Transcript Unit 3-7 Digestive System Notes File

Depth and Rate of Breathing: PCO2
Changing PCO2 levels are monitored by chemoreceptors of the brain stem
Carbon dioxide in the blood diffuses into the cerebrospinal fluid where it is hydrated
Resulting carbonic acid dissociates, releasing hydrogen ions
PCO2 levels rise (hypercapnia) resulting in increased depth and rate of breathing
•Hyperventilation – increased depth and rate of breathing that:
Quickly flushes carbon dioxide from the blood
Though a rise CO2 acts as the original stimulus, control of breathing at rest is
regulated by the hydrogen ion concentration in the brain
•Hypoventilation – slow and shallow breathing due to abnormally low PCO2 levels
Apnea (breathing cessation) may occur until PCO2 levels rise
Arterial oxygen levels are monitored by the aortic and carotid bodies
Substantial drops in arterial PO2 (to 60 mm Hg) are needed before oxygen levels become a major
stimulus for increased ventilation
If carbon dioxide is not removed (e.g., as in emphysema and chronic bronchitis), chemoreceptors
become unresponsive to PCO2 chemical stimuli
In such cases, PO2 levels become the principal respiratory stimulus (hypoxic drive)
Depth and Rate of Breathing: Arterial pH
Changes in arterial pH can modify respiratory rate even if CO2 and O2 levels are normal
Increased ventilation in response to falling pH is mediated by peripheral chemoreceptors
Respiratory Adjustments: Exercise
Respiratory adjustments are geared to both the intensity and duration of exercise
During vigorous exercise:
Ventilation can increase 20 fold
Breathing becomes deeper and more vigorous, but respiratory rate may not be significantly
changed (hyperpnea)
Exercise-enhanced breathing is not prompted by an increase in PCO2 or a decrease in PO2 or pH.
These levels remain surprisingly constant during exercise
As exercise begins:
Ventilation increases abruptly, rises slowly, and reaches a steady state
When exercise stops:
Ventilation declines suddenly, then gradually decreases to normal
Neural factors bring about the above changes, including:
Psychic stimuli
Cortical motor activation
Excitatory impulses from proprioceptors (relationship receptors) in muscles
Respiratory Adjustments: High Altitude
The body responds to quick movement to high altitude (above 8000 ft) with symptoms of acute
mountain sickness – headache, shortness of breath, nausea, and dizziness
Acclimatization – respiratory and hematopoietic adjustments to altitude include:
Increased ventilation – 2-3 L/min higher than at sea level
Chemoreceptors become more responsive to PCO2
Substantial decline in PO2 stimulates peripheral chemoreceptors
Digestive System
Where enzymes are used to break down:
– Protein
– Carbohydrates
– Triglycerides
– Nucleic Acids
Stages of Food Processing
Digestive systems of vertebrates are variations on a common plan
1. Ingestion / Propulsion: eating
2. Digestion: breaking down food to smaller molecules {so they can get into the cell}
• Mechanical digestion= chewing
• Chemical digestion= digestive enzymes
3. Absorption: uptake of small nutrient molecules by the cells lining digestive tract
4. Egestion / Defecation: elimination of undigested material we eat
Types of Digestive Systems / Digestive compartments
Animals must have a digestive compartment to
avoid digesting themselves!
1. Intracellular / Food Vacuole: simplest
• Intracellular
• Organelle filled with digestive enzymes
• Once food is broken down it passes through
the membrane to nourish the cell
• Unicellular organisms
2. Gastrovascular Cavity:
• Single opening that functions both as an
entrance for food & an exit for undigested
wastes
• Enzymes are secreted into cavity
• All Cells are in proximity to absorb nutrients
• Ex: sponges, hydras, jellies
3. Complete Digestive Systems:
complete alimentary canal
aka “Alimentary Canal” or “gut”
oral opening  passes through a tube  anal opening
• Food enters through a mouth
• Moves through special regions that digest &
absorb nutrients
- Has several accessory organs
• Undigested wastes are eliminated from tube as
feces via the anus
Examples:
• Vertebrates: such as humans, birds
• Invertebrates: earthworms, grasshoppers
EXTRACELLULAR! But INTERNAL
4. External Digestion
Assassin Bug for example:
• Inject powerful digestive enzymes into their pray
• This liquefies the insides of the pray
• The insides are sucked out by the assassin bug!
