Digestive system Flashcards
What are nutrients?
substances that are ingested, digested, absorbed and metabolised to maintain homeostasis through their roles in structure and function within the body
What is anabolism?
the building up or synthesising of large and complex molecules which requires energy
What is catabolism?
the breaking down of substances to provide energy and raw materials for anabolism
What are the two main groups of nutrients?
macronutrients and micronutrients
What are the four macronutrients?
water, carbohydrates, proteins, lipids
What are the three micronutrients?
vitamins, minerals, trace elements
Carbohydrates are composed of which three elements?
carbon (C), oxygen (O), hydrogen (H)
What are the four main categories of carbohydrates?
monosaccharides - easily digested and absorbed in the body (e.g. glucose, galactose and fructose)
disaccharides - consist of 2 monosaccharides so take longer to break down (e.g. sucrose and lactose)
oligosaccharides - consist of 3-9 monosaccharides used to make glycoproteins and glycolipids
polysaccharides - consist of 10+ monosaccharides used as an energy store (e.g. glycogen in liver and muscle cells)
What are the benefits of soluble fibre in the diet?
as it dissolves in water, it ferments and produces gaseous by-products
forms viscous solutions that prolong gastric emptying which prevents the movement of glucose, triglycerides and cholesterol across the intestinal wall into the blood stream
enhances glucose tolerance
adds bulk to faeces to improve intestinal motility
What are the benefits of insoluble fibre in the diet?
adds bulk to faeces and promotes elimination of toxins and waste by stimulating peristalsis
insoluble fibre from cereal and vegetable sources reduces the risk of coronary heart disease and cardiovascular disease
Proteins are composed of which four elements?
carbon (C), oxygen (O), hydrogen (H), nitrogen (N)
Proteins may also contain which other minerals?
sulphur, iron, copper, zinc and phosphate
On average, how many grams of dietary protein do adults require daily?
70-100 g
What are proteins made of?
amino acids
How many amino acids are there in the human body?
20
What is the function of globular amino acids in the body?
to make enzymes and hormones
What is the function of fibrous amino acids in the body?
used in structures (e.g. cell membranes, collagen, and keratin in skin and hair)
What are essential amino acids?
amino acids that cannot be synthesised in the body
What are the essential amino acids?
isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine, histidine (infants)
What are conditionally essential amino acids?
their production can be inhibited by certain pathophysiological conditions
What are the conditionally essential amino acids?
arginine, cysteine (from methionine), glycine, glutamine, proline, tyrosine (from phenylalanine)
What are non-essential amino acids?
amino acids that can be synthesised in the body
What are the non-essential amino acids?
alanine, asparagine, aspartate (aspartic acid), glutamate (glutamic acid), serine
What is the difference between a complete protein and an incomplete protein?
complete protein contains all 20 amino acids
incomplete protein is missing one or more amino acids
Proteins are an important source of which element in the body?
nitrogen
What is nitrogen needed for in the body?
to form non-essential amino acids
Lipids are composed of which three elements?
carbon (C), oxygen (O), hydrogen (H)
Where are fats stored in the body?
in the form of triglycerides in adipocytes in adipose tissue
What are the functions of lipids in the body?
energy source
energy store
source of fat-soluble vitamins
dissolved in lipids for absorption across the intestinal wall into the bloodstream
essential component of the cell membrane and the myelin sheath
What are essential fatty acids?
fatty acids that cannot be synthesised in sufficient amounts in the body
What are saturated fats?
solid at room temperature
no double bonds between carbon atoms
come from animal sources
What are unsaturated fats?
liquid at room temperature
one or more double bonds between carbon atoms
come from plant and vegetable sources
What are the three main types of lipids?
triglycerides, phospholipids, lipoproteins
What is the structure and function of triglycerides?
consist of one glycerol molecule and three fatty acids
provide insulation and energy storage in the body
What is the structure and function of phospholipids?
consist of one glycerol molecule, two fatty acids, and a phosphate group
form the structural component of cell membranes
What is the structure and function of lipoproteins?
triglycerides and cholesterol units combined with proteins and phospholipids
recognised in the body and remain in suspension in the blood
What are the four classifications of lipoproteins?
