Chapter 5 Digestion, Absorption, and Metabolism 1

Chapter 5

Digestion, Absorption, and Metabolism

1

Ch 5Key Concepts:Whole foods must be changed to simpler

substances in order to be useful to the body. The body must free the food nutrients and re-form and re-route them to meet special life needs.

Special organ structures and functions conduct these tasks through the successive parts of the overall system.

2

ObjectivesIdentify the

principles of digestion

Describe the absorption process

Identify the human water balance system

Identify human acid – base balance system

3

Digestion

Basic principles:Food can not be used as eatenFood must be changed to a simpler form for absorption

Digestion is the process in which food is broken down in the gastrointestinal tract

Absorption is the process in which these nutrients are taken into the cell lining the gastrointestinal tract

4

5

Principle of WholenessThe parts of this overall process of change do not

occur separately but make one continuous whole

Mechanical and chemical changes – Food must go through a series of mechanical and chemical changes

Together these two types of actions make up digestion:Mechanical digestion: gastrointestinal motilityChemical digestion: gastrointestinal secretions

6

I

Gastrointestinal system

7

Mechanical Digestion

Motility = the ability to move spontaneously

Starting in the mouth, muscles and nerves in the walls of the GI tract coordinate their actions to provide the needed motility for digestion to proceed

8

Mechanical DigestionMuscles – Layers of muscles provide two types of movement Muscle Tone - tonic contraction ensures the continuous passage of the food mass and valve control along the way

Periodic muscle contraction and relaxation – rhythmic waves that mix the food mass and move it forward

9

Mechanical Digestion

Nerve - Complex nerves in the gastrointestinal wall extend from the esophagus to the anus Control muscle tone

in the wallRegulate the rate

and intensity of muscle contractions

Coordinate all of the various movements

10

Chemical DigestionA number of secretions work together to

make chemical digestion possible

5 types of substances involved:Hydrochloric acid and buffer ions –

needed to produce the correct pH which is necessary for enzyme activity

Digestive Enzymes – digestive enzymes are proteins, specific in kind and quantity for breaking down specific nutrients

Mucus- lubricate and protect the mucosal tissues; help mix food mass

11

Chemical Digestion

Water and electrolytes – products of digestion are carried and circulated through tract and into tissues by water and electrolytes

Bile – made in liver and stored in gallbladder, bile breaks down fats into smaller pieces to assist fat enzymes

12

Chemical Digestion Secretory Cells: in the

GI tract produce each of the preceding substances.

The action of these cells is stimulated by:Presence of foodNerve impulsesHormones specific

for certain nutrients

13

Digestion in the Mouth and EsophagusMechanical Digestion:

Mastication (chewing) breaks down food into small particles

Muscles at the base of the tongue facilitates swallowing

Food is swallowed and passes down the esophagus through peristaltic waves and gravity

14

Digestion in the Mouth and Esophagus

Gastroesophageal sphincter at the stomach entrance relaxes, allowing food to enter, then constricts to retain food

GERD (Gastroesphageal Reflux Disease) – Reflux – Heartburn If the gastroesophageal sphincter is

not working properly, it may allow the acid-mixed food to seep back into the esophagus a uncomfortable feeling of “heartburn”

15


GERD cont.“Heartburn” –

unrelated to cardiacCalled this

because the sensations are perceived as originating in the area of the heart

16

Digestion in the Mouth and EsophagusChemical Digestion

Salivary glands secrete material containing salivary amylase or ptyalin – a “starch-splitting” enzyme in the mouth

Ebner’s serous glands at the back of the tongue secrete a lingual lipase

17


Salivary glands also secret a mucous material to lubricate and bind food particles, facilitating the swallowing of the food bolus (lump)

Secretions from the mucous glands in the esophagus help move food toward the stomach

18

Digestion in the StomachMechanical digestion:

Under sphincter control, the food enters the upper portion (fundus) of the stomach as individual bolus lumps

The muscles knead, store, mix, and propel the food mass forward in a slow controlled movement

19

Digestion in the StomachMechanical digestion cont.

By the time the food mass reaches the lower portion of the stomach, it is a semi-liquid acid/food mix called chyme.

