Digestive System

Functions of small intestine:

*Digestion* -Neutralizes acid from stomach -Adds digestive enzymes and bile -Breaks down proteins, carbohydrates, and lipids to -absorbable materials (monomers) *Absorption* -90% of food is absorbed in small intestine (Due to its large surface area)

What is the role of the stomach?

-Food storage -Digestion of proteins -Regulation of delivery of partially digested food into the small intestine

Mucosa adaptations increase surface area to facilitate absorption: how?

-Large folds, Villi, Microvilli:

What are the metabolic functions of the liver?

-Metabolic functions Storage, synthesis of proteins, and chemical processing

Composition of saliva:

-Mucin -Salivary amylase: begins digestion of starch -Bicarbonate: maintains pH (6.5-7.5) for optimal amylase activity -Lysozyme: inhibits bacterial growth

Digestive enzymes in the Pancreas include:

-Proteases: trypsin, chymotrypsin, & carboxypeptidase -Pancreatic amylase: digests starch -Lipase: digests lipids

Explain feedforward control in digestion.

Feedforward reflexes originate in the brain and originate by stimuli such as sight, sound, smell, or thought of food, and prepares digestive system for food that is anticipated. The reflex creates a feedforward response (AKA cephalic phase of digestion) which activates neurons in medulla oblongata which sends an efferent signal through autonomic neurons to salivary glands and ENS; as a response stomach, intestine, and accessory organs start secreting and increasing motility in anticipation of food

Describe the primary function of the digestive system.

To move water, nutrients, and electrolytes from the external environment to the internal environment.

Map the processes and control pathways of the gastric phase.

When food arrives from esophagus, stomach relaxes and expands to hold increased volume (receptive relaxation). Digestive activity in the stomach begins with the long vagal reflex of the cephalic phase.

Smooth muscle and glands of the GI tract are under _________ control (for long reflexes)

autonomic

Mucosa:

innermost layer, mucous membrane in  contact with the lumen

Long reflexes:

integrate in the central nervous system (CNS), ENS sends sensory info to the CNS and receives info from the CNS through autonomic neurons, can originate outside the GI tract, smooth muscle and glands of GI tract are under autonomic control, parasympathetic division is excitatory and enhances GI functions, include feedforward and emotional reflexes that originate in the brain

Short reflexes:

integrate in the enteric nervous system (ENS), reflex begins, is integrated, and ends completely in the GI tract, no outside input. Receives sensory info from the lumen of the gut and the ENS integrates it to initiate a response.

Endopeptidase digests

internal peptide bonds

Submucosa:

layer of connective tissue containing  blood vessels, lymph vessels, nerves

Cephalic reflexes

long reflexes; originate outside the digestive system; include feed-forward and emotional reflexes (originate in the brain)

Villi:

microscopic projections containing blood and lacteal capillaries

Fats Digestion= triglycerides are broken down to _________ &_______ ______ by lipase and colipase

monoglycerides fatty acids

*Role of bicarbonate(secreted by the pancreas):

neutralize stomach acids

Nucleic acids are digested into

nitrogenous bases and monosaccharides

Trace a piece of undigested food from mouth to anus.

oral cavity (mouth and pharynx) - GI tract which consists of the esophagus, stomach, small intestine, and large intestine, and then leaves - the anus.

Serosa:

outermost layer

How does the Chloride get secreted

1. Chloride from the ECF enters the cell via NKCC transporters, then exit into the lumen via a gated Cl- channel → called cystic fibrosis tramembrane conductance regulator (CFTR channel) 2. Movement of Cl- into the lumen draws Na_ down its gradient through leaky cell junctions 3. Water follows Na+ along the gradient → results in secretion of isotonic saline solution

what is the role of the large intestine?

Absorbs nutrients and water AND Temporarily stores and eliminates waste

How are fats digested and absorbed?

