WEEK 10 - DIGESTION & METABOLISM

Question 1

The Digestive Tract = gastrointestinal (GI) tract = alimentary canal

Answer 1

The purpose of digestion is to breakdown food into small molecules so it can be absorbed into the body and used for energy

Question 2

What are the main structures of the GI?

Answer 2

Includes the mouth, pharynx, oesophagus, stomach, small intestine, and large intestine

Question 3

The Digestive Tract (GI) cont.

Answer 3

Continuous ‘tube’ with external environment, i.e. food within the tract is technically outside the body. Food inside the lumen (internal canal) does not enter the body until it is absorbed.

Question 4

What are the accessory organs of the GI?

Answer 4

Accessory organs for digestion include = teeth/tongue, salivary glands, liver, gallbladder, and pancreas

Question 5

Chemical digestion of food (revision)

Answer 5

Macro-nutrients (large molecules) must be chemically digested (with enzymes) into their single unit in order to be absorbed into the blood or lymphatic vessels from the GI tract.
If someone is deficient in an enzyme, like lactase, which breaks down the disaccharide ‘lactose’ (in dairy), they will not be able to absorb it.

“Nutrient” = a molecule from food that is necessary to sustain life (‘nourishment’): Carbohydrates, Proteins, Fats, Vitamins, Minerals and Water

Question 6

Name the 7 major processes involved in digestion

Answer 6

1. INGESTION = taking food into the body (eating). Food forms into a bolus. Swallowing = deglutition (pharynx)
2. PROPULSION = moving food forward. The major type = peristalsis (occurs in oesophagus, stomach, small and large intestines = mass movement)
3. MECHANICAL DIGESTION = physical break down of food into smaller pieces. Begins with chewing (mastication) and continues with churning of the stomach and segmentation in the small intestine
4. CHEMICAL DIGESTION = breaking down food into simpler molecules by enzymes in the mouth/stomach and small intestines
5. SECRETION = digestive enzymes and other substances liquefies food and alters the pH. Hormones impact GI activity.
6. ABSORPTION = movement of simple molecules/nutrients from GI tract to blood/lymphatic vessels.
7. DEFECATION = elimination of undigested food through the anus

Question 7

PERISTALSIS

Answer 7

PUSHES FOOD FORWARDS (primarily propulsive, although some mixing may occur)
Alternating waves of contractions and relaxations of smooth muscle that lines the walls of the oesophagus, stomach and intestines

Question 8

SEGMENTATION

Answer 8

PUSHES FOOD FORWARDS AND BACKWARDS (primarily mixes food and breaks down food mechanically; some propulsion may occur)
Mixes partially digested contents (chyme) with brush border enzymes in the small intestine to complete chemical digestion

Question 9

Regulation of GI Tract Activity

Answer 9

GI tract is controlled by:
Hormones
There are many hormones and peptides released from different structures like the brain (i.e. galanin, peptide Y), fat cells (i.e. leptin), pancreas (insulin and glucagon) and GI tract itself (i.e. CCK, gastrin, secretin) that have an effect on how the GI tract functions.

Autonomic NS

LONG REFLEX
Parasympathetic NS (↑ activity)
Sympathetic NS (↓ activity)

SHORT REFLEX
Enteric NS (ENS) = ‘The gut brain’ (‘enter’ = gut): nerves embedded in the lining of the tract runs the entire length of the tract in ‘plexus’ - branching network of interconnecting nerves. Contains one hundred million neurons (1/1000 of the brain). A stimulus in one part of the tract is processed in the ENS and produces an effect in another part of the tract, with no input from the brain or brain stem

Question 10

The Enteric Nervous System (ENS) = ‘the gut brain’

Answer 10

Stimuli
Stretch = distension, Osmolarity, pH, presence of ‘food chemicals’

Receptors
Mechanoreceptors, Osmoreceptors, Chemoreceptors

Effectors
Smooth muscle - stimulated to ↑ or ↓ contractions
Exocrine glands - stimulated to ↑ or ↓ secretions into the GI tract. i.e. salivary and gastric glands, and glands in gallbladder and pancreas
Endocrine glands - secretes hormones into blood stream

Question 11

What describes the ENS?

