Wold 3 Flashcards by Tyler Brown

But, there are only 2

stomach**

2 glandular regions in the stomach.

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There are 3 sections of the

stomach.

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Pepsinogen is a

zymogen (i.e., it is a proenzyme that needs to be cleaved to become active)

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Parietal cells produce

intrinsic factor

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pancreas is an

exocrine gland

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pancreas

Exocrine

bicarbonate ions
  digestive enzymes (many, many, many digestive enzymes)

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CCK Secretion is potentiated by

*Pancreas

secretin

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Pancreatic Bicarbonate secretion is essentially the same

as HCl secretion in reverse.

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Regulation of Pancreatic Bicarbonate Secretion:

Hormone regulation by

secretin

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Regulation of Pancreatic Bicarbonate Secretion

Feedback regulation by

acidity

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CCK strongly potentiates the effects of

secretin

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Liver:

Secretes bile into small ducts called

bile canaliculi

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Liver

Canaliculi converge and drain into

larger bile ducts.

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Small Intestine types of muscle contractions:

Peristalsis: progressive contractions of successive sections of

circular smooth muscle

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Small Intestine types of muscle contractions

Segmentation: closely spaced contractions of

circular muscle layer. When this is rhythmic (i.e., sites of the circular contractions alternate between contraction and relaxation), chyme/bolus is mixed and slowly moved downward.

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Bacteria can ferment some of these undigested substances, which will then be absorbed in the

large intestine, but this comes at a price…

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motility in large intestine occurs as slow

“segmentation” contracts

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large intestine; peristaltic-like contractions occur

3 – 4 times per day

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How do we digest a meal?

Mouth

Chewing, salivation, amylase, lipase

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How do we digest a meal?

Stomach

HCl, pepsinogen, lipase, gastrin

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How do we digest a meal?

Small Intestine

Hormones—Secretin, Cholecystokinin

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How do we digest a meal?

Accessory Organs

Pancreas—proteases, lipase, amylase, HCO3-

Liver—bile

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How do we digest a meal?

Small Intestine

Absorption—carbohydrates, proteins, fats, vitamins, minerals

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How do we digest a meal?

Large Intestine

Absorption—Na+, Cl-, water

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Chyme has to ------- in order for us to digest a meal

keep moving

Stomach churning | Rate is determined by

pacemaker cells,

Stomach churning magnitude determined by

excitatory stimuli

Stomach | Peristalsis from

body to pyloric sphincter

Small Intestine Segmentation and Peristaltic contractions Peristalsis controlled by

migrating myoelectric complex (initiated by motilin)

Primary purpose of the large intestine is to actively transport

Na+ from lumen to blood.

Large intestine | Also, bicarbonate secretion is coupled to

Cl- ion absorption.

Bacterial fermentation of some of the food bolus contents maximizes the absorption of nutrients from food in the

large intestine

Small Intestine Motility | During absorption ------- occur.

segmentation contractions

Small Intestine Motility | After most absorption has occurred,

peristaltic contractions occur.

Small Intestine Motility Peristalsis is driven by a

migrating myoelectric complex.

peristalsis in small intestine | Starts in

lower stomach and travels about 2 feet prior to dying out

peristalsis in small intestine | Initiated by the intestinal hormone

motilin

Absorption is maximized when the

chyme/bolus is continuously brought into contact with the intestinal tissue (constant movement)

The liver synthesizes, about

20-60 mg of bile salts a day

-------- of bile salts are secreted a day.

1,200-3,600 mg

Bile salts are recycled through the

enterohepatic circulation

Bile

Bile salts (and other salts) Lecithin Cholesterol Bile pigments (like bilirubin) and other metabolic end products Trace metals Bicarbonate ions (secreted by epithelial cells of ducts, stimulated by secretin)

In general, the same factors that affect HCl secretion also affect

gastric motility/emptying.

Enterogastrone is a hormone produced by

intestinal cells

Enterogastrone | inhibit the secretion or motility in the

stomach.

Secretin, and CCK are

enterogastrones.

Regulation of HCl Secretion | Intestinal phase---initially enhances

HCl secretion. But, later in digestion, the intestinal phase is inhibitory.

Regulation of HCl Secretion when pH of chyme < 3, =

secretin produced

Regulation of HCl Secretion | High H+, amino acids, and fatty acids stimulate

CCK, which ↓HCl production

Regulation of HCl secretion | Gastric phase---

distension, proteins, peptides and amino acids. Increase in gastrin secretion.

Somatostatin is a potent inhibitor of

HCl secretion via 2 mechanisms Effects on G Cell Effects on Parietal Cell

Inputs to Parietal Cells Regulate

Acid Secretion

Histamine is the strongest

HCl stimulant.

Histamine release can be triggered by

gastrin or Ach

Gastrin and Ach can have direct effects on

parietal cells.

Parietal cells: In all cases, activation of receptors results in second messenger activation that increases the ability of

parietal cell to release H+ and Cl-

Mucous—Mucous Neck Cells

Throughout all sections of stomach

HCO3-—Epithelial Cells

Throughout all sections of stomach

Gastrin—G Cells

Antrum (Pyloric Gland Area)

Somatostatin—D cells

Throughout the stomach at the base of the gastric glands

Histamine—

Enterochromaffin-like cells

HCl—Parietal Cells

Fundus and Body (Oxyntic Gland Area)

Intrinsic Factor—Parietal Cells

Fundus and Body (Oxyntic Gland Area)

Pepsinogen—Chief Cells

Body and Antrum (Oxyntic and Pyloric Gland Area)

stomach | Delivers the chyme to the duodenum at a rate compatible with

The secretion rate of bile salts, bicarbonate ions, and digestive enzymes Rate of enzymatic breakdown of proteins, lipids, and carbohydrates Small intestine transit

Phases of GI Control

cephalic, gastric, intestinal

Cephalic Phase

(parasympathetic nerve fibers affecting ENS)

Cephalic phase Initiated when

receptors in head are stimulated

Gastric Phase

``` (short and long neural reflexes and gastrin) Distension Acidity Amino acids Peptides ```

Intestinal

``` (short and long neural reflexes, secretin, CCK, and GIP) Distension Acidity Osmolarity Various digestive products ```

Each GI hormone participates in a

feedback control system to regulate some aspect of the GI lumen e.g., CCK stimulated by fatty acids, stimulates enzyme production by pancreas, which will reduce CCK and reduce pancreas activation

Each GI hormone affects more than

one type of target cell | e.g., CCK can stimulate pancreas, liver, gallbladder, and inhibit stomach emptying

GI Hormones can have

``` synergistic effects (one can potentiate the other) e.g., secretin enhances the effects of CCK ```

Four hormones play a large role in regulating digestion | Gastrin

Cholecystokinin (CCK) Secretin Glucose-dependent insulinotropic peptide (GIP)

Receptors GI

Mechanoreceptors Osmoreceptors Chemoreceptors

GI: changes Initiated by

Distension of the wall Chyme osmolarity Chyme acidity Chyme concentrations

ENS Motor Neurons

Muscle Contractions | Gland Function

ENS: Interneurons | Regulate

interactions between different layers of the GI tissue

GI Neurons

Autonomic nervous system | Enteric nervous system

Paracrine mediators | Produced by

local cells | Reach target cells via diffusion

Hormones | Produced by

endocrine cells | Reach gut via the blood