Basic Info
- Evolved to extract energy from nutrients taken in, excreting any indigestible remains
- Ingestion = process of taking food into the body via the mouth
- Digestion = process of breaking down food into small chemical units via the stomach + small intestine
- Absorption = process in which the chemical units pass into the blood and to the liver via the small intestine
- Metabolism = process in which the chemical units are converted into energy via the liver
- Excretion = removal of any remaining indigestible material
- Consists of:
- Oral cavity
- Pharynx
- Oesophagus
- Stomach
- Small Intestine
- Large intestine
- Accessory glands - salivary glands, pancreas, gall bladder + liver
Oral Cavity
- AKA mouth/buccal cavity = tongue, teeth, salivary glands
- Function = pick up food (prehension, lips + tongue), break up food (mastication, tongue, teeth + cheeks), lubrication of food (mucus + saliva)
- Bones = incisive + maxilla (upper jaw), palatine (roof of mouth, hard palate), mandible (lower jaw, joining of these at the midline is the mandibular symphysis > articulate temporal bones of the skull = temporomandibular joint > have a scissor-like action to shear flesh off bone)
- enter oral cavity lined in a layer of MM, reflecting on the jawbones as the gums and covering the hard palate as well as the soft palate which divides the pharynx in oro- and naso- (pharynx)*
Tongue:
Function = aid ingestion of food, carry receptors (gustation), help in formation of food bolus (swallowing), groom fur (cats), assist thermoregulation [saliva cools skin when fur is licked, also panting (dogs)], produce vocalisation
Location = lies on floor of oral cavity, attached at root of tongue to hyoid bone + mandible sides
Structure = made of striated muscle fibres, covered in MM (dorsal surface is thicker + covered with rough papillae, food bolus + grooming, but some become taste buds), underside of tongue - paired sublingual veins + arteries
Teeth:
-Hard structures embedded in upper + lower jaw
-Dogs + cats = brachydontic (fairly low in profile, cease growth at final size)
-Jaws form dental arch/dentary (4 in total)
-Teeth pierce gums, sit in sockets/alveoli
-Gingival/Periodontal membrane covers gums
Basic Structure = pulp cavity (young animals - large, shrivels as the animals develops, changes only occur after this due to wear)
Function = carnivore, shearing + tearing flesh from bones, 4 types of tooth (incisors, canines, premolars and molars)
Dentition = deciduous (milk/temporary teeth, present at birth + erupt as animal grows, smaller + whiter than permanent and they fall out once adult teeth push through) + permanent (adult teeth, larger than milk teeth + shows signs of wear over time)
Incisors - fine nibbling, cutting meat, delicate grooming
Canines - hold prey in mouth
Premolars - shearing + grind flesh
Molars - shearing + grinding
CARNASSIAL TOOTH ONLY FOUND IN CARNIVORES
Dogs = Permanent (I 3/3 C 1/1 P 4/4 M 2/3 x2 = 42 Teeth)
Deciduous (I 3/3 C 1/1 P 3/3 x2 = 28 Teeth)
Cats = Permanent (I 3/3 C 1/1 P 3/2 x2 = 30 Teeth) Deciduous (I 3/3 C 1/1 P 3/2 x2 = 26 Teeth)
Salivary Glands:
-Paired glands lying around the oral cavity
-Secrete saliva, pours into cavity ducts = 99% water, 1% mucus
-No enzymes in salvia of dog + cat
-Zygomatic = close to eyeball, within orbit
-Parotid = base of ear + mandibular glands, palpated
-Sublingual = medial to mandible, under mucosa (tongue underside)
-Mandibular = caudal to angle of jaw, palpated (sometimes mistaken for lymph nodes)
Production = continuous but certain factors may affect production (inc. at sight/smell of food, fear, pain + irritant gases)
Function = lubricate food (mastication + swallowing easier), thermoregulation (panting, licking fur during groming cools blood in capillaries beneath the surface, dec. in core body temperature), omnivores > parotid gland secretes amylase for digestion of carbohydrates
Pharynx
- Crossover point between respiratory and digestive systems
- Muscular tube lined with MM
- Connect back of nasal + oral cavities with oesophagus, larynx + trachea
- Soft palate extends caudally to the epiglottis, dividing the pharynx into the naso- and oro- (pharynx)
- Contains lymphoid diffuse areas (tonsils, protect against disease)
- Eustachian tube connects pharynx to middle ear
- Conveys food from mouth to oesophagus by deglutition/swallowing:
1) Food rolled into bolus by tongue and cheeks, base pf tongue passes it to back of mouth