chylomicrons
high-density lipoproteins (HDLs)
low-density lipoproteins (LDLs)
very-low-density lipoproteins (VLDLs)
What are chylomicrons?
easily absorbed substances created in the small intestine to aid absorption
converted to triglycerides in adipocytes
What are high-density lipoproteins (HDLs)?
collect cholesterol in the blood and bring it to the liver to be eliminated in bile
What are low-density lipoproteins (LDLs)?
largely composed of cholesterol and absorbed by cells when they need cholesterol
cells use enzymes to break down the lipoprotein to release the cholesterol
What are very-low-density lipoproteins (VLDLs)?
precursors to LDLs
their triglycerides are removed in adipocytes and become LDLs
What is considered the optimum HDL level in the blood?
> 1 mmol/L
What is considered the optimum LDL level in the blood?
<3 mmol/L
How do statins work to reduce cholesterol levels?
these drugs inhibit HMG-CoA reductase
this enzyme is responsible for cholesterol formation, and subsequently an increased synthesis of LDL receptors
LDL clearance increases which lowers LDL levels and cholesterol mobilisation
How can cholesterol levels in the blood be lowered naturally?
reduce saturated fat and favour non-saturated fat in the diet
increase soluble fibre in the diet to lower cholesterol absorption
increase oat content in the diet - beta-glucan in oats lower LDLs
choose nuts as a snack - nuts with skins (pecans, walnuts, almonds, peanuts and pistachios) are high in soluble fibre and unsaturated fats
choose natural foods with natural plant stanols and sterols to lower LDLs
exercise and stop smoking to raise HDLs
What are vitamins?
substances required in small quantities and necessary for metabolism
What is unusual about vitamin D?
can be synthesised in the body
considered a hormone as well as a vitamin
What are the two main groups of vitamins?
water-soluble vitamins - readily absorbed into the body and any excess excreted, largely in urine
fat-soluble vitamins - absorption is dependent on fat absorption and they are stored in the body, but in excess they can be harmful
What are the fat-soluble vitamins?
A, D, E, K
What are the water-soluble vitamins?
C, B1, B2, B3, B6, B12, folate
What are the functions of vitamin A (retinol)?
necessary for cell division, and healthy skin and hair
role in low vision in light, promoting GI health (particularly in children), and an optimal immune system
involved in the maintenance of epithelial cellular integrity
What are the functions of vitamin D?
promotes absorption and use of calcium and phosphate for healthy bones
role in controlling the activity of monocytes, macrophages, lymphocytes, and epithelial cells to protect the body from pathogens
influences expression of genes that have roles in preventing neurological and mental health disorders, most notably MS
What is the function of vitamin E?
antioxidant that protects cell membranes
recently identified as neuroprotective, preventing neurotoxicity
What is the function of vitamin K?
required for blood coagulation and synthesis of proteins found in plasma, bone, and the kidneys
What is the function of vitamin C (ascorbic acid)?
antioxidant used in the synthesis of collagen and protects cells from the damage of oxygen free radicals
supports vitamin D in its role in immunity
What are the functions of vitamin B1 (thiamine)?
needed for the release of energy from carbohydrate metabolism
supports neural function, particularly as neurons use carbohydrate as an energy source
What are the functions of vitamin B2 (riboflavin)?
necessary for carbohydrate and protein metabolism
has a role in tissue homeostasis, particularly in the skin and eyes
What are the functions of vitamin B3 (niacin)?
needed for the release of energy from carbohydrate metabolism
inhibits cholesterol production and aids fat breakdown
What are the functions of vitamin B6 (pyridoxine)?
amino acid metabolism and synthesis of non-essential amino acids
protective role in reducing damage caused to cells by oxidative stress
What is the function of vitamin B12 (cobalamin)?
essential in nucleic acid synthesis, and the formation and maintenance of myelin
What is the function of folate (folic acid)?
essential in nucleic acid synthesis and cell division
has a distinct role in neural tube development in the embryo
What are minerals?
chemical substances needed for a variety of reasons in the body, largely to do with structure, fluid balance, nervous and muscular activity and blood clotting
What are the functions of calcium?
