16.1 General Aspects of digestion Flashcards
How does the digestive system contribute to homeostasis (the primary function of it)?
By transferring nutrients, water, and electrolytes from the external environment to the internal environment, not regulating their concentrations but rather optimizing conditions for digestion and absorption.
How long is the GIT?
4,5 m long
What is the other name of the digestive system
Gastrointestinal tract (GIT)
The GIT starts from the mouth and ends with?
the anal sphincter
What are the four basic processes that the digestive system performs?
MSDA
1. Motility
2.Secretion
3.Digestion
4.Absorption
What is motility?
The muscular contraction that mix and move forward the contents within the GIT
The smooth muscle in the walls of the GIT is?
Phasic meaning it displays action potential-induced bursts of contraction and it also maintains tone
what is tone?
Maintenance of a constant low level of contraction. In order to maintain a steady pressure of the contents of GIT and preventing permanent stretching following distension.
What are the two basic types of phasic digestive motility ?
Propulsive movements and Mixing movements
What do propulsive movements do?
Propel or push the contents forward through the GIT
what are the two functions of mixing movements?
- Mixing food ingested with the digestivie juices, thus promoting digestion
- Facilitate absorption by exposing all the parts of the intestinal contents to the absorbing surfaces of the GIT
Why is it that for the ends of the GIT (Mouth and External anal sphincter) smooth muscle contraction doesn’t occur?
This is because the ends are controlled by skeletal muscle which is primarily voluntary, think about chewing, swallowing and defecation. Motility by smooth muscle is accomplisehd by involuntary mechanisms think about peristalsis.
What are digestive exocrine gland cells?
They are specialised epithelial cells found in the lining of the digestive organs as well as that of accessory digestive organs. They secrete digestive juices into the digestive tract in response to neural and hormonal cues.
What are the two types of secretions of the digestive system?
Exocrine and endocrine
give me the 4 steps of the digestive secretion process
- Secretory cells extract water and raw materials from plasma
- Energy is required for: Active transport for some materials to enter the cells, and synthesis of secretory products
- Digestive juices are normally reabsorbed into the blood after digestion
- Failure to reabsorb (due to diarrhea, vomitting) results in fluid loss.
The endocrine tissue of the GIT is organised as?
single, individual cells scattered throughout the entire GIT
What are the two types of signal proteins that the endocrine tissue produce?
GI hormones
GI peptides
What are the three primary categories of energy-rich foodstuffs consumed by humans?
Carbohydrates
proteins
fats
Macronutrients vs Micronutrients in terms of energy
Macronutrients provide energy for the body while micronutrients assist as co-factors for providing energy (but they cannot provide energy themselves).
What is the purpose of digestion in terms of foodstuffs
To chemically break down the structurally complex foodstuffs of the diet into smaller, absorbable units.
What are the carbohydrate monosaccharides(simple sugars)? AKA soluble units of carbohydrates
Glucose, fructose and galactose (very rare in diet)
Carbohydrate Polysaccharides consist of……..
interconnected glucose chains
What are the 3 main types of carbohydrate polysaccharides, name their components and explain them briefly
- Starch from plants:
Amylose- Unbranched glucose chain
Amylopectin- Branched glucose chain - Glycogen (from meat): Highly branched glucose storage form in muscle
- Indigestible fiber(from plant walls):
Insoluble fiber like cellulose
Soluble fiber like pectin
What are the carbohydrate disaccharides?
galactose + glucose = lactose
fructose + glucose = Sucrose
glucose + glucose = maltose
(less common in diet)
Dietary proteins consist of various combinations of……
Amino acids held together by peptide bonds
Through the process of digestion, Proteins are broken down into their absorbable units being?
amino acids and small peptides
Most dietary fats are in the form of?
Triglycerides (neutral fats)
What is a triglyceride made up of?
A glycerol and 3 fatty acids attached
What are the absorbable units of fats after digestion has broken down triglycerides?
monoglycerides and free fatty acids ( 2 from each triglyceride molecule)
What process ensures that digestion of all foodstuffs is accomplished
Enzymatic hydrolysis
Where does most digestion occur
Stomach
Where does most absorption occur
Small intestine
What is the purpose of absorption in terms of the foodstuffs
To ensure that the absorbable units of the foodstuff after digestion along with vitamins, water and electrolytes are absorbed from the digestive tract lumen and into the blood or lymph
What are the three exocrine digestive accessory organs?
- salivary glands
- biliary system (liver and gallballder)
- Exocrine Pancreas
the digestive tract includes?
Mouth
Pharynx
Esophagus
Stomach
small intestine (duodenum, jejunum, ileum)
Large intestine (cecum, appendix, colon, rectum)
anus
Only after ____________ are substances viewed as part of the body. Before then they are foreign.
Absorption
What are some of the characteristics of the GIT that are beneficial for digestion but would be harmful in any other part of the body? (4)
- The stomach’s pH being 2 due to HCl
- Enzymes that hydrolyse proteins remain inactive until needed for digestion
- The millions of microorganisms
- foodstuffs are complex foreign particles that would be attacked by the immune system elsewhere.
What are the four layers of the digestive tract wall (inner to outer)
- Mucosa
- Submucosa
- Muscularis Externa
- Serosa
What is the mucosa?
It is the layer that lines the luminal surface of the digestive tract.
what are the three layers of the mucosa? (inner to outer)
- Mucous membrane (primary component)
- Lamina propria
- Muscularis mucosa
What is the mucous membrane and what does it do?
