BIOH122 Human Biological Science 2 · BIOH122 Human Biological Science 2 ... o Absorption and...

© Endeavour College of Natural Health endeavour.edu.au

BIOH122Human Biological Science 2

Session 15

Digestive System 3 –

Small Intestine & Large Intestine

Bioscience Department

© Endeavour College of Natural Health endeavour.edu.au 2

Session Plan

o Absorption in Small intestine

o Large intestine – anatomy and histology

o Large intestine – mechanical and chemical digestion

o Absorption and Faeces Formation in the Large Intestine

• The Defecation Reflex

o Phases of digestion

o Aging and the digestive system


Review: Digestion of

Carbohydrates

o Mouth: Salivary amylase

o Oesophagus and stomach: no enzyme released for

carbohydrates but salivary amylase active for about an

hour.

o Small intestine:

• Pancreatic amylase

• Brush border enzymes (ɑ-dextrinase, maltase,

sucrase and lactase) act on disaccharides

o Production of monosaccharides - fructose, glucose and

galactose for absorption


Review: Digestion of

Carbohydrates

Marieb, and Hoehn, 2011


Review: Digestion of Proteins

o Mouth and Oesophagus: No digestion

o Stomach:

• HCl

• Pepsin

o Small Intestine:

• Pancreatic peptidases: trypsin, chymotrypsin,

carboxypeptidase, and elastase

• Brush border peptidases: aminopeptidase and dipeptidase

o Digestive enzymes - split peptide bonds between different

amino acids for absorption.


Review: Digestion of Proteins



Review: Digestion of Lipids

o Mouth: Lingual lipase (activated in stomach)

o Stomach: Gastric lipase (has a limited role in the adult)

o Small Intestine:

• Bile salts

• Pancreatic lipase

• No enzymes in brush border

o Triglycerides are broken down into long/short chain fatty

acids and monoglycerides for absorption


Review: Digestion of Lipids



Digestion of Nucleic Acids

o Mouth, oesophagus and Stomach: No enzymes for

digestion

o Small intestine:

o Pancreatic nucleases: Ribonuclease and

deoxyribonuclease

o Brush border enzymes: nucleosidase and phosphatase

o Nucleic acids are broken down into nucleotides and

then into pentoses, phosphates, and nitrogenous bases

for absorption.


Digestion of Nucleic Acids



Absorption in Small intestine


Absorption in the Small Intestine

o Absorption: The passage of the end products of

digestion from the GI tract into the blood or lymph

o Occurs by:

• Simple diffusion

• Facilitated diffusion

• Osmosis

• Active transport

o Most absorption occurs in Small intestine


Absorption in Small Intestine


Absorption of Monosaccharides

o Essentially all carbohydrates are absorbed as

monosaccharides.

o Absorption into epithelial cell

• Glucose and Galactose - sodium glucose symporter

(active transport)

• Fructose - facilitated diffusion

o Movement out of epithelial cell into bloodstream

• by facilitated diffusion


Absorption of Monosaccharides



Absorption of Amino Acids and

Dipeptides

o Absorption of Amino Acids, Dipeptides, and Tripeptides

o Absorption into epithelial cell

• active transport with Na+ or H+ ions (symporters)

o Movement out of epithelial cell into blood

• diffusion


Absorption of Amino Acids

and Dipeptides



Absorption of Lipids

o Dietary lipids are all absorbed by simple diffusion.

o Absorption into epithelial cell:

• Long-chain fatty acids and monoglycerides: absorbed as part of

micelles

• Small fatty acids: simple diffusion

o Movement out of epithelial cell

• Long-chain fatty acids and monoglycerides: chylomicrons into

the lacteal of a villus.

• Small fatty acids: simple diffusion into blood



o From the lacteal the chylomicrons enter the lymphatic

system and then pass into the cardiovascular system,

finally reaching the liver or adipose tissue.

o The plasma lipids: fatty acids, triglycerides, cholesterol -

are insoluble in water and body fluids.

o Lipoproteins: The combination of lipid and protein

transporters in order to transport lipids in blood and

utilized by body cells.



o Lipids enter cells by

simple diffusion leaving

bile salts behind in the

lumen.

o Bile salts are

reabsorbed into blood

and reformed into bile in

the liver



Absorption of Electrolytes

o Many of the electrolytes absorbed by the small intestine

come from gastrointestinal secretions and some are part

of digested foods and liquids.

o Absorption: by diffusion and secondary active transport

• Sodium: diffusion, secondary active transport, Na+/K+

pumps (active transport)

• Chloride, iodide and nitrate: active or passively

transport

• Iron, magnesium, potassium and phosphate ions:

active transport

• Calcium: active transport, stimulated by calcitriol.


