Flashcards in Human Digestive System Deck (90):
What are the four components of a tooth?
Dentin + pulp
What is enamel and what is its function?
It consists of crystalline rods/prisms of calcium phosphate and carbonate, but contains no cells
It is the hardest tissue in the body, allowing its longevity, and has no sensation
It's the top layer of teeth and provides protection and a hard surface for chewing.
It's formed when the tooth erupts initially, but can't undergo repair
What is dentin/pulp and its function?
Dentin has a structure similar to bone but contains odontoblasts, not osteoblasts. These are located in the pulp, rather than the dentine.
It is connected to the underlying pulp, a very soft tissue containing blood vessels, nerves and lymphatics. It supplies the tooth and removes waste as well as having nerve endings
What is cementum and its function?
It is a calcified connective tissue covering the root of the tooth, anchoring it to the gums
What is the periodontal ligament and its function?
It consists of collagen fibres.
It links the bone of the socket (alveolar bone) to the cementum
It has a rapid turnover
How does scurvy affect the teeth?
As vitamin C is important for collagen, its deficiency erodes the periodontal ligament, allowing the teeth to fall out and gum disease to occur.
What are the three predominate functions of the tongue?
Manipulation and movement of food to form an easily swallowed bolus
Speech and phonetics
Taste and smell
What are the 3 muscle types making up the tongue and what are their locations?
The longitudinal muscle runs from the front to back of the tongue, and exists in layered sheets
The vertical muscles are between the top and bottom longitudinal muscles, and run from top to bottom of the tongue.
These alternate with columns of transverse muscle, which run from side to side.
What are the three types of papillae and their functions?
Fungiform papillae are oblong and contain tastebuds on their tips and sides
Filiform are ridged and spiky and contain no tastebuds- they are important for keeping the mouth clean
Circumvallate are large with tastebuds on their sides, and there are only 8-12 in the mouth, near the back of the tongue.
How do tastebuds work and how many are there?
There are about 10k tastebuds in the mouth, with a quick turnover.
They have gustatory pores, into which saliva with food particles flows, allowing them to sample it and send information to the brain via an afferent nerve
What tastes can be sensed and where on the tongue?
Salty, sweet, bitter, sour and umami (glutamate).
There are no specific regions responsible to any one taste type
What and where are the 3 salivary gland pairs, and what type of secretion do they produce?
The parotid glands run underneath the ears and produce a serous (watery) secretion
The Sublingual glands lie underneath the tongue and produce a mucous (viscous) secretion
Submandibular glands lie beneath the jawbone (mandible) and produce a mixed solution
What encourages salivary glands to secrete?
Parasympathetic stimulation due to interaction with or thought of food
What is saliva made up of?
Water, mucus and enzymes
What are the enzymes in saliva, and what are their functions?
Salivary amylase breaks down starchy carbohydrates for chemical digestion
Lysozyme is an antibacterial enzyme that keeps the mouth clean
What is the pathway of food (and associated organs) through the digestive system?
Mouth teeth and tongue (salivary glands) --> pharynx --> esophagus --> stomach --> duodenum (pancreas and gallblader) --> jejunum --> ileum --> cecum (appendix) --> ascending colon --> transverse colon --> descending colon --> sigmoid colon --> rectum --> anal canal --> anus
What are the four ways the digestive tract increases its surface area?
Circular inner folds
Fingerlike projections into the lumen
Simple tubular glands (project out of the lumen, fingerlike)
What are the four tunics of the gut tube?
Mucosa (mucus membrane)
Muscularis externa (external smooth muscle)
What does the mucosa consist of and what are their functions?
- Epithelium specialized for absorption, secretion of enzymes/mucus and/or protection
- Lamina propria (loose connective tissue connected to epithelium, carries nerves, blood and lymph for the feeding and absorption from lumen)
- Muscularis mucosae, two layers of smooth muscle: inner circular and outer longitudinal. These provide the mucosa with movement independent from that of the whole tube, allowing secretions to be squeezed etc.