Human Digestive System
“complete digestive system”
Has several accessory organs:
– Salivary gland: secretes digestive enzymes via ducts into mouth
– Stomach: enzymes & HCl
– Pancreas: digestive enzymes for all 3 types of foods- into small intestines
– Liver: secretes bile to breakdown fats- into small intestines
Digestive System: Overview
Alimentary canal / gastrointestinal (GI) tract
• digests and absorbs food
Alimentary canal
mouth
pharynx
esophagus
stomach
small intestine
large intestine
Accessory digestive organs
Teeth
Tongue
Gallbladder
salivary glands
Liver
pancreas
Segmentation
Digestive Processes in the Mouth
Food is ingested
Mastication - Mechanical digestion (chewing)
Propulsion is initiated by swallowing
Salivary amylase
-chemical breakdown of starch
Saliva: Source and Composition
Three pairs of extrinsic glands –
Sublingual
parotid (cheeks)
gland
submandibular (under jaw)
sublingual (under tongue)
Intrinsic salivary glands (buccal glands)
scattered throughout the oral mucosa (epithelium)
97-99.5% water, hypo-osmotic, slightly acidic solution containing
Electrolytes – Na+, K+, Cl–, PO42–, HCO3–
Digestive enzyme – salivary amylase & lingual lipase
Proteins – Lubrication –mucin, anti-microbial - lysozyme, defensins, and IgA
Metabolic wastes – urea and uric acid
Deglutition (Swallowing)
Peristalsis moves food through the pharynx 
esophagus  stomach
Peristalisis: sequential contraction of smooth muscle to
propel substances
Stomach
Esophageal Sphincter to Pyloric Sphincter
• Holds ingested food
• Degrades this food both physically and
chemically - food is converted to chyme
Pepsin Enzymatically digests proteins
• Secretes intrinsic factor required for
absorption of vitamin B12
• Delivers chyme to the small intestine
Parotid
gland
Submandibular
gland
Gastric Contractile Activity
• Most vigorous near the pylorus
• Chyme is either:
- Delivered in ~ 3 ml spurts to the duodenum, or
- Forced backward into the stomach
Pyloric
valve
closed
Pyloric
valve
closed
Pyloric
valve
slightly
opened
Musculature –additional oblique (outside) layer
Allows the stomach to churn, mix, and pummel food physically
Breaks down food into smaller fragments
Chemical digestion in the stomach
Epithelial lining is composed of:
1. Goblet cells that produce a coat of alkaline (basic) mucus
- The mucous surface layer traps a bicarbonate-rich fluid beneath it
2. Gastric glands
• Parietal cells secrete HCl and
intrinsic factor
• Chief cells – produce pepsinogen
Pepsinogen is activated to
pepsin (digesting enzyme) by:
- HCl in the stomach
- Pepsin itself via a positive
feedback mechanism
3. Enteroendocrine cells – secrete gastrin, histamine, endorphins, serotonin, cholecystokinin (CCK), and
somatostatin into the lamina propria
Stomach Lining
The stomach is exposed to the harshest conditions in the digestive tract
To keep from digesting itself, the stomach has a mucosal barrier with:
A thick coat of bicarbonate-rich mucus on the stomach wall
Epithelial cells that are joined by tight junctions
Gastric glands that have cells impermeable to HCl
Damaged epithelial cells are quickly replaced
Small Intestine: Gross Anatomy
Runs from pyloric sphincter (stomach to intestine) to the ileocecal valve
Has three subdivisions:
Duodenum, Jejunum, Ileum
The bile duct and main pancreatic duct:
Join the duodenum at the hepatopancreatic ampulla
Are controlled by the sphincter of Oddi
The jejunum extends from the duodenum to the ileum
The ileum joins the large intestine at the ileocecal valve
Structural modifications of the small intestine wall increase surface area