essential for neural functioning, including the release and generation of neurotransmitters
essential in bone structure and density
necessary for erythrocyte production
What are the functions of phosphorus?
key role in nervous impulses
essential in bone structure and density
used in balancing the blood pH, assisting in alkalising when necessary
What are the functions of sodium?
essential for muscular contraction and transmission of nervous impulses
helps to maintain fluid balance
What are the functions of potassium?
involved in protein synthesis, intestinal mobility, and generation of nervous impulses
essential for muscular contraction and transmission of nervous impulses
helps to maintain fluid balance
necessary for erythrocyte production
What is the function of magnesium?
necessary for erythrocyte production
What are the functions of iron?
necessary for the transport of oxygen in the blood
needed for nucleic acid synthesis
role in activating enzymes modulating neurotransmitters
What are the functions of iodine?
involved in the synthesis of thyroxine and triiodothyronine (hormones necessary for metabolism, growth and development)
essential for the development and function of the nervous system
What are the functions of zinc?
necessary for cell division, replication and enzyme activity
promotes optimal functioning of immune cells
What are the five key activities of the digestive system?
ingestion, propulsion, digestion, absorption, elimination
What is ingestion?
the taking in of food into the system
What is propulsion?
moving food along the full length of the GIT
What is digestion?
breaking down food into individual nutrients both mechanically and chemically
What is absorption?
once nutrients are broken down to a state where they can be absorbed, they cross the intestinal wall into the blood and lymphatic circulation
What is elimination?
the removal of undigested and unabsorbed food from the DT in the form of faeces
What are the four layers of the tubular structure of the digestive tract?
mucosa (innermost layer), submucosa, muscular, serosa (outermost layer)
What is the structure and function of the mucosa?
involved in secreting enzymes for digestion in parts of the DT and in absorbing nutrients
in the small intestine, absorption is maximised by villi which increase the surface area
when the stomach is empty, the mucosa falls into folds (rugae) which are stretched flat when the stomach is full
mucosa is divided into 3 sublayers: epithelium, lamina propria (connective tissue), and the muscularis mucosae (muscular layer that maintains the folds and enables secretion of enzymes through movement)
What is the structure and function of the submucosa?
structural layer
contains connective tissue with blood and lymphatic vessels and nerves passing through it
What is the structure and function of the muscular?
contains smooth muscle arranged in circular and longitudinal layers
this muscle is essential for mixing the contents and for propulsion within the DT
the two types of movement in the gut are peristalsis and segmentation
peristalsis involves ripple-like waves created by the relaxation and contraction of muscle along the DT
PNS stimulates peristalsis, achieved through the vagus nerve and the enteric nervous system
segmentation mixes contents of the gut to promote digestion
What is the structure and function of the serosa?
fibrous and elastic layer
over the small and large intestine, this layer is the peritoneum and is composed of two serous membranes - the parietal (lining the walls of the abdomen), and visceral (covering the abdominal organs)
this layer is absent over the oral cavity, pharynx, oesophagus and rectum
What is the structure of the mouth?
formed by the hard and soft palates at the roof, the tongue at the floor and the rest covered by the oral mucosa as part of the tubular structure of the digestive system
How does the mouth digest nutrients?
the tongue manipulates food into position for mastication (chewing) to mechanically digest food
this increases the surface area for enzymes to make contact with nutrients to start chemical digestion
the tongue moves the bolus back into the oropharynx for swallowing, has a role in speech and is the organ for taste
What is the function of teeth?
used for biting, chewing (mastication) and grinding food
the upper and lower teeth are moved against each other by the muscular movement of the mandible (jaw)
What are the muscles used in mastication?
masseter, temporalis, medial, and lateral pterygoid muscles
Teeth are composed of which four primary layers?
enamel, dentine, cementum, pulp
What is enamel?
a hard, bone-like structure that protects the tooth
above the gum line it forms the crown, and below it is called cementum
What is dentine?
hard and porous
protected by enamel
makes up the bulk of the tooth
What is cementum?
a hard, bone-like tissue but softer than enamel and dentine
a thin layer covers the root of the tooth and helps to anchor the tooth within bone
What is pulp?
