renal vein) • Urine: substances not absorbed drain from collecting ducts into renal pelvis (where they are then pushed by waves of contraction to the urinary bladder to be stored)

Ascending limb Descending limb

Urine Formation Glomerular filtration •Takes place in renal corpuscle • Fluid is forced of the blood into the Bowman’s capsule – efferent arteriole is smaller then afferent arteriole, creating pressure in glomerulus. Turns to filtrate. • Blood in capillaries is separated from 2 layers: wall of capillary and wall of capsule • Filtrate = water + salts + amino acids + fatty acids + glycerol + urea + uric acid + creatinine + hormones + toxins + various ions Reabsorption (and selection) •Takes place in renal tubule (by internal cells) • Absorbed materials: water + glucose + amino acids + ions i.e. Na+, K+, Ca2+, Cl- and HCO3- (bicarbonate) + some wastes i.e. urea • Active transport of water can be regulated under hormonal control (facultative reabsorption) Tubular secretion (passive or active) • Adds K+, H+, creatinine and drugs • Controls pH of blood by secreting H+ and NH4 + into filtrate

microvilli

capillaries lacteal absorptive cell secretory cell

intestinal gland

artery vein lymphatic vessel

Region of nephron

Activities taking place

Renal corpuscle

Filtration of blood from capillaries of glomerulus Formation of filtrate in the Bowman’s capsule

Proximal convoluted tubule and loop of Reabsorption of Na+, K+, Ca2+, Cl- and Henle HCO3Reabsorption of glucose Passive reabsorption of water by diffusion Distal convoluted tubule

Reabsorption of Na+ Active transportation of water depending on body’s needs Secretion of K+, H+, creatinine and certain drugs i.e. penicillin

Collecting duct

Active reabsorption of water depending on body’s needs

Liver – Excretion • Proteins are metabolised when other sources of energy are used up – deamination (the removal of the amino group NH2 from amino acids) occurs in liver with help of enzymes • Once NH2 is removed, it is converted by liver cells to urea • The remaining part of amino acid is converted to carbohydrate (which is broken down to energy, CO2 and water by cells) enzymes

amino acid + O2 → carbohydrate + ammonia • Ammonia is water-soluble and is highly toxic in large quantities to cells • It is converted to urea by liver cells, then eliminated through urine or as sweat energy + CO2 + ammonia → urea + water