Absorption of Vitamins

o Fat-soluble vitamins (A, D, E, and K):

• travel in micelles and are absorbed by simple

diffusion

o Water-soluble vitamins (B and C):

• absorbed by diffusion

o B12 combines with intrinsic factor before it is transported

into the cells

• receptor mediated endocytosis


Absorption of Water

o Water absorption: Osmosis from the lumen of the

intestines through epithelial cells and into blood

capillaries.

o Depends on the absorption of electrolytes and nutrients

to maintain an osmotic balance with the blood.

o Small intestine reabsorbs 8.3 liters

o Absorption is by osmosis through cell walls into vascular

capillaries inside villi


Absorption of

Water o 9.3 liters of fluid dumped

into GI tract each day

o Small intestine reabsorbs

8.3 liters

o Large intestine reabsorbs

90% of that last one liter

o Absorption is by osmosis

through cell walls into

vascular capillaries inside

villi

24


Where will the absorbed nutrients go?


Large intestine – anatomy and

histology


Large Intestine

o The large intestine: The terminal portion of the GI tract

(1.5 m long by 6.5 cm in diameter), extends from the

ileocaecal sphincter to the anus.

o Functions:

• Haustral churning, peristalsis, and mass peristalsis drive

contents of colon into rectum.

• Bacteria in large intestine convert proteins to amino acids, break

down amino acids, and produce some B vitamins and vitamin K.

• Absorbing some water, ions, and vitamins.

• Forming faeces and defecating (emptying rectum).


Anatomy of the Large Intestine

o Anatomical subdivisions:

• Caecum

• Colon

• Rectum

• Anal canal

o Appendix


Anatomy of the Large Intestine

o Anal canal

• Anus:

– An internal anal

sphincter

– An external anal

sphincter


Histology of the Large Intestine

o Layers of large intestinal wall:

• Mucosa

• Submucosa

• Muscularis

• Serosa



o Mucosa: composed of:

o The epithelial layer: Simple columnar epithelium

• Contains Absorptive and Goblet cells

o Intestinal glands: Extend the full thickness of the mucosa

o Lamina propria: Areolar connective tissue

• Solitary lymphatic nodules

o Muscularis mucosae: smooth muscle

o Structural adaptations: No circular folds or villi; however,

microvilli are present on the absorptive cells



Cells of Intestinal glands:

• Absorptive cell

• Goblet cell



o Submucosa: Areolar connective tissue.

• Solitary lymphatic nodules: may extend through the

muscularis mucosae into the submucosa

o Muscularis: Consists of 2 layers of smooth muscles.

• An internal layer: circular smooth muscle.

• An external layer: longitudinal smooth muscle

– Taeniae coli

– Haustra

o Serosa: Visceral peritoneum

• Omental appendices


Large intestine – mechanical and

chemical digestion


Mechanical Digestion in the

Large Intestine

o Mechanical movements:

• Haustral churning: Slow haustral contractions that occur about

every 30 minutes and last approximately 1 minute.

• Peristalsis: 3 to 12 contractions per minute

• Mass peristalsis: 3 or 4 times a day, during or immediately after a

meal

• Gastroileal reflex: when stomach is full, gastrin hormone

relaxes ileocecal sphincter so small intestine will empty

and make room

• Gastrocolic reflex: when stomach fills, a strong peristaltic

wave moves contents of transverse colon into rectum


Chemical Digestion in the

Large Intestine

o Large intestinal glands: only mucous, no enzymes are

secreted

o Bacterial fermentation:

• Undigested carbohydrates: into hydrogen, carbon dioxide and

methane gas

• Flatulence

• Undigested proteins: into indole, skatole, hydrogen sulfide, and

fatty acids

• Fecal odour and Urinary excretion

• Bilirubin: into simpler pigments- Stercobilin

• Fecal colour

• Vitamins K and B produced in the colon


Absorption and Feces Formation in

the Large Intestine


Absorption in the Large Intestine

o Absorption:

o Water: by osmosis through cell walls into vascular

capillaries inside villi

• Of the 0.5–1.0 litre, 90% reabsorbed Only 100 -200 ml of water

is excreted in the faeces each day.

o Some electrolytes: sodium and chloride ions

o Some vitamins


Feces Formation in the Large

Intestine

o Feces formation: After 3 to 10 hours, due to water

absorption from chyme.

o Feces: Solid or semisolid chyme consist of

• water, inorganic salts, sloughed-off epithelial cells

from the mucosa of the gastrointestinal tract, bacteria,

products of bacterial decomposition, unabsorbed

digested materials, and indigestible parts of food.


The Defaecation Reflex

o Defaecation: The elimination of faeces from the rectum

o The defaecation reflex: action aided by voluntary

contractions of

• The diaphragm,

• The abdominal muscles and

• The external anal sphincter

o The external anal sphincter can be voluntarily controlled

(except in infants) to allow or postpone defaecation.


The Defaecation Reflex



Defaecation Problems

o Diarrhoea: • Increased motility of the intestines.

• Caused by: lactose intolerance, stress, and microbes that irritate the gastrointestinal mucosa.