Bile salts from liver coat the fat droplets Pancreatic lipase and colipase break down the fats into monoglycerides and fatty acids- stored in micelles (happening in lumen of small intestine) either cholesterol is transported into the cell or he monoglycerides and fatty acids move out of the micelles and enter cells(of small intestine) by diffusion Absorbed fats combine with cholesterol and proteins in the intestinal cells to form chylomicrons chylomicrons are then removed by lymphatic system

Long reflexes integrated in

CNS

Diagram the digestion and absorption of carbohydrates, proteins, and fats.

Carbs: *Amylase* breaks down carbs. Digestion starts in mouth and then resumed in intestine. Glucose enters cell with Na+ on SGLT and exits on GLUT2. Also moves galactose. Fructose doesnt move wiht Na+, enters on GLUT5 and exits on GLUT2. Proteins: *Endopeptidases digest internal peptide bonds*, also called proteases, and are found in the stomach, SI and pancreas. *Exopeptidases release amino acids by cutting the ends of peptides*, but play a lesser role in digestion. *Products of protein digestion are amino acids, dipeptides, and tripeptides*. Most free a.a.s are carried by Na+-dependent cotransport proteins, some are H+ dependent. Di/tripeptides are carried into cells on oligopeptide transporters that are H+ dependent. When inside the cells, the peptides are furhter digested into aas which are then transported across basolateral membran and enter circulation, or can be transported as peptides across basolateral membrane by H+ dependent exchanger. Some larger peptides are absorbed by transcytosis after binding to membrane receptors on luminal surface of intestine. Fats: *Bile salts break down the coarse emulsion of large fat droplets so that there is greater surface area and they can be enzymatically digested*. Lipases digest fat by removing two fatty acids from each triglyceride molecule. Colipase is also needed and is secreted by the pancreas to allow lipase to access the fats inside the bile salt coating. Phospholipase digests phospholipids. *Free cholesterol is not digested.* Fatty acids and monoglycerides are absorbed by simple diffusion into the cells. *They then move to the smooth endoplasmic reticulum where they recombine to form triglycerides, which then join with cholesterol to form chylomicrons which are packed into vesicles and leave cell by exocytosis and absorbed by lymphatic system*. Shorter fatty acids

Explain the neuronal and hormonal control of the intestinal phase of digestion.

Chyme entering intestine activates ENS which decreases gastric motility and secretion and slows gastric emptying. Secretin, CCK, and gastric inhibitory peptide (GIP) are hormones that reinforce the decrease in motility. Secretin is released by presence of acidic chyme in duodenum and inhibits acid production and decreases motility. It also stimulates production of pancreatic bicarbonate to neutralize acidic chyme. CCK is secreted when fats are present. It decreases motility and acid secretion. GIP is released when carbs are present. It feeds forward to promote insulin release by pancrease so that it is prepared for glucose that will be absorbed. It also slows entry of food into intestine by decreasing motility and acid secretion. The mix of acid, enzymes, and food in chyme forms hyperosmotic solution. Sensors in the intestine wall are sensitive to osmolarity and are stimulated by high osmolarity, inhibiting gastric emptying.

What happens in the Gallbladder?

Concentrates and stores bile (delivers it to the duodenum via the bile duct)

Pepsinogen

Converted to pepsin by acid Begins protein breakdown

Carbohydrate digestion: =Starch, glycogen> Digested by=______ Into: Maltose= ______+______ Sucrose=_______+______ Lactose=_______+______

Digested by = amylase Maltose=2 glucose Sucrose= 1 glucose +1 fructose Lactose = 1 glucose + 1 galactose

Describe and compare secretion, digestion, absorption, and motility.

Digestion: chemical and mechanical *breakdown* of food into smaller particles that can move across the intestinal epithelium into the body. Absorption: movement of substances from the *lumen of the GI tract to the extracellular fluid*. Secretion: can mean the movement of water and ions from the *ECF to the GI tract lumen* (opp of absorption) or the release of substances made by GI epithelial cells into either the lumen or the ECF. Motility: movement of material in the GI tract as a result of muscle contraction

Map the processes and control pathways of the cephalic phase.