2) Pharyngeal muscles contract, forcing bolus forward towards oesophagus
3) Epiglottis is closed, prevents food enter larynx
4) Peristalsis pushes food down oesophagus
5) Epiglottis opens once food has passed through the pharynx - respiration starts again
Oesophagus
- Simple tube carrying food from pharynx to stomach
- Dorsal to trachea, passes through the thoracic cavity and runs within the mediastinum
- Enters abdominal cavity via oesophageal hiatus of diaphragm (separates thorax + abdomen)
- Lined with stratified squamous epithelium, prevents damage by food, allowing widthways expansion for the bolus to move
- Walls contain circular + longitudinal smooth muscle bands (contractions = peristaltic waves)
- Food can move backwards (antiperistalsis) as seen when vomiting
Abdominal Cavity
Stomach
- Majority of digestive system lies within
- Divided into 3 parts:
- Stomach > store + mix digested food
- Small intestine > main site for enzymatic digestion + absorption of nutrients
- Large intestine > water absorption site including electrolytes + water-soluble vitamins (indigestible remains pass out anus as faeces)
Stomach:
-Simple digestive system is monogastric
Function = reservoir for food, break up food, mix with gastric juices, begin protein digestion process
Structure = lies to the left side of the cranial abdomen, food enters via cardiac sphincter + leaves via pyloric sphincter, inner sac curve > lesser curvature, outer sac curve > greater curvature, stomach covered in mesentery (layer of visceral peritoneum) like all abdominal cavity organs, lesser curvature (lesser omentum), greater curvature (greater omentum, spleen lies within layers), stomach walls are thick, easily distended (swollen due to inside pressure) > full = occupies 1/3 of abdomen, empty = beneath the ribs
Wall Structure = cardia, fundus, pylorus (3 regions), fundic region (gastric glands, gastric mucosa = MM lining the stomach in longitudinal folds, RUGAE, gastric pits = 3 cell types responsible for secretion of gastric juices, goblet + chief + parietal >
GOBLET all parts of stomach, secrete mucus for food lubrication + protect stomach from digestive enzymes (autodigestion)
CHIEF within fundus, secrete pepsinogen (precursor for pepsin, responsible for breaking down proteins with peptides)
PARIETAL within fundus, secrete HCl to create an acid pH enabling pepsin to work effectively - gastrin is secreted from stomach distension = production of gastric juices
Muscular Movements - peristalsis > push food through stomach, rhythmic segmentation > breaks up + mixes food boluses
Gastric Emptying - food in stomach is broken up + partially digested = soup-like liquid, acidic pH (chyme), chyme released in spurts through the pyloric sphincter into duodenum (digestion continues)
Abdominal Cavity
Small Intestine
- Major site of enzymic digestion + absorption
- Long + narrow tube
- Food mixed with digestive juices during peristalsis + rhythmic segmemntation
- 3 parts:
- Duodenum* U shaped, pancreas with U, fixed position due to mesentery (mesoduodenum) being short, pancreatic + common bile ducts lead here, Brunner’s glands (secrete succus entericus) - leads to jejunum
- Jejunum + Ileum* difficult to distinguish, long tube but not fixed in place as mesojejunum + mesoileum are long, Crypts of Lieberkuhn (digestive glands), ileum ends at ileocaecal junction
- Intestinal Wall Structure = epithelial layer (folded into villi, inc. SA to maximise digestive + absorptive efficiency), microvilli (brush border to further inc. SA), villus contains (blood capillary network - hepatic portal vein take sin the small molecules from carbohydrate/protein breakdown in the liver + lacteal - lymphatic capillary to carry chyle)
Abdominal Cavity
Large Intestine
-Short tube, wide diameter
-No lumen or digestive glands
-More goblet cells > secrete mucus, lubricates passing faeces
>Caecum< short, blind-ending tube, joins ileum and ascending colon at ileocaecal junction
>Colon< ascending, transverse, descending (position in peritoneal cavity), water + electrolytes + vitamins are absorbed
>Rectum< colon in pelvic cavity
>Anal sphincter< end of digestive tract, muscular ring controlling the passage of faeces - internal = smooth muscle, involuntary, external = striated muscle, voluntary - constricted, lined with deep longitudinal folds of MM