the core layer of the tooth
contains a rich supply of blood and nerves entering from the root of the tooth through a gap called a foramen (root end opening)
the pulp is needed for the growth of a tooth and for sensation
What are the four types of teeth?
incisors, canines, premolars (bicuspids), molars
What are incisors?
the sharpest teeth, for biting into food
What are canines?
located next to incisors
contain large, cone-shaped crowns to hold, grab and tear food
What are premolars?
only present in permanent teeth
contain rounded crowns to hold, crush and grind food
What are molars?
the largest teeth
contain large, wide crowns to grind food
third molars are called ‘wisdom teeth’ because they usually erupt aged 17-25
What are the three pairs of salivary glands in the mouth?
parotid, submandibular, sublingual
What is the function of salivary glands?
to produce and secrete saliva (a slightly acidic liquid) into the mouth through ducts
saliva has a pH of 6.2-7.6 (average 6.7)
What is saliva composed of?
water, enzymes, hormones, antibodies and antimicrobial substances
What are the functions of the components of saliva?
amylase digests starch to maltose
lipase digests fats, activated in the stomach by hydrochloric acid
enables food swallowing and tasting through lubrication
lubricates the tissues of the oral cavity
antibacterial substances (lysozyme and immunoglobulins) maintain tooth integrity and oral hygiene
antibacterial and antiviral protection
What are the three stages of swallowing?
oral - the food bolus is moved onto the oropharynx by the tongue and cheeks
pharyngeal - involuntary contraction of the muscles in the oropharynx moves the bolus into the oesophagus
oesophageal - the bolus is moved by peristalsis, the cardiac sphincter relaxes and food enters the stomach
How is the swallowing process coordinated?
a sensory input travels along five pairs of cranial nerves (V, VII, IX, X and XI), relaying signals to nuclei in the brainstem
these signals are interpreted and generate a motor response through six pairs of cranial nerves (V, VII, IX, X, XI and XII) to the oropharyngeal muscles, contracting them to move the bolus into the oesophagus
the soft palate rises during swallowing and the epiglottis prevents food entering the trachea
What is the structure and function of the oesophagus?
a muscular tube that connects the oropharynx to the stomach
18-25 cm long
lined with mucous membrane to lubricate the passage of food by secretion of mucus
food moves through the oesophagus by peristalsis which is innervated by the vagus nerve
the cardiac sphincter muscle is located at the entrance to the stomach
it permits the bolus to move into the stomach and prevents it from refluxing back into the oesophagus
What is the stomach?
a bean-shaped sac containing food for chemical and mechanical digestion
as the bolus moves into the stomach, the pyloric sphincter (at the exit from the stomach) closes to retain the food for digestion
How does the stomach enable mechanical digestion?
the muscular layer has three strata of muscle compared to two in the rest of the GIT
this allows the stomach to churn the contents, maximising digestion by breaking down the bolus and mixing it with gastric juices
How does the stomach enable chemical digestion?
gastric juices are secreted by the mucosal layer of the stomach
parietal cells produce hydrochloric acid and intrinsic factor (required for absorption of vitamin B12 from the lower part of the small intestine)
chief cells secrete pepsinogen and rennin
What are the components and functions of gastric juice?
water - a liquid medium for chemical reactions and acts as a lubricant
hydrochloric acid - converts pepsinogen to pepsin, lowers pH to inactivate amylase and promote pepsin function, kills ingested pathogens
pepsin (from pepsinogen) - begins digestion of proteins
rennin - digests milk proteins in infants
intrinsic factor - enables absorption of vitamin B12 in the ileum
mucus - protects the mucosal layer from chemical damage by hydrochloric acid and digestive enzymes, lubricates food
The control of secretion of gastric juices occurs in which three stages?
cephalic, gastric, intestinal
What happens in the cephalic phase?
the sight, smell and taste of food are the stimuli for action of the vagus nerve on the stomach
What happens in the gastric phase?
the presence of food in the stomach stimulates production of gastrin (a hormone produced in the pylorus of the stomach and the duodenum)
gastrin passes into the blood stream, circulates back to the stomach and enhances gastric juice secretion until the stomach pH is 1.5
What happens in the intestinal phase?