What does the submucosa consist of and what is its function?
It consists of loose connective tissue with larger blood vessels, lymph vessels and nerves called the submucosal plexus.
Its function is to connect the mucosa with the muscularis externa, while allowing some movement independent of one another
What does the muscularis externa consist of and what is its function?
It contains an inner circular layer and an outer longitudinal layer with the myenteric nerve plexus between.
This is important for generating waves of motion for movement of food/fluid, called peristalsis
What does the serosa consist of and what is its function?
It is the slippery outer covering of the gut tube, formed by outer mesothelium on a bed of connective tissue.
It's also known as the visceral peritonium
Where does secretion occur in the digestive tract?
Saliva in mouth
Enzymes in stomach through to mid-SI
Mucus starting in end of SI and through to LI
Where does digestion occur in the digestive tract?
Some mechanical/chemical in mout
Chemical from stomach to end of SI
Where does absorption occur in the digestive tract?
Some in stomach
Nutrients absorbed through SI
H2O absorbed in LI
Where does transport occur in the digestive tract?
From the back of the mouth all the way through
NB the stomach has a secondary transport function (mixing)
What are the 4 main functions of the digestive system?
What is the esophagus?
A muscular tube, 25cm long, from pharynx to stomach and posterior to the trachea
Normally collapsed when empty, and expands to accommodate food and water
What does the esophagus do?
Transports food to stomach (5s) and water to stomach (1s)
Protects surrounding structures from thick or sharp boluses of food
Does NOT absorb or digest, but secretes some mucus close to the stomach
What is unusual about the epithelium of the mucus membrane of the esophagus?
It is thick with many layers of stratified squamous epithelia.
The outer layers are for protection, and are sacrificial
The cells divide in the basal layers and slowly migrate outward, with old cells sloughed off.
It takes about 7 days to renew the esophagus
What is unusual about the external muscle of the esophagus?
There is some skeletal muscle in the upper third of the esophagus, for voluntary control of swallowing
What is the function of the inner/outer layer of muscle in the esophagus?
Inner circular layer encircles the tract, allowing boluses to be squeezed from above and below for peristalsis
Outer longitudinal layer allows the esophagus to shorten during swallowing
What is unusual about the serosa of the esophagus?
It doesn't have one as it doesn't lie within a body cavity giving it no need of a serous membrane as it doesn't move around. Instead, it has a fibrous adventitial, attaching it to neighbouring organs
What is the stomach?
J-shaped bag with a capacity of approx. 1.5L
Muscled sphincters at inlet (cardiac) and outlet (pyloric)
Lined with longitudinal folds called rugae when empty
What are the functions of the stomach?
Secretion of HCl, enzymes and mucus to make chyme (food, acid and enzymes)
Digestion of proteins using pepsin
Absorption of water, ions and some drugs
Transport (especially mixing via churning waves every 20s)
Protection from its own secretions and microbes (by dissolving them in acid)
What are the four parts of the stomach?
Cardia (area around the cardiac sphincter)
Fundus (superior bulge)
Body (majority of the stomach)
Pylorus ('flattish' bit before the pyloric sphincter)
Which parts of the stomach secrete what?
The cardia and pylorus secrete mucus
The fundus and body secrete mucus, acid and enzymes
What is unusual about the epithelium (mucus membrane) of the stomach?
It forms many pits that are lined with mucus secreting cells, as well as gastric glands opening into the pits
What is unusual about the external muscle of the stomach?
Instead of two layers, there are three- the innermost layer contains an oblique layer of muscle
How is pepsin secreted and why?
It is secreted in its inactive form, to prevent the digestion of the organ walls.
It's activated by the acidic conditions in the stomach
What are the two parts of the indentations in the stomach wall?
The gastric pit and the gastric gland (beneath)
What are the 6 cell types within the gastric pits and glands?
Surface mucous cells
Mucous neck cells
Where are the surface mucous cells and where are they?