Plicae circulares: deep circular folds of the mucosa and submucosa
Villi – fingerlike extensions of the mucosa
Microvilli – tiny projections of absorptive mucosal cells’ plasma membranes
Contain Brush boarder enzymes – enzymes attached to membrane
Pancreas
Exocrine (secretes into ducts) function
• Secretes pancreatic enzymes as zymogens
(inactive precusor)
Endocrine function – release of insulin and glucagon
• Islets of Langerhans – area containing hormone
secreting cells
• Alpha cells – secrete glucagon in response to low
levels of blood glucose
• Beta cells – secrete insulin in response to high
levels of blood glucose
Liver The largest gland in the body
Hepatocytes’ functions include:
• Production of bile
• Processing bloodborne nutrients
• Storage of fat-soluble vitamins
• Detoxification
Secreted bile flows between hepatocytes toward the bile ducts in the portal triads
Central vein
hepatocytes
Bile
Canaliculi
(small
passageway)
Portal vein
macrophages
in sinusoid walls
The Gallbladder
• Thin-walled, green muscular sac on the ventral surface of the liver
• Stores and concentrates bile by absorbing its water and ions
Digestion Flowchart
Enzymes are written over the chemical reaction they control
All enzymes active at pH 7 except pepsin active at pH 2
Carbohydrates
Amylase
Starch
Oligosaccharrides
salivary or
pancreatic
ie – smaller fragments
Dextrinase
Glucoamylase
Maltose
Brush border
Enzymes
Sucrose
Lactose
sucrase
Maltase
Brush border
Enzymes
Glucose
Glucose & Fructose
Brush border
Enzymes
lactase
Glucose & Galactose
Brush border
Enzymes
Proteins
Pepsin *
only @ pH 2
stomach
Protein
Trypsin *
Chymotrypsin *
Protein
fragments
only @ pH 7
pancreatice
dipeptidase
aminopeptidase
carboxypetidase*
Individual
amino acids
Brush border enzymes
Carboxypeptidase also pancreatic
* - Activation of proteases
Zymogen
Active Enzyme
HCl
pepsinogen
trypsinogen
Brush border Enzymes
& Trypsin
chymotrypsinogen
procarboxypeptidase
Lipids
pepsin
Trypsin
Trypsin
trypsin
chymotrypsin
carboxpeptidase
Triglycerides only, other lipids don’t require digestion
Bile needed to separate triglycerides into smaller globs – no chemical digestion
Tryiglycerides
Lipase
= 1 glycerol &
3 fatty acids
pancreatic
Monoglyceride
& 2 fatty acids
Nucleic Acids
DNA
Brush border enzymes
deoxyribonuclease
Deoxynucleotides
pancreatic
RNA
deoxyribonuclease
pancreatic
nucleosidases
phosphatases
riobonucleotides
nucleosidases
phosphatases
Phosphate
Bases
Pentose sugars
Chemical Digestion: Carbohydrates
Enzymes used: salivary amylase, pancreatic amylase, and brush border enzymes
Absorption
• Secondary active transport (cotransport) with Na+
• Facilitated diffusion of some monosaccharides
Enter the capillary beds in the villi
Transported to the liver via the hepatic portal vein
Chemical Digestion: Proteins
Enzymes used: pepsin in the stomach
Enzymes acting in the small intestine
1. Pancreatic enzymes – trypsin,
chymotrypsin, and
carboxypeptidase
2. Brush border enzymes –
aminopeptidases,
carboxypeptidases, and
dipeptidases
Absorption: similar to carbohydrates
Chemical Digestion: Fats
Chemicals used: bile salts – emulsify fats
Enzymes used: pancreatic lipase
Absorption:
• Form micelles with bile that fuse to
intestinal cells
• Inside, combine with proteins and extrude
(exocytosis) chylomicrons (lipid-protein
complex)
• Enter lacteals (lymph structure) and are
transported to systemic circulation via lymph
Glycerol and short chain fatty acids are:
•Absorbed into the capillary blood in villi
•Transported via the hepatic portal vein
Includes fat-soluble vitamins (A, D, E, and K)