Digestive processes begin before food enters becuase of smelling, seeing, or thinking about food. Mouth waters, the reflexes begin in the brain to create a feedforward response. Anticipatory stimuli/ stimulus of food in mouth activate neurons in medulla oblongata that then sends signal to salivary glands and through vagus nerve to enteric nervous system. Stomach, intestine and accessory organs begin secretion and increase motility. Chemical and Mechanical digestion starts in the mouth.

Where does the Hepatic portal system go?

Drains blood from the DIGESTIVE tract to the LIVER(blood picks up nutrients from gastrointestinal tract and goes to liver, the capillaries form a portal system)

3 parts of the small intestine

Duodenum: most digestion occurs here Jejunum: absorption Ileum: absorption

Short reflexes integrated in

ENS (the enteric nervous system shares features with the CNS)

Protein Digestion= name the two enzymes:

Endopeptidase and Exopeptidase

Functions of the Pancreas?

Exocrine functions and Secretes digestive enzymes(protease, amylase, lipase) and produces sodium bicarbonate (NaHCO3) (it delivers these products to the duodenum via the ducts)

Describe the gastric secretions and their major actions.

G cells secrete gastrin into blood (hormone). It short reflexes its release is stimulated by amino acid and peptide presence in the stomach and distension of stomach. Also released by neural reflexes. Short reflexes are mediated by ENS neurotransmitter gastrin-releasing peptide. In cephalic reflexes, parasympathetic neurons from vagus nerve stimulate G cells to release gastrin into blood. Gastrin is used to promote acid release by acting of parietal cells and indirectly stimulating histamine release. Parietal cells secrete gastric acid into lumen of stomach to create low pH. It causes release and activation of pepsin, which digests proteins. Also triggers somatostatin release from D cells. It also denatures proteins by unfolding them so that it is easier to digest by pepsin. Kills bacteria and undigested microorganisms, and inactivates salivary amylase to stop carbohydrate digestion. Pepsin carries out initial digestion of proteins. It is secreted in its inactive form pepsinogen by chief cells. Acid simulates pepsinogen release through short reflex. H+ in lumen clease pepsinogen into pepsin. Gastric lipase is co-secreted with pepsin to break down triglycerides. Histamine is a paracrine signal secreted by ECL cells in response to gastrin or Ach. It diffuses to parietal cells to stimulate acid secretion by combining with H2 receptors on the cells. Intrinsic factor is also secreted by parietal cells and complexes with vitamin B12 so that it can be absorbed in the intestin. Somatostatin is secreted by D cells in stomach. It is a negative feedback signal for gastric phase secretion by decreasing gastrin and histamine secretion. It also inhibits pepsinogen secretion.

gastric juice contains=

Hydrochloric acid, Intrinsic factor, Mucus, Pepsinogen

Compare and contrast digestion and motility in the large and small intestine.

In the small intestine, there is slow motility by peristaltic and segmental contractions, which allow the chyme to mix with enzymes and so that digested nutrients are exposed to epithelium for absorption. It has to be slow so digestion and absorption can be fully completed. Parasympathetic innervation and gastrin and CCK promote intestinal motility. Most chemical digestion happens in SI. in large intestines, chyme is mixed by segmental contractions and depends on mass movement, which is a wave of contraction that sends a substantial bolus of material forward. Bacteria in the colon break down undigested complex carbs and proteins by fermentation. Some products can be absorbed by simple diffusion. Vitamin K is also produced and absorbed.