gastric juice production stops when partially digested food (chyme) moves into the small intestine
the acidic, fatty chyme in the small intestine stimulates release of the hormones secretin and cholecystokinin from the duodenum into the blood stream
when they reach the stomach, they inhibit gastric juice production and reduce gastric contractions to slow stomach emptying
What happens after food enters the stomach?
the proximal stomach relaxes and functions as a reservoir, enabling a large amount of food to enter without pressure
the distal stomach mixes and grinds food by contractions, followed by peristalsis which creates a controlled flow of chyme into the duodenum via the pyloric sphincter
What are the three parts of the small intestine?
duodenum - the first section about 25 cm long
jejunum - the middle section about 2.5 m long
ileum - the last section about 3.5 m long
What is the function of the duodenum?
to receive digestive enzymes from the pancreas and gallbladder (accessory organs for digestion)
gastric juices are neutralised
What is the structure and function of the pancreas?
contains both endocrine and exocrine tissue
islet cells secrete insulin and glucagon
acinar cells secrete up to 1.5 L of pancreatic juice daily
pancreatic juice is secreted into ducts that merge to form the pancreatic duct
this duct eventually merges with the common bile duct and enters the duodenum at the hepatopancreatic ampulla when release of bile and pancreatic juice is controlled by the sphincter of Oddi (a ring of muscle around the opening)
What is the structure and function of the gallbladder?
a small sac beneath the liver
it stores bile after secretion by the liver and before release into the duodenum via the bile duct and hepatopancreatic ampulla
the mucosa of the gallbladder absorbs water and salts from bile which concentrates it
While cholecystokinin decreases gastric juice secretion, it has which three other actions?
it triggers contraction of the gallbladder which releases bile
it triggers secretion of pancreatic juices
it triggers the hepatopancreatic sphincter to relax which releases bile and pancreatic juices into the duodenum
What are the components of pancreatic juice?
trypsinogen, chymotrypsinogen, procarboxypeptidase, pancreatic amylase, pancreatic lipase, ribonuclease, deoxyribonuclease, sodium bicarbonate
What is the function of trypsinogen?
converted to active trypsin by enterokinase (secreted by duodenal wall)
trypsin digests polypeptides into tripeptides, dipeptides and amino acids (proteolysis)
it activates chymotrypsinogen and procarboxypeptidase
What is the function of chymotrypsinogen?
converted to active chymotrypsin by trypsin
chymotrypsin digests polypeptides into tripeptides, dipeptides and amino acids
What is the function of procarboxypeptidase?
converted to active carboxypeptidase by trypsin, carboxypeptidase digests polypeptides into tripeptides, dipeptides and amino acids
What is the function of pancreatic amylase?
digests polysaccharides into disaccharides
What is the function of pancreatic lipase?
breaks triglycerides into two fatty acids and glycerol
What is the function of ribonuclease?
digests RNA into nucleotides
What is the function of deoxyribonuclease?
digests DNA into nucleotides
What is the function of sodium bicarbonate?
neutralises pH of chyme which enables amylase and lipase to work optimally
What are the components of bile?
bilirubin, cholesterol, lecithin and bile acids
What is the function of bilirubin?
converted to urobilinogen by bacteria in the large intestine
it gives faeces its brown colour
What is the function of cholesterol?
secretion of cholesterol in bile enables excretion of cholesterol from the body
What is the function of lecithin and bile acids?
emulsifies fats and cholesterol
How is the small intestine specialised for maximum absorption?
the mucosal wall is structure into villi and microvilli (brush border)
this increases the surface area for absorption and produces enzymes
final digestion occurs at the surface of microvilli through the action of brush border enzymes
villi and microvilli are highly vascular, they contain blood vessels (to carry all nutrients except lipids to the liver) and lacteals (to drain lipids into lymph vessels)
What happens in final carbohydrate digestion?
dextrinase and glucoamylase digests oligosaccharides
maltase digests maltose to glucose
sucrase digests sucrose
lactase digests lactose
carbohydrates are then ready for absorption through the walls of the small intestine
the intestine contains two main groups of probiotic bacteria (Bifidobacterium and Lactobacillus species) which are saccharolytic as they digest carbohydrates anaerobically into short chain fatty acids
What happens in final protein digestion?