• Chyme passes too quickly through intestine

• Decreased absorption by the intestines.

o Constipation:• Decreased intestinal motility

• Faeces remain in the colon for prolonged periods, too much water is absorbed

• Caused by poor habits, spasms of the colon, insufficient fibre in the diet, inadequate fluid intake, lack of exercise, emotional stress, and certain drugs


Dietary fibers

o Dietary fibers: indigestible plant carbohydrates

– cellulose, lignin, and pectin—found in fruits, vegetables,

grains, and beans

– affect the speed of food passage through the GI tract

o Insoluble fiber: insoluble in water

– woody parts of plants (wheat bran, veggie skins)

– may help protect against colon cancer

o Soluble fiber: soluble in water

– in beans, oats, barley, broccoli, prunes, apples, and citrus

fruits.

– lowers blood cholesterol by preventing reabsorption of bile

salts so liver has to use cholesterol to make more


Phases of Digestion


Phases of Digestion

o Three overlapping phases:

• Cephalic phase

• Gastric phase

• Intestinal phase


Ph

ase

s o

f D

ige

stio

n


Cephalic Phase

o The cephalic phase: prepares the mouth and stomach

for food that is about to be eaten.

o Activated by: The smell, sight, thought, or initial taste of

food

o Neural centres: in the cerebral cortex, hypothalamus,

and brain stem.

o The facial and glossopharyngeal nerves:

• stimulate the salivary glands to secrete saliva

o Vagus nerve:

o stimulate the gastric glands to secrete gastric juice


Gastric Phase

o The gastric phase: Begins when food arrives in the

stomach.

o Promote gastric secretion and gastric motility.

o Two regulations:

• Neural control

• Hormonal control


Gastric Phase

o Neural control:

• Stretch receptors and chemoreceptors: in the wall of stomach

• Submucosal plexus

• Vigorous peristalsis and gastric juice secretions

• Gastric emptying


Gastric Phase

o Hormonal control:

o Gastrin: released from G cells secretion of gastrin into

bloodstream

o Stimuli:

• Distension of the stomach by chyme,

• Partially digested proteins in chyme,

• High pH of chyme due to the presence of food in the stomach*

• Acetylcholine released from parasympathetic neurons.

* Gastrin release inhibited when pH of gastric juice drops below 2.0


Gastric Phase

o Roles of Gastrin:

• Stimulates gastric glands to secrete large amounts of

gastric juice.

• Strengthens the contraction of the lower oesophageal

sphincter to prevent reflux of acid chyme into the

oesophagus,

• Increases motility of the stomach, and relaxes the

pyloric sphincter, which promotes gastric emptying.


Intestinal Phase

o The intestinal phase: Begins when food enters the small

intestine.

o Slow the exit of chyme from the stomach. Promote

digestion of food in small intestine

o Two regulations:

• Neural control

• Hormonal control


Intestinal Phase

o Neural control:

o Stretch receptors, fatty acids or sugar: Signal Enteric NS

and CNS (medulla)

o ↑ Sympathetic nerves ↓Parasympathetic nerves: Slow

stomach motility / emptying

o The enterogastric reflex: stretch of duodenum causes

inhibition of gastric motility and increases the contraction

of the pyloric sphincter to decrease gastric emptying.


Intestinal Phase:

Neural control

Start here


Intestinal Phase

o Hormonal control:

• Secretin: Stimulates the flow of pancreatic juice rich in

bicarbonate, and inhibits the secretion of gastric juice.

• Cholecystokinin (CCK): decreases stomach emptying

and stimulates the secretion of pancreatic juice rich in

digestive enzymes, and increase the flow of bile.


Aging and The Digestive System


Aging and the Digestive System

o Changes associated with ageing:

• Decreased secretory mechanisms

• Decreased motility of the digestive organs

• Loss of strength and tone of muscular tissue

• Changes in neurosensory feedback

• Diminished response to pain and internal stimuli



Pathologies with Ageing:

o Upper GIT:

• mouth irritations and sores,

• loss of taste,

• periodontal disease,

• difficulty in swallowing,

• hiatal hernia, gastritis, and peptic ulcer disease.

o Small intestine:

• Duodenal ulcers,

• malabsorption, and maldigestion



Pathologies with Ageing:

o Large intestine:• Constipation

• Haemorrhoids

• Diverticular disease

• Cancer of the colon or rectum

• Bowel obstructions and impactions

o Other pathologies:• Appendicitis,

• Gallbladder problems,

• Jaundice, and cirrhosis,

• Acute pancreatitis


Readings and Resources

o Tortora, GJ & Derrickson, B 2014. Principles of Anatomy and Physiology, 14th edn, Wiley.

o Harris, P, Nagy, S & Vardaxis, N 2010, Mosby’s Dictionary of Medicine, Nursing and Health Professions, 2nd edn, Mosby Elsevier.

o Guyton, AC & Hall, JE 2011, Textbook of Medical Physiology, 12th edn, Saunders Elsevier.

o Marieb, EN & Hoehn, K 2011, Human Anatomy and Physiology, 9th edn, Benjamin Cummings Pearson.

o Moore, KL, Dalley, AF & Agur, AMR 2010, Clinically Orientated Anatomy, 6th edn, Lippincott, Williams & Wilkins.


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