They are located within the gastric pit, and secrete and insoluble alkaline mucus, to protect the mucosa from acidic conditions and peptin, as well as keeping chyme poist
What are undifferentiated cells and where are they?
They are located between the gastric pit and the gastric glands, and divide to generate new epithelium, moving to either the gland or the pit depending on requirements
What are parietal cells and where are they?
They secrete HCl to kill microbes and cells
Secretes intrinsic factor, crucial for the absorption of B12 in the small intestine
What are mucous neck cells and where are they?
They are in the middle of the gastric gland processes, and secrete soluble acidic mucus when eating occurs
What are chief cells and where are they?
They are located within the bottom of the gastric glands, secreting pepsinogen and gastric lipase to begin the digestion of fats.
(Pepsinogen is converted to pepsin by acidic conditions in the gland lumen)
What are gastrin cells and where are they?
They secrete gastrin into the blood surrounding the stomach as a hormone, to stimulate the secretion of acid and pepsinogen, increase contractions of the stomach and relax the pyloric sphinceter
It is located in the bottom of the gastric glands.
How is gastrin regulated?
The cells are stimulated by stretch after the stomach becomes full, and has a negative feedback mechanism with pH- at levels below 3, they are inhibited
What is the epithelial shape within the stomach?
What is the liver?
A very large gland made up of epitheliacl cells called hepatocytes
What do hepatocytes do?
Glycogen/glucose, vitamin and mineral storage, recycling of RBCs, bile synthesis and secretion, synthesis of albumins, globulins and proteins for blood plasma and removal of drugs, metabolites and poisons from the blood
What do hepatocytes require?
Access to nutrient-laden blood from the intestinal wall
Access to oxygenated blood from the systemic circuit
Access to ducts draining the bile to the gall bladder
How are hepatocytes arranged together?
If they're like bricks, they're held together with 'mortar', creating little dents between them in which the bile ducts lie.
Outside of the columns of hepatocytes, the blood and lymph vessels surround them, allowing diffusion in and out.
How are hepatocytes arranged?
They have sinusoid (largely fenestrated) blood vessels surrounding them, with the lymph space of Disse betwen them and the vessels.
Between tight junctions holding hepatocytes together are the bile canalliculi
Lymph flows into the hepatocytes, where the components needed are removed
How are liver lobules arranged?
They are hexagonal, each 2mm long and 1mm wide.
They are made of radiating vertical 'plates' of hepatocytes surrounding the central veins in the middle
At each corner, they have 'portal triads'
What is a portal triad?
3 vessels groups on the corner of a liver lobule: a branch of the hepatic portal vein, a bile canaliculus and a hepatic artery
What substances enter the liver lobules and through what vessels?
O2 blood enters via the hepatic arteries
Nutrient-laden blood enters via the hepatic portal vein
What substances leave the liver lobules and through what vessels?
Outgoing blood leaves via the central vein
Outgoing bile leaves via the bile canaliculi, which fule into the bile duct
What separates lobes of the liver?
The interlobular septum
What and where is the pancreas?
It is a gland with both endocrine and exocrine functions, located behind the stomach
What is the exocrine function of the pancreas?
It manufactures inactive digestive enzymes in an alkaline solution (due to bicarbonates). This is called pancreatic juice.
Once the enzymes reach the duodenum, they are converted to their active form, and digest proteins, carbohydrates, lipids and nucleic acids.
What is the endocrine function of the pancreas?
Islets of langerhans (1% of pancreas!) secretes insulin and glucagon into the bloodstream to help regulate blood-glucose levels
What is the structure of the pancreas from inside areas outward?
The main pancreatic duct forms the 'trunk' of the pancreas. Interlobular ducts bud from these, followed by the intercalated ducts
The intercalated ducts have many acini budding from them, which contain the secretory cells of the pancreas (serous type)
What is the small intestine?
A tube about 3cm in diameter, a little over 3m long
What are the three parts of the small intestine and their respective functions?