It is the inner epithelial layer of the mucosa that serves as a protection surface. In some regions of the GIT, it is modified for secretion and absorption (meaning it has exocrine gland cells for the secretion of digestive juices, endocrine gland cells for secreting GI hormones, and specialised epithelial cells for absorbing digested nutrients)
What is the lamina propria
It is the thin middle layer of the mucosa made up of connective tissue on which the epithelium rests. It houses the gut-associated lymphoid tissue (GALT)
What is the function of GALT?
Gut-associated lymphoid tissue defends the GIT against disease-causing intestinal bacteria.
What is the muscularis mucosa?
It is the sparse outermost layer of the mucosa made of smooth muscle
The small intestine has a highly folded………….to maximise…….
Mucosa, Absorption
What is the submucosa
It is a thick layer of connective tissue that provides the digestive tract with its distensibility.
What is the submucosal plexus
a nerve network that lies within the submucosa
What is the muscularis externa?
The major smooth muscle coat of the GIT surrounding the submucosa.
What two layers does the muscularis externa have in certain regions of the GIT?
- Inner circular layer- Adjacent to the submucosa
- Outer longitudinal layer
What do the contractions of the inner circular layer fibers of the muscularis externa do?
they decrease the diameter of the lumen, constricting it
What do the contractions of the outer longitudinal layer fibers of the muscularis externa do?
Shorten the tube
The contractile activity of the two smooth muscle layers of the muscularis externa……..
bring about propulsive and mixing movements
What nerve network is situated in between the two smooth muscle layers of the muscularis externa
myenteric plexus
What is the collective function of the submucosal plexus, myenteric plexus, GI hormones and local chemical mediators
They help regulate local gut activity
What is the serosa?
It is the outer connective tissue covering the digestive tract and secretes serous fluid
What does the serous fluid (watery slippery fluid) do?
it lubricates and prevents friction between the digestive organs and surrounding viscera.
What suspends the digestive organs from the inner wall of the abdominal cavity like a sling to keep them in the right fixed position?
the mesentery
What are the 4 factors that regulate the digestive system function? (motility and secretion are carefully regulated in order to maximise digestion and absorption)
- Autonomous smooth muscle function
- Intrinsic nerve plexuses
- Extrinsic nerves
- GI hormones
What is the main electrical activity of the GIT smooth muscle
Basic electrical rhythm (BER) / slow-wave potentials
What determines the frequency of the BER
Pacemaker cells (interstitial cells of Cajal)
What are interstitial cells of Cajal
They act as pacemaker cells located in the muscularis externa. They generate slow-wave potentials that propagate to adjust smooth muacle cells via gap junctions
true or false: slow-wave potentials are action potentials and directly cause muscle contraction
false
What are slow-wave potentials
They are rhythmic fluctuations in membrane potential that bring it closer or farther from threshold.
If threshold is reached, it triggers action potentials that cause for muscle contraction.
Factors that influence contractile activity
- hormonal, neural, mchanical factors affecting starting point of slow-wave oscillations
-absence/presence of food
Contractile activity rate and intensity depends on:
- rate (frequency) depends on pacemaker cells
- intensity (strength) depends on the number of action potentials when slow-wave potential reaches threshold
What is the role of calcium when slow-wave potentials reach threshold
the Ca2+ voltage gate channels open causing for an influx of calcium. The influx is responsible for action potential rising phase and triggers contractile response
high cystosolic Ca2+ concentration=
sttronger more intense contractions
What is the name of the unique nervous system of the digestive system ?
Enteric nervous system(ENS)
What are the two nerve networks that make up the ENS
submucosal plexus and myenteric plexus
What are thr three types of neurons of the enteric nervous system
- Intrinsic primary afferent neurons (sensory neurons)
- Intrinsic efferent neurons
- Interneurons
What is the function of the sensory neurons
reaspond to specific local stimuli in the GIT
what is the function of the intrinsic efferent neurons
control smooth muscle and exocrine and endocrine cells
What is the function of interneurons
they modulate the communication between sensory neurons and efferent neurons
The intrinsic nerve plexuses influence all aspects of ………….
GIT activity
What are the extrinsic nerves
They are the nerve fibers from both branches of the autonomic nervous system that originate outside the GIT and regulate GIT function
What is the function of the extrinsic (autonomic) nerves ?
They influence GIT motility and secretion, altering GI hormones secretion, or acting directly on the smooth muscle and glands
Distinguish between the antagonistic roles of sympathetic and parasympathetic nerves in digestion
1.The sympathetic nerves are dominant during “fight or flight” situations. They supress or slow down GIT secretion and motility because these activities are not the highest priority when the body faces an emergency.
2. The parasympathetic nerves are dominant during “rest and digest” when the general maintenance type of activities such as digedtion function optimally. Supplied to the GIT via the vagus nerve, they increase motility and secretion of digestive elements.
Where are GI hormones produced and what is their function
They are produced by specilaised endocrine cells within the mucosa of certain regions of the GIT. They exert either excitatory or inhibitory influences on digestive smooth muscle and exocrine gland cells
Receptor activation alters digestive activity through ____________
Neural and hormonal pathways thus modifying motility, secretion of digestive juices and GI hormone
What are the three types of sensory receptors of the digestive tract wall that respond to local changes in the GIT
- Mechanoreceptors (stretch receptors)- sensitive ot stretch or tension within the wall
- Chemoreceptors- sensitive to chemical components within the lumen
- Osmoreceptors- sensitive to the osmolarity of the luminal contents
What are the two types of neural reflexes that receptor activation may bring out
short reflexes and long reflexes
Explain what short reflexes
They take place when all elements of the of the reflex are located within the wall of the GIT itself (when the intrinsic nerve networks influence local motility or secretion in response to specific local stimulation)
Explain long reflexes
The autonomic reflexes involve long pathways between the CNS and the digestive system.