Answer 11

mesh-like system of neurons that governs the function of the gastrointestinal system

Question 12

Functions of saliva

Answer 12

Secreted from salivary glands ~1 - 1.5 litres per day
Cleanses mouth, protects against microorganisms and dissolves food chemicals (so we can taste)
Moistens food and helps to form a bolus (‘ball of food’)
Amylase begins carbohydrate digestion

Question 13

OESOPHAGUS

Answer 13

Smooth muscular tube that secretes mucus
Provides passageway of food/fluid between mouth and stomach, which is propelled by gravity and peristalsis.
When food passes through the pharynx (throat), the epiglottis closes over the trachea (windpipe) to prevent food going into the lungs
Separated from stomach by gastroesophageal sphincter, that prevents stomach contents coming back into the oesophagus (‘reflux/heart burn’)

Question 14

STOMACH

Answer 14

Expandable, muscular ‘storage’ bag (minimal absorption takes place here, i.e. water, some drugs/ alcohol)
Layers of muscle allows for mixing of contents - performs peristalsis and churning
Separated from small intestine by pyloric sphincter
Digested stomach contents now called chyme
Gastric pits and glands are located in the stomach lining, which consist of different cells that secrete ‘gastric juice’

PEPSINOGEN + HCL = PEPSIN

Question 15

PEPTIC ULCERS

Answer 15

Food in stomach → stretch and chemo receptors activated → → short reflex → ENS → ↑ stomach secretions and motility

Question 16

SMALL INTESTINE

Answer 16

3 sections: duodenum → jejunum → ileum
Secretes sodium bicarbonate to neutralise stomach acid (HCL)
Surrounded by smooth muscle to promote peristalsis and segmentation
Villi and microvilli line the lumen and increase surface area for absorption
MAIN SITE OF ABSORPTION (95% of water is also absorbed here - when molecules are absorbed into blood/lymph vessels, water follows via osmosis)
Microvilli = ‘brush border’ contains enzymes that complete chemical digestion of proteins and carbohydrates

Question 17

Phases of Gastric Secretion: CEPHALIC PHASE

Answer 17

CEPHALIC PHASE (STIMULATORY):
- Triggered by: taste, sight, smell
- ↑ volume of enzymes, acid production
- LONG REFLEX (PARASYMPATHETIC)
- ↑ GASTRIC SECRETIONS & MOTILITY

Question 18

Phases of Gastric Secretion cont. : GASTRIC PHASE

Answer 18

GASTRIC PHASE (STIMULATORY)”
- Arrival of food in stomach
- Stomach distension activates stretch receptors and food chemicals and ↓ pH activates chemoreceptors
- SHORT REFLEX (ENS)
- ↑ GASTRIC SECRETIONS & MOTILITY

Question 19

Phases of Gastric Secretion cont. : INTESTINAL PHASE

Answer 19

INTESTINAL PHASE (INHIBITOY)
- When chyme enters duodenum, activates stretch and chemoreceptors, inhibitory signals sent back to stomach to slow down gastric secretion and emptying so chyme has time to be fully digested
- SHORT REFLEX (ENS)
- ↓ GASTRIC SECRETIONS & MOTILITY

Question 20

Accessory Organs: Liver, gall bladder and pancreas

Answer 20

Liver produces bile

Gall bladder stores and concentrates bile
BILE salts emulsifies fats (break large fat droplets into smaller particles).

Pancreas secretes pancreatic juice which contains primarily:
Enzymes (like lipase, amylase, trypsin etc.). Bicarbonate
neutralises HCL)

Pancreas also releases hormones insulin and glucagon to control blood glucose levels
The presence of acidic fatty chyme in the duodenum, initiates mechanisms that secrete pancreatic juice and bile to help digestion.

Question 21

LARGE INTESTINE

Answer 21

Different regions including ascending, transverse, descending and sigmoid colons, appendix, rectum and anus
Water absorption
No villi but ‘haustra’ (pouches)
Contains microbiome (trillions of bacteria)

Question 22

LAREGE INTESTINE cont.

Answer 22

Presence of food in stomach stimulates MASS MOVEMENT - powerful waves of contraction which push contents (now called faeces) towards the rectum for elimination = defecation.

If faeces are not evacuated from the body, they are pushed back into the descending colon by reverse peristaltic waves.