Intrinsic factor

Made by same cells that make acid; needed to absorb vitamin B12

Phasic contractions:

Migrating motor complex (MMC) Peristalsis moves bolus forward Segmentation mixes

So the gastric gland (in the stomach) has many different cells that secrete different things into the lumen: Name a couple

Mucus, Bicarbonate(buffer), Gastric acid (HCl)(activates pepsin; kills bacteria), intrinsic factor(Complexes with vitamin B12 to permit absorption), Histamine(stimulus gastric acid secretion), Pepsin(ogen)(digests proteins), Gastric lipase(digests fats) , Somatostatin(inhibits gastric acid secretion), Gastrin(stimulates gastric acid secretion)

What accessory Organs Aid Digestion and Absorption?

Pancreas, Liver, and Gallbladder

Describe the major secretions of the pancreas and liver.

Pancreas: Trypsin is released when there is distention of the small intestine, food in the intestine, signals from neurons, and CCK (hormone that breaks down fats and proteins). Bicarbonate neutralizes acid entering from the stomach, and is secreted into the duodenum. Carbonic anhydrase is needed to produce bicarbonate Liver: Bile is secreted by liver cells(hepatocytes), and is made up of bile salts which digest fat, and bile pigments, such as bilirubin, which are a waste product of hemoglobin degredation, and cholesterol, which is excreted by feces. Bile travels to the gallbladder where it is stored and them released into the duodenum.

Describe and compare peristalsis, segmentation, and the migrating motor complex.

Peristalsis: progressive waves of contraction that move from one section of GI tract to the next, *circular muscles contract right behind the mass (bolus) of food, pushes food forward* to where muscles are relaxed, contributes to food mixing Segmental contractions: short segments of intestine alternately contract and relax, *circular muscles contract while longitudinal muscles relax, can happen randomly along intestine or at regular intervals , mix intestinal contents so that they are in contact with absorptive epithelium* Migrating motor complex: series of contractions of stomach that each take 90 min to reach large intestine, *happens between meals when stomach is empty, sweeps food remnants and bacteria out of upper GI tract into large intestine*

Hydrochloric acid in the gastric juice:

Produces a pH of about 2; breaks down large bits of food

What is produced by the Liver?

Produces bile, which emulsifies lipids(digestion of fats)

Mucus

Protects stomach lining from acid

Saliva:

Secreted by salivary glands

How does the Pancreas secrete bicarbonate and enzymes?

Secretion bicarbonate requires high level of carbonic anhydrase. Steps: 1. Cells that produce bicarbonate have a lot of carbonic anhydrase (creates bicarbonate) a. CO2 and water is secreted by exchanger of apical side of the cell b. H+ ions produced leaves via Na+/H+ exchangers c. The reabsorption of H+ thus allows for balance between bicarbonate concentration (sodium enters the cell in exchange for the H+ ion to leave) 2. Chloride enters cells by indirect active transport and leaves the apical side through a CFTR channel. *Cl- then reenters the cell in exchange for HCO3- (movement of Cl- allows for bicarbonate to enter the lumen of intestine or pancreas)* a. Defects in the CFTR channel results in cystic fibrosis 3. Leaky junctions allow paraceullular movement of ions and water. Negative ions in the lumen attract Na+ by the paracellular pathway. Water follows a. Movement of negative ions from the CF to the lumen creates a gradient that attracts Na+ molecules b. They will enter the cell via osmotic gradient and water will follow its movement

Describe single-unit smooth muscle, slow wave potentials, tonic and phasic contractions.

Single-unit smooth muscle: groups of cells *electrically connected by gap junction*, diff regions exhibit diff types of contractions Slow-wave potentials: *cycles* of *smooth muscle contraction and relaxation* associated with *cycles of depolarization and repolarization. Originate in interstitial cells of Cajal (ICCs) which lie between smooth muscle layers and intrinisic nerves, may act as intermediary between neurons and smooth muscle, function as pacemakers for slow wave activity of GI tract* Tonic contractions: *sustained for minutes or hours*. Occur *in some smooth muscle sphincters and in anterior portion of stomach* Phasic contractions: *contraction-relaxation cycles last seconds, occur in posterior region of stomach and in small intestine*

where is there Large surface area for absorption?