Chemical Digestion: Nucleic Acids
Enzymes used: pancreatic ribonucleases and deoxyribonuclease in the small intestines
Absorption: active transport via membrane carriers
Absorbed in villi and transported to liver via hepatic portal vein
Electrolyte Absorption
Most ions are actively absorbed along the length of small intestine
Na+ is coupled with absorption of glucose and amino acids
Ionic iron is actively transported into mucosal cells where it binds to ferritin
Anions passively follow the electrical potential established by Na+
K+ diffuses across the intestinal mucosa in response to osmotic gradients
Ca2+ absorption:
Is related to blood levels of ionic calcium
Is regulated by vitamin D and parathyroid hormone (PTH)
low calcium prompt PTH release which stimulates kidneys to activate vitamin D.
Active Vitamin D accelerates calcium absorption in intestine
Vitamins
•
•
•
•
Only vitamins D, K, and B are synthesized in the body; all others must be ingested
Water-soluble vitamins (B-complex and C) are absorbed in the gastrointestinal tract
B12 additionally requires gastric intrinsic factor to be absorbed through endocytosis
Fat-soluble vitamins (A, D, E, and K) bind to ingested lipids and are absorbed along with fatty acids
Water Absorption
95% of water is absorbed in the small intestines by osmosis
Water moves in both directions across intestinal mucosa
Net osmosis occurs whenever a concentration gradient is established by active transport of solutes
into the mucosal cells
Water uptake is coupled with solute uptake, and as water moves into mucosal cells, substances follow
along their concentration gradients
Large Intestine
Has three unique features:
• Teniae coli – three bands of longitudinal smooth muscle
• Haustra – pocketlike sacs caused by the tone of the teniae coli
• Epiploic appendages – fat-filled pouches of visceral peritoneum
Is subdivided into the cecum, appendix, colon, rectum, and anal canal
Regulation of Gastric Secretion
Neural and hormonal mechanisms regulate the release of gastric juice
Stimulatory and inhibitory events occur in three phases
1. Cephalic (reflex) phase: prior to food entry
Excitatory events include:
Sight or thought of food
Stimulation of taste or smell receptors
Inhibitory events include:
Loss of appetite or depression
Decrease in stimulation of the parasympathetic division
2. Gastric phase: once food enters the stomach
Excitatory events include:
Stomach distension - Activation of stretch receptors (neural)
Activation of chemoreceptors by peptides, caffeine, and rising pH
Release of gastrin to the blood
Inhibitory events include:
A pH lower than 2
Emotional upset that overrides the parasympathetic division
3. Intestinal phase: as partially digested food enters the duodenum
Excitatory phase
low pH; partially digested food enters the duodenum and encourages gastric gland
activity
Inhibitory phase
distension of duodenum, presence of fatty, acidic, or hypertonic chyme, and/or irritants
in the duodenum
Initiates inhibition of local reflexes (stomach action to push chyme into intestine)
and parasympathetic branch
Closes the pyloric sphincter (sympathetic branch)
Releases enterogastrones that inhibit gastric secretion
Regulation and Mechanism of HCl Secretion
HCl secretion is stimulated by ACh (parasympathetic), histamine (enteroendocrine cells), and
gastrin through second-messenger systems ( ie –cAMP therefore slow)
Release of hydrochloric acid:
Is low if only one ligand binds to parietal cells
Is high if all three ligands bind to parietal cells
Antihistamines block H2 receptors and decrease HCl release