↑ fibre or bulk - ↑ strength of contraction.

Question 23

Bacterial Flora = ‘microbiome’

Answer 23

Resident bacteria ferment indigestible CHOs (cellulose), synthesise B vitamins and vitamin K (for blood clotting).

Question 24

Defecation Reflex

Answer 24

1. Faeces move into the rectum, distending it.
   This stimulates stretch receptors which send signals along sensory neurons to the spinal cord. Sensory neurons synapse directly to motor neurons = reflex.
2. Parasympathetic motor neurons send action potentials to:
   the walls of the sigmoid colon and rectum and signal it to contract.
   the internal anal sphincter and signal it to relax.
   Faces are pushed into the anal canal.

If convenient to defecate, you can voluntarily relax your external anal sphincter (skeletal muscle), so faeces may pass.

Question 25

Metabolism

Answer 25

‘Chemical processes that occur within a living organism in order to maintain life’

Question 26

Name and describe 2 types of metabolism

Answer 26

ANABOLISM
Energy-requiring building phase of metabolism in which simple molecules are combined to form more complex substances.

CATABOLISM
Process where complex substances are broken down into simpler molecules.
Think “C” is for ‘cutting’….

Question 27

The Energy Cycle

Answer 27

Stored complex molecules + nutrients (from food) are broken down to simple compounds. This releases heat and ADP uses free phosphate to resynthesise ATP.
When ATP splits off a phosphate group, energy is released which can be used for cellular processes (‘work’), and also to build simple compounds into complex ones for storage.

Question 28

Absorptive State (anabolism)

Answer 28

Nutrients from the digestive tract are absorbed and if not required are stored for later use:
glucose as glycogen in liver and skeletal muscle
amino acids in skeletal muscle
triacylglycerols (TAGs) in adipose tissue

Genesis = ‘new’
Glycogenesis (formation of glycogen)
= glucose → glycogen
Lipogenesis (formation of lipids)
= glycerol and fatty acids → TAGs

Question 29

Post-absorptive state (catabolism)

Answer 29

No nutrients from the digestive tract are being absorbed, so nutrients are broken down from their stored form so they can be used for energy

Lysis = ‘breakdown/split’
Glycogenolysis (breakdown of glycogen stores) = glycogen → glucose
Lipolysis (breakdown of lipids) = TAGs → glycerol and fatty acids
Gluconeogenesis (formation of glucose) = amino acids, lactic acid and glycerol → glucose
NOTE: this process is anabolic in nature! - exception to the rule!

Question 30

What is ABSORPTIVE METABOLISM?

Answer 30

Absorptive metabolism is ANABOLIC

Question 31

What is POST-ABSORBTIVE METABOLISM?

Answer 31

Post-absorptive metabolism is CATABOLIC

Question 32

Glucose Sparing

Answer 32

IMPORTANT: The brain is only able to use glucose (or ketones) to make energy

During the post-absorptive state, peripheral tissues always derive energy from fat, rather than glucose, which is ‘spared’ for the brain.

Essential step is lipolysis (fatty acids are used to make energy). The liver uses fatty acids to make ketones. This prolongs the energy source for many tissues including the brain.

During prolonged periods of starvation, once all the fat is used (= loss of subcutaneous fat), the body begins making glucose from amino acids found in skeletal muscle, ‘gluconeogenesis’ (this leads to severe muscle wastage).
These metabolic adaptations allow the brain a consistent supply of glucose, even during starvation

Question 33

ABSORPTIVE STATE (anabolism):

Answer 33

ABSORPTIVE STATE (anabolism):
When blood glucose rises, beta cells of the pancreas detect this and release insulin.
Insulin acts on target tissues to enhance membrane transport of glucose into cells, or send it to the liver to be converted to glycogen.
This leads to a ↓ in BGC (to normal ‘fasting levels’)

Question 34

POST-ABSORPTIVE STATE (catabolism):

Answer 34

POST-ABSORPTIVE STATE (catabolism):
When blood glucose falls, alpha cells of the pancreas detect this and release glucagon.
Glucagon stimulates the breakdown of glycogen to glucose in the liver and releases into the bloodstream.
This leads to a ↑ in BGC (to prevent it dropping below ‘fasting value’)