carboxypeptidase, aminopeptidase and dipeptidase are brush border enzymes
they complete protein digestion and release amino acids from the beginning, end and middle of peptide chains
What happens in final fat digestion?
lipase digests fats
bile emulsifies fats and breaks them down into smaller globules for lipase to be effective
What happens in final nucleic acid digestion?
nucleic acids (DNA, RNA), now in the form of nucleotides, are digested to phosphate ions, ribose and deoxyribose by nucleosidases and phosphotases
How are carbohydrates absorbed in the small intestine?
monosaccharides are transported across the intestinal mucosal wall and into the ECF by transport proteins
they pass into capillaries by facilitated diffusion or by following water moving by osmosis (solvent drag)
How are amino acids absorbed in the small intestine?
similar to carbohydrate absorption
some amino acids are sodium-dependent in terms of transport
dipeptides and tripeptides may move across the plasma membrane but are broken down to amino acids in mucosal cells before reaching the circulation
How are lipids absorbed in the small intestine?
micelles release lipids (free fatty acids, glycerol, cholesterol and other fatty acids) at the microvilli of the brush border
they move into mucosal cells by diffusion or active transport
SER recreates the lipid components back into triglycerides
Golgi apparatus converts triglycerides to chylomicrons (a lipoprotein created in the small intestine to aid absorption) by adding cholesterol and coating them in phospholipids and proteins
chylomicrons are transported out of the cell and absorbed into lacteals
this collection of chylomicrons is called chyle and drains into lymph vessels
some free fatty acids are directly absorbed into blood capillaries
How are phosphate ions, ribose and deoxyribose absorbed in the small intestine?
they are transported by membrane carriers across the plasma membrane and into blood capillaries
How are vitamins absorbed in the small intestine?
either absorbed with fats (fat-soluble vitamins) or by diffusion (water-soluble vitamins)
B12 binds with the intrinsic factor from gastric juice and is taken up by receptor-mediated endocytosis in the terminal ileum
How are minerals absorbed in the small intestine?
sodium - co-transported with sugars and amino acids
chloride - transported by an ion exchange pump in exchange for bicarbonate ions
potassium - diffusion
iron (ferrous ions) - active transport
calcium - absorbed from the duodenum into epithelial cells by diffusion and pumped out for absorption into the circulation by calcium-ATPase, calbindin (a carrier protein created under the influence of vitamin D) enhances this process
What are the functions of the large intestine?
to absorb water and minerals
to form faeces for excretion
What is the first part of the large intestine called?
caecum
What valve separates the small intestine from the large intestine?
ileocaecal valve
What causes the ileocaecal valve to open?
contraction of the small intestine stimulated by the presence of food in the stomach - gastroileal/gastrocolic reflex
this reflex is stimulated by the SNS
What is the vermiform appendix?
a blind tube attached to the caecum
about 9 cm long
contains a large amount of lymphoid tissue
What are the main components of the large intestine?
caecum, vermiform appendix, ascending colon, hepatic flexure of colon, transverse colon, splenic flexure of colon, descending colon, sigmoid colon, rectum, anus
What is the function of the rectum?
to store faeces until elimination through the anus
What is the internal sphincter?
a thickened layer of muscle that joins the rectum to the anus
it covers the first three quarters of the anal canal
What is the external sphincter?
surrounds the whole of the anal canal
it works with the internal sphincter to regulate defaecation
What is chyme entering the large intestine largely composed of?
cellulose, water and minerals
the colon absorbs water and minerals by osmosis as mucosal epithelial cells have a high sodium concentration
Why is there a large amount of bacteria present in the large intestine?
the small intestine provides an environment that is conducive to bacterial growth
Why do a significant amount of bacteria die in the colon?
there are few available nutrients
What is faeces composed of?
dead bacteria and cellulose
short chain fatty acids (fermented in the colon by probiotic bacteria which contribute to replenishing the colonic mucosa)
What is the structure and function of the colon?
contains small pouches (haustrations) that give it a segmented appearance
this maximises surface area in the colon for absorption of water and minerals
as they fill with chyme, they distend which causes them to contract approx. every 25 mins (haustral churning)
this propels colonic contents from one haustration to another but retains faecal content to maximise water and mineral absorption
high- and low-amplitude contractions move content along the colon towards the rectum and anus
What are high-amplitude peristaltic contractions?