Small intestine

Explain the functions of saliva and the process by which it is secreted.

Soften and moisten food: the water and mucus in saliva softens and lubricates food so it is easier to swallow Digest starch: salivary amylase chemically digests starch into maltose when enzyme is activated by Cl- in saliva. Taste: saliva dissolves food so we can taste it Defense: lysozyme is an antibacterial salivary enzyme, and immunoglobins in saliva disable bacteria and viruses. Saliva also washes teeth and keeps tongue free of food particles Chewing starts mechanical digestion to create a soft, moistened mass that is easily swallowed. Is an exocrine secretion by salivary glands. It is first secreted as an isotonic NaCl solution and as it passes through the duct to the oral cavity, NaCl gets reabsorbed and K+ and bicarbonate is secreted until it is hyposmotic. Parasympathetic innervation stimulates saliva secretion.

List the steps of the deglutition (swallowing) reflex.

Stimulus for swallowing is the pressure created by tongue pushing bolus against the soft palate and the back of the mouth. The pressure activates sensory neurons that run through the cranial nerve to a swallowing center in the medulla oblongata. Output from somatic motor neurons of swallowing center control muscles of pharynx and upper esophagus, as well as autonomic neurons that act on lower part of esophagus. As reflex starts, the soft palate rises to close off nasopharynx. Muscle contractions move larynx up and forward to close off trachea and open upper esophagal sphincter. As bolus moves toward esophagus, epiglottis folds down to complete the closure of upper airway to prevent food from entering airway. When bolus reaches esophagus, esophagal sphincter relaxes. Wave of perstaltic contractions pushes bolus to stomach with the help of gravity. Lower esophagal sphincter relaxes so bolus can enter stomach.

Tonic contractions:

Sustained Occur in smooth muscle sphincters and stomach Keep bolus from moving backwards

Amino acid transport = need________....where as small peptides (di or tri amino acids) only need ______ to be transferred....

amino acids= need sodium small peptides = need protons

Teeth(function): Types: Children: Adults: Structure:

bite and chew food incisors, canines, premolars, molars 20 teeth 32 permanent teeth crown, root

Epiglottis:

closes airway temporarily so food will not enter the trachea

Pharynx:

common passageway for air and food

Segmental:

contractions promote mixing

Microvilli:

cytoplasmic projections of epithelial cells of the villi

Food passes through lower ________ _________ into the stomach

esophageal sphincter

Food moves through _______ with the help of peristaltic contractions

esophagus

GI peptides:

excite or inhibit GI motility and secretion, can be hormones, neuropeptides, and cytokines; can be secreted into lumen to bind to receptors on epithelium to elicit a response, or can be secreted into ECF to act on neighboring cells; can also act outside GI tract such as the brain to enhance feeling of satiety; GI hormones are secreted into blood and transported through body→ can act on pancreas and brain, GI peptides can act as feedforward signals for insulin release→ act on pancreas

Intestine absorbs vitamins and minerals: Fat-soluble vitamins absorbed with _____ Water-soluble vitamins by ______ ______ Ex. Vitamin B12 complexed to intrinsic factor and absorbed in ileum Minerals by _______ _______(ex. Calcium or iron have to be transported in)

fats mediated transport active transport

Tongue:

positions and tastes food Skeletal muscle Moves food in mouth (also adds digestive enzymes) Taste receptors are located on the tongue(molecules have to be dissolved in the saliva- if its dry u wont taste it) Important for speech

Peristalsis:

promotes forward movement

Feed-forward reflexes begin with

stimuli such as sight, smell, sound or thought of food and they prepare the digestive system

Exopeptidase digests

terminal peptide bonds to release amino acids

Secretion of intestinal bicarbonate occurs when:

there is a drop in pH bc bicarbonate is added to neutralize acid in the digestive tract

Sphincters:

thick muscular rings that separate some of the organs

Muscularis:

two or three layers of smooth muscle, responsible for movement, motility