Digestive System Flashcards
Name all the parts of the GI tract in order
Oral cavity –> pharynx –> oesophagus –> stomach –> duodenum –> jejunum –> illium –> caecum –> ascending colon –> transverse colon –> descending colon –> sigmoid colon –> rectum
What are the four major functions of the GI system?
Digestion - chemical and mechanical breakdown of food into absorbable units
Absorption - movement of material from GI lumen to ECF
Motiltiy - movement of material through the GI tract as a result of muscle contraction
Secretion - movement of material from cells into lumen or ECF
Describe the genreal structure of the GIT wall
All digestive organs have the same four basic layers:
- Mucosa
- Submucosa
- Muscularis externa
- Serosa
Explain how each of the components of the GIT wall contribtue to the four basic functions of the GIT
Modicfications in the mucosa increase the SA for absorption
Muscularis externa contain 2 different smooth muscle orientations to allow for lumen constriction (circular) and extension/contraction (longitudinal). This allows for digestion and motility via phasic contractions.
Compare and contrast tonic vs phasic contractions and paristalsis vs segmentation and explain how they each contribute to GI motility
Tonic contractions occurs in some smooth muscle sphinters and the anterior portion of the stomach. They are sustained for minutes or hours and function to stop bolus from moving backwards (exception anus)
Phasic contractions occur in the posterior region of the stomach and in the small intestine. They last a few seconds.
Peristalisis and segmentation are both types of phasic contractions. Peristalisis is responsible for forward movements via successive contractions, moving food, while segmentation is responsible for mixing via alternate contractions, moving but also breaking down food
What is meant by the enteric nervous systyem?
The GI tract has its own nervous system, reffered to as the enteric nervous system, also called the gut brain
Digestive activity is provoked by a range of mechanical and chemical stimuli:
- Stretch (distention)
- Changes in osmolarity and pH
- Presence of substrate and end products of digestion
Effectors of digestive activity are smooth muscle and glands
- Upon stimulation, receptors initiate reflexes that stimulate smooth muscle to mix and move lumen contents
Neurons (intrinsic and extrinsic) and hormones control digestive activity
- Both nervous system and hormonal (endocrine) control
Explain the differnece between long and shot reflexes in the GIT
Short reflexes- mediated by enteric nerve plexuses (gut brain); respond to stimuli in GI tract
Long reflexes- respond to stimuli arising inside or outside of the gut, such as from autonomic nervous system
- The parasympathetic system enhances the digestive process
- The sympathetic system inhibits digestion
LIst the internal and external stimuli to the GIT that affect GIT functions via the enteric nervous system
What way do the elements of the upper GIT move in swallowing to pervent food going down the trachea?
Deglutition (the swallowing reflex) makes sure food goes into the oesophagus. Its steps are:
- tongue pushes blous against soft palate and back of mouth, triggering swallowing reflex
- breathing is inhibited as the blous passes the closed airway
- food moves downward into the oesophagus, propelled by perostaltic waves and aided by gravity
List the 3 functions of the stomach
- Storage - regulates passage of food into the intestine
- Digestion - of lipids and protions, by acid and enzymes, regulates by paracrine signal molecules and hormones
- Defence against swallowed pathogens
Describe the six types of secretory cells of the stomach mucosa
Exocrine cells:
- Mucous cells
- Parietal cells
- Chief cells
Endocrine/paracrine cells:
- Enterochromaffin-like (ECL) cells
- D cells
- G cells
Explain the role of mucus and bicarbonate in protecting the gastric mucosa
Mucus (physical barrier) and bicarbonate (chemical barrier) secretion in the stomach by mucus cells protect the stomach wall from being digested
Breakdown of these protective measures can result in inflammation (gastritis) and peptic ulcer as pepsin and HCL erode the stomach wall
The acid-resistant bacterium Helicobacter pylori causes inflammation of the gastric mucosa leading to gastritis and peptic ulcer disease
Explain the difference between the cephalic and gastric phase of digestion
Cephalic phase is anticipatory. Slight, smell, taste and thought of food trigger long reflexes via vagus nerve which serves to prepare the stomach to receive food
Gastric phase is responsive. Presence of food triggers short reflexes via enteric plexus wherin distension of the gut wall and presence of peptides and amino acids in the gastric lumen activate endocrine cells and enteric neurons
Describe how acid is secreted by parietal cells
Parietal cells pump H+ (from carbonic acid breakdown) into stomach lumen via H+/K+ ATPase (proton pumps).
HCl is secreted by parietal cells under stimulatation from 3 chemicals: Ach, gastrin and histamine. All three chemicals need to be present and bind to initate large production of HCl. HCl activates pepsinogen (a zymogen [enzyme precursor]), converting it to pepsin
Acid contributes to nonspecific disease resistance by destroying most ingested pathogens
1-3L secreted per day
Name the major excitatory neurotransmitter in the enteric nervous system
Acetylcholine
Describe the GI hormones
How does bile assist in the digestion and absorption of fats?
Bile is an alkaline solution containing bile salts, bile pigments, cholesterol, neutral fats, phospholipids, and electrolytes. Bile salts emulsify large fat particles into tiny particles which can be attacked by lipiases. The liver produces and secretes 600-1200ml/day
Describe the types of cells within the pancreas
Exocrine cells: chemical digestion
- Acinar cells - clusters of secretory cells in acini whihc contain zymogen granules with digestive enzymes
- Ductal cells (epithelial cells) - produce alkaline secretion (bicarbonate) and transport all secretions
Endocrine cells of pancreatic islets (Islets of Langerhans)
- Secrete insulin and glucagon into bloodstream
Describe the different types of motility in the lower GIT
Chyme is released from the stomach in a controlled manner. Little chemical digestion has occurred so need to avoid overwhelming the SI. Slow passage maximises chances of absorption of nutrients
Both segmental and peristaltic contractions occur in the SI and serve to mix and move chyme
Migrating motor complex (MMC): Contractions that begin in the empty stomach and end in the large intestine; Housekeeping function - serves to ‘sweep out’ foor and bacteria from the upper GIT into the colon
Mass movement: 3-4/day in large intestine; Waves of contraction that send bolus of material forward
Defecation
List the factors which stimulate hunger and cause satiety
Two competing behavioural states: appetite and satiety
Hypothalamus contains two control centres: Feeding centre and Satiety centre
Sensory input from the nervous system also impacts regulation of food intake: Psychological factors and Stress
2 theories:
- Glucostatic theory - glucose metabolism by hypothalamic centres regulates food intake
- Lipostatic theory -signals from fat stores to the brain modulate eating behaviour to maintain a particular weight
Explain the role of leptin and neuropeptide Y in the regulation of food intake
Give an overview of how nutrients are absorbed in the body
Macromolecular nutrients are broken down by enzymes to their component parts prior to absorption
Hydrophilic nutrients are taken up into the blood stream
Lipophilic nutrients are taken up into the lymphatic system
Indigestible fibre is not broken down or absorbed so passed out in faeces
How are monosaccharides and peptides absorbed?
Why and how are digestive enzymes acitivated after release into the duodenum?