Defecation = peristalsis moves faecal matter along as does antiperistalsis, rhythmic segmentation and mass movements (infrequent) = involuntary, pelvic cavity push against rectal wall, abdominal contractions > strain = voluntary
Faecal Composition
-Water + Fibre
-Dead + Living Bacteria
-Mucus
-Sloughed Intestinal Cells
-Anal sac contents
-Stercobilin (pigment from bile = faecal colour)
Pancreas + Gall Bladder
Pancreas:
- Pale pink, lobular gland
- U of duodenum
- Outside digestive tract
- Extrinsic gland
- Endocrine + Exocrine = mixed gland, exocrine > secretes digestive enzymes + bicarbonate into duodenum via pancreatic duct
Gall Bladder:
- Within lobes of liver
- Reservoir for bile (produced by liver)
- Bile > bile salts for fat digestion, stained yellow-green by pigment (bilirubin) > breakdown of old erythrocytes
- Secreted via common bile duct
Digestion
-Food ingested > proteins, carbohydrates + fats
-Mammals use enzymes to breakdown large molecules into smaller ones = easily absorbed
-4 types of digestive juices that source enzymes:
Gastric Juice secreted by gastric pits, response to gastrin, mucus (goblet cells, lubricated food + protects from autodigestion), HCl (parietal cells, denatures proteins, facilitates protein digestion with acidic pH), pepsinogen (chief cells, converts pepsin to peptides by hydrolysis), liquid form = chime
Pancreatic Juice exocrine part, response to secretin + cholecystokinin (duodenum wall) + gastrin, bicarbonate (neutralises acid effects in chyme), digestive enzymes (inactive precursors, prevents autodigestion + pancreas destruction) > 1 = proteases (proteins) trypsinogen (converts to trypsin by enterokinase, present in succus entericus > trypsin (activates other enzyme precursors, acts on peptides) > 2 = lipases (bile salts) > fats to fatty acids + glycerol > 3 = amylases (starches) > convert to maltose
Bile Salts chyme in duodenum, gall bladder contracts = bile, bile emulsifies fat globules, larger SA on which enzymes act + can activate lipases
Intestinal Juice secretion stimulated by secretin, response to chyme passing through pyloric sphincter, produced by Brunner’s glands in duodenum (succus entericus) + Crypts of Lieberkuhn in jejunum + ileum, enzymes present = maltase (maltose>glucose), sucrose (sucrose>glucose + fructose), lactase (lactose>glucose + galactose), enterokinase (trypsinogen>trypsin), aminopeptidase (peptides>amino acids), lipase (fats>fatty acids + glycerol)
Result of digestive process (small molecules) leads to absorption > villi of small intestine, efficiency inc. by: length, SA + blood capillary network, amino acids + simple sugars = liver, fatty acids + glycerol = lacteals - chyle + carried to cisterna chyli
Liver
- Largest body gland (cranial abdomen)
- Cranial aspect = convex, contact with diaphragm
- Caudal aspect = concave, contact with stomach, duodenum + right kidney
- Divided into several lobes > falciform ligament in centre
- Gall bladder inbetween lobes of caudal aspect
- Hepatocytes > responsible for liver function, arranged in lobules surrounded by connective tissue
- Sinusoids > carry blood from hepatic portal vein
- Hepatocytes covered in plasma, drains to central vein, hepatic vein, caudal vena cava
- Arterial blood > hepatic artery
- Bile canaliculi > bile is secreted, forms an interconnecting network (drains into gall bladder)
Function
Carbohydrate Metabolism:
-glucose= source of energy but only used with insulin present (secreted by pancreas)
-excess glucose stored as glycogen in liver but broken down when blood glucose levels are low
Protein Metabolism
-plasma protein formation= albumin (balance of body fluids), fibrinogen + prothrombin (blood clots), globulins (immune mechanism)
-regulation of amino acids= 20 essential AA, liver cells use these to make new proteins, transamination converts non-essential AA into something useful
-urea production= liver converts surplus AA in to NH3 then urea (deamination) + urea is excreted in urine
Fat Metabolism
-fat= source of energy
-liver= converts fatty acids + glycerol into phospholipids (cell membranes) and cholesterol (bile salts)
Bile Formation
-stored in gall bladder
RBC Destruction (old)
-Haemoglobin excreted as bilirubin (bile)
RBC Formation
-liver carries this out in foetus
Vitamin Storage
-fat soluble (ADEK) but some water soluble
Iron Storage
Body temperature Regulation + Heat Production
Detoxification of Substances (Alcohol)
Detoxification + Conjugation (Steroid Hormones)
