Alimnetary system

Digestive Enzymes

• Trypsin and carboxypeptidase are enzymes that digest proteins
• Nucleic acid enzymes digest DNA and RNA
• Pancreatic amylase is an enzyme that digests polysaccharides

Small Intestine

• Chemical digestion, absorption, and secretion occur in the small intestine
• Pancreatic juice, bile, and interstitial juice are involved in chemical digestion
• The small intestine is lined with microvilli, which increase the surface area for absorption
• The mucosa of the small intestine is highly folded and lined with villi, which also increase the surface area for absorption

Large Intestine

• The large intestine consists of the caecum, ascending colon, transverse colon, descending colon, sigmoid colon, and rectum
• The large intestine is involved in the fermentation of complex carbohydrates, absorption of salts and water, and excretion of faeces
• Midgut fermentation occurs in the ascending colon, where commensal bacteria break down cellulose

Gut Functions

• Motility, secretion, digestion, absorption, and storage are the functions of the gut
• The gut has intrinsic and extrinsic control mechanisms, including hormonal and neuronal regulation

Gut Control

• The enteric nervous system and hormones such as secretin, gastrin, CCK, GIP, and motilin regulate gut function
• The vagus and splanchnic nerves also regulate gut function

Gut Structure

• The gut consists of four layers: mucosa, submucosa, tunica muscularis, and serosa/adventitia
• The myenteric plexus and submucosal plexus are two types of nerve plexuses that regulate gut function

Digestive Secretions and Motility

• Parasympathetic stimulation increases digestive secretions and motility, while sympathetic stimulation decreases them
• Hormonal signalling involves endocrine, neurocrine, paracrine, and autocrine signalling

Phases of Digestion

• The cephalic phase involves salivation, which is stimulated by parasympathetic innervation
• The gastric phase involves acid and enzyme production and secretion, stimulated by gastrin and parasympathetic stimulation
• The intestinal phase involves stimulation of motility and secretion, regulated by hormones such as secretin and CCK

Hormones and Satiety

• Ghrelin is the "hunger hormone", stimulated by an empty stomach and released by the stomach and pancreas
• Leptin is the "satiety hormone", produced by adipocytes and opposing the action of ghrelin
• Cholecystokinin, peptide YY, and GLP-1 are satiety hormones that stimulate hunger and satiety centres in the hypothalamus

Layers of the Gut

• Mucosa: site of absorption and secretion
• Submucosa: contains blood vessels and lymphatic vessels
• Tunica muscularis: layer of smooth muscle
• Serosa/Adventitia: outermost layer

Luminal Receptors

• Mechanoreceptors: detect stretch and stimulate contractions
• Chemoreceptors: sense chemical composition of the chyme and regulate pH
• Osmoreceptors: sense osmolarity and control motility and secretion

Hormonal Signaling

• Endocrine: secretes into blood
• Neurocrine: secreted by nerves
• Paracrine: secreted by adjacent cells
• Autocrine: secreted by self
• Gut endocrine cells: columnar epithelial cells with narrow apex (sensory) and wide base (secretory)

Phases of Digestion

Cephalic Phase

• Salivation, stimulated by parasympathetic innervation (cranial nerves)
• Increased blood flow to glands
• Brainstem regulates visual or olfactory salivation

Gastric Phase

• Acid and enzyme production and secretion
• Sphincter is relaxed as well as stomach and duodenum to accommodate chyme
• Stimulation of acid: Gastrin produced by duodenum, stomach, and pancreas
• Increases acid, pepsinogen secretion

Intestinal Phase

• Stimulation of motility
• Motilia: increased motility/peristalsis in the gut, 'clears' tract between meals
• Cholecystokinin: decreases gastric emptying

Hunger and Satiety

• Hypothalamus: Ventro Medial area = "satiety centre", Lateral area = "appetite centre"
• Low blood sugar sensory input appetite stimulated
• Ghrelin: "hunger hormone", stimulated by empty stomach, released by stomach and pancreas, stimulates hunger
• Leptin: "satiety hormone", opposes action of Ghrelin, produced by adipocytes, crosses blood-brain barrier to reach hypothalamus

Satiety Hormones

• Cholecystokinin, Peptide YY, Glucagon-like Peptide 1 (GLP-1)
• GLP-1: secreted by gut cells and neurons, increases Insulin, decreases Glucagon, GI motility, gastric emptying, food and water intake

Pancreatic Endocrine Cells

• Islet of Langerhans: A cells (Glucagon), B cells (Insulin), D cells (Somatostatin)
• A cells: secreted when blood glucose falls, triggers breakdown of glycogen into glucose in the liver
• B cells: secreted when blood glucose is high, triggers storage of glucose in liver, skeletal muscle, and fat cells
• D cells: secretion induced by low pH, suppresses release of hormone from A and B cells, inhibits gut motility and exocrine secretion

Obesity

• BMI over 25 = overweight
• BMI over 30 = obese
• BMI under 18.5 = underweight
• Ideal BMI = 18.5-24.9
• BMI = Weight (kg) / Height^2 (m)

Drugs

• Orlistat: inhibits pancreatic and gastric lipases, prevents around 1/3 of fat from food being absorbed
• Semaglutide (diabetes medication): binds to and activates GLP-1 receptor

Bariatric Surgery

• Restrictive (Reduces stomach size and dietary intake)
• Malabsorptive (bypassing the stomach and small intestine)
• Gastric band: restrict amount you eat
• Gastric bypass (Roux-en-Y): bypasses stomach and duodenum by attaching jejunum to gastric pouch so food is not absorbed at stomach

Small Intestine

• Chemical digestion: Pancreatic juice, Bile, and interstitial juice
• Absorption: Proteins, Fats, and Carbohydrates
• Duodenum: C-shaped, encircles pancreas, contains Brunner glands which secrete alkaline mucus
• Jejunoileum: microvilli are lined with membrane-bound enzymes for final breakdown of nutrients for absorbable constituents

Colon / Large Intestine

• Food moves in this direction
• Caecum, Ascending colon, Transverse colon, Descending colon, Sigmoid colon, Rectum

Midgut Fermentation

• Cellulose digestion by commensal bacteria in the ascending colon
• Only in Grass eaters
• Animals have simple single-chambered stomachs
• Smooth mucosa (no villi)
• Fermentation of complex carbohydrates
• Absorption of salts + water
• Excretion of faeces

Rectum + Anus

• Pelvic part of GI tract
• Supports and stores faecal mass before defection
• Build-up pressure against walls
• Internal sphincter relaxes to open and allow faeces into canal
• Relaxation of internal sphincter permits defecation