occur 2-14 times daily
achieve mass movement of faecal content
stimulated by colonic and gastric distention, eating, short chain fatty acids and some medications (cholinergic drugs and laxatives)
What are low-amplitude peristaltic contractions?
aka long spike bursts
less frequent and contribute to the urge to defaecate
stimulated by the gastrocolic reflex due to gastric distention
Where is faeces collected and stored ready for defaecation?
rectum
What happens once the rectum is full?
mechanoreceptors sense the distention
they stimulate a reflex contraction of the muscles in the rectum and along the colonic walls
internal sphincter relaxes which allows movement of faeces into the anal canal
the urge to defaecate becomes conscious
What happens if the urge to defaecate is not consciously responded to?
reverse peristalsis occurs and faecal content returns to the colon
more water is absorbed which may contribute to constipation
What happens if a person makes the voluntary choice to defaecate?
intra-rectal pressure increases
anal canal muscles relax
external sphincter opens
pelvic floor relaxes
intra-abdominal pressure increases (due to contraction of abdominal muscles)
faeces are expelled from the digestive tract
What are the four hormones that increase appetite/hunger?
ghrelin, orexins, neuropeptide Y, melanin-concentrating hormone
What is ghrelin?
produced primarily by endocrine cells in the stomach and hypothalamus
stimulates the hypothalamus (opposite effect to leptin) increasing appetite
increases blood glucose and inhibits insulin secretion
highest level before a meal
stimulates neuropeptide Y, potent appetite/hunger stimulators
What are orexins?
released when blood glucose levels fall and cease when food is in the stomach
create a hyperphagic response (making you hungry) in the hypothalamus
What is neuropeptide Y?
produced in the hypothalamus
promotes meal initiation and delays the onset of satiety which causes hyperphagia
What is melanin-concentrating hormone?
produced in the hypothalamus
acts on the limbic system
this increases food intake and causes hyperphagia
What are the five hormones that decrease appetite/hunger?
leptin, cholecystokinin, glucagon-like peptide-1 and peptide YY 3-36, amylin, serotonin
What is leptin?
released by adipocytes when fat storage reaches a particular level
activates leptin receptors in the hypothalamus which causes increased energy expenditure and reduction in appetite
inhibits appetite-stimulating neurons and enhances effects of cholecystokinin
What is cholecystokinin?
released in the intestinal phase of gastric juice secretion
stimulates the brainstem to signal meal termination and early satiety
released about 25 mins after starting a meal and levels do not fall for approx. 3 hrs
What are glucagon-like peptide-1 and peptide YY 3-36?
secreted by distal cells of the small intestine in response to carbohydrate and fat digestion
release insulin, inhibit glucagon, delay gastric emptying, and reduce appetite via vagal nerve stimulation
What is amylin?
released from beta cells when blood glucose levels rise
this circulates in the blood to signal to the brainstem which slows gastric emptying and contributes to the sensation of satiety
inhibits neuropeptide Y
What is serotonin?
stimulated satiety during and after meals by stimulating the hypothalamus
this occurs in response to fat digestion and carbohydrate intake
How does the nervous system increase appetite/hunger?
the sight and smell of food are often the first triggers of appetite
once we start to eat, taste receptors (chemoreceptors) enhance this feeling of hunger by relaying through the cranial nerves to the brainstem
mechanoreceptors detect the stretching of the stomach wall once food enters the stomach
chemoreceptors also detect the presence of nutrients and both of these sensors in the stomach relay this to the CNS via the vagus nerve
How does the nervous system decrease appetite/hunger?
once nutrients are absorbed their levels are detected by chemoreceptors peripherally and in the CNS
mechanoreceptors in the intestinal wall detect distention
chemoreceptors detect nutrients in the intestinal tract
blood glucose levels stimulate the brainstem via the vagus nerve
How is the limbic system involved in appetite regulation?
it processes the reward/pleasure experience of food and how that influences appetite
the taste and pleasure derived from food are considered to be the key motivators of appetite