Duodenum: 25cm long. Receives bile from the liver/gallbladder and pancreatic juices from the pancreas
Jejunum: 1m long. Primary area of chemical digestion and nutrient absorption
Ileum: 2m long. Does some digestion and absorption
These enter the large intestine via the ileocecal valve
What is unique about the mucosa and submucosa of the small intestine?
The mucosa contains villi to increase SA
The submucosa just downstream of the pyloric sphincter contains mucus glands (glands of brunner)
How does the SI increase its surface area? 4 ways
1. The SI is heavily convoluted
2. The submucosa is arranged in plicae, or circular ridges, with a covering of mucosa
3. The mucosa on the plicae is arranged in villi, with a core of lamina propria and a covering of epithelium
4. The epithelium of the villi are covered with microvilli, to form a brush border. These have a core of cytoplasm and a covering of cell membrane.
What are the 7 different components of a villus?
What do columnar enterocytes do in the villus?
They absorb the small molecules released by digestion
(also called absorptive cells). Replaced every 2-3 days
What do goblet cells do in the villus?
What do the enteroendocrine cells do in the villus?
They secrete secretin, gastrin and CCK into the capillaries of the lamina propria
They are released in response to the arrival of acidic chyme.
Secretin promotes release of pancreatic juice, as does CCK
Gastrin stimulates the production of secretions from gastric glands in the stomach
What do the undifferentiated cells do in the villus?
They divide to generate new epithelium, either moving up or down the villus as needed
What do paneth cells do in the villus?
They secrete batericidal enzymes including lysozyme, and are phagocytic, removing any foreign bodies surviving the acidity of the stomach
What do blood vessels do in the villus?
Transport charbohydrates (monosaccharides), proteins (amino acids), water and electrolytes to the liver; the venules merge to give rise to the hepatic portal vein
Also transport of hormones from enteroendocrine cells
What do lacteals do in the villus?
They absorb the majority of digested fats, which are then returned to the venous system.
How does the epithelium of the villus renew itself?
Cell division occurs in the undifferentiated cells, forcing the whole sheet to move upwards except for the paneth cells.
Old cells are shed from the tip, and the whole journey lasts about 5 days
How is lymph fluid transported out of the villus?
It is 'milked' out by contraction of the muscularis mucosae in the mucous membrane.
What are the 2 parts of a villus and how do you differentiate them?
There is the 'villus' and the crypt
The crypt begins once the brush border of microvilli ends.
What are the 7 parts of the large intestine?
What are the functions of the large intestine?
Absorption of salt and water (but only 1 L, SI absorbs 8)
Conversion of chyme to feces
Fermentation of carbohydrates by bacteria
Productions of vit. K and B, to be absorbed
Secretion of Mucus to lubricate feces
Storage of feces
Which valve controls the flow of chyme into the cecum?
What is the cecum?
A pouch containing bacteria for the digestion of fibre.
This also produces fatty acids, which are important for the protection of LI cells.
There are no enzymes here!
What is the composition of feces?
30% bacteria, 30% undigested fibre, cells shed from intestinal lining and mucus
What is unusual about the mucosa of the large intestine/
No villi, but many intestinal glands (crypts of lieberkuhn)
Surface epithelial cells are enterocytes
Contain goblet cells but no paneth cells
Entire epithelium replaced every 4-5 days
Contains many clusters of lymphocytes (lymphoid nodules) to prevent escape of digestive bacteria
What is unusual about the external musculosa of the large intestine?
The outer longitudinal layer forms three thick strips (teniae coli) that contract to pull the tube into pouches called haustra coli.
What is the structure of the rectum/anus?
Rectum makes up 20cm of tube
Anal canal makes up 2cm
Rectum-anal canal closed by smooth muscle sphincter
Anal-external opening controlled by voluntary anal sphincter
How does the urge to defecate develop?
Commenced when the rectum fills to 25% of capacity.
This initiates reflex contraction of teniae coli, putting more pressure on the rectum and opening the internal anal sphincter
How does the urge to defecate get controlled?
If the external anal sphincter is not relaxed, the reflex subsides