Chyme is released slowly into the first section of the small intestine (duodenum) by the pyloric valve

20

Digestion in the StomachChemical digestion:

Gastric secretions contain acid, mucus and enzymes which aid in digestion

Hydrochloric Acid – parietal cells secrete HCL to activate enzymes

Mucus – protect stomach lining from the erosive effect of the acid, and bind and mix the food mass and help it move along

21

Digestion in the StomachChemical digestion cont.

Enzymes - inactive form (pepsinogen) is secreted by the stomach cells and activated by HCl to become pepsin, a protein-splitting enzyme

22

Digestion in the Small Intestine

The structural parts, synchronized movements and array of specific enzymes of the small intestine are highly developed for the final task of mechanical and chemical digestion.

23


Mechanical Digestion: Under the control of nerve impulses,

stomach walls stretch from the food mass and the intestinal muscles produce several types of movements:

Peristaltic waves – push food forwardPendular movements –small, local

sweeps back and forth, stirring the chyme

24

Digestion in the Small Intestine Mechanical digestion cont.Segmentation rings –

alternating contractions and relaxations of circular muscles

Longitudinal rotation – long muscles roll slowly moving food in spiral motion

Surface villi motions – mix chyme at intestinal walls – exposing additional nutrients for absorption

25

Digestion in the Small Intestine Chemical digestion

Pancreatic enzymesCarbohydrate Pancreatic amylase

converts starch to disaccharides, maltose and sucrose

Protein – Trypsin and chymostrypsin split large protein molecules into smaller peptide fragments and single amino acid

Fat – Pancreatic lipase converts fats to glycerides and fatty acids

26

Napping: what you do after lunch!

27


Intestinal enzymes :

Carbohydrate Disaccharidases (maltase, lactase, and sucrase) convert their respective disaccharides (maltose, lactose, and sucrose) to monosaccharides (glucose, fructose, and galactose)

28


Lactose IntoleranceA large percentage of the world’s population does not produce enough lactase to digest lactose (milk sugar

As a result, these individuals cannot tolerate milk and milk products unless they are in the predigested form – e.g. yogurt, buttermilk, aged cheese, or lactase-treated milk. May also take commercially available oral lactase

29

Digestion in the Small Intestine Protein

The enzyme enterokinase activates trypsinogen from the pancreas and turns it into trypsin (the protein-splitting enzyme).

Fat Intestinal lipase splits fat in to glycerides and fatty acids

30

Digestion in the small IntestineMucus- large quantities of mucus

protect the mucosal lining from irritations and erosion caused by the acidic gastric contents

Bile – emulsifying agent - aids fat digestion and absorption [made in the liver, stored in the GB]

31

Digestion in the small IntestineHormones

Secretin – produced by the mucosal

glands – this controls the acidity and secretion of enzymes from the pancreas.

Cholecystokinin – secreted by intestinal

mucosal glands when fat is present; triggers the release of bile from gallbladder to emulsify the fat

32

I

Summary of digestive processes

33

I

Factors influencing the GI tract

34

Accessory Organs The “accessory

organs” (to the duodenum) make up the “biliary system”

The liver is sometimes called the “Metabolic Capital” of the human body

Metabolizes all converging nutrients

35

Role of the LiverProcesses blood immediately after it

leaves the gastrointestinal tractBile productionSynthesis of protein and blood

clotting factorsRegulation of blood glucoseMetabolism of hormones and

medications

36

37

Absorption and Transport

When digestion is complete, food has been changed into simple end-products that are ready for cell use

Transport- is the movement of nutrients through the circulatory system from one area of the body to another

38

Absorption and Transport“Simple” end-products:

Carbohydrates: reduced to simple sugars (glucose, fructose, galactose)

Fats: changed into fatty acids and glycerides

Proteins: changed into single amino acids

Vitamins and minerals: liberated from food

39

Absorption in the Small IntestineSpecial absorbing structures – [p. 72, Fig.5-

6]Mucosal folds - surface of the small intestine piles into folds

Villi – small finger-like projections, cover the mucosal folds, increasing the area of exposed intestinal surface

Microvilli – electron microscope reveals a covering of multiple small projections on the surface of the villi. This is the brush border because it looks like bristles on a brush

40

I

Villi and microvilli

41

Absorption ProcessesMoving vital nutrients across the inner

intestinal wall into body circulation.

Simple diffusion – the force by which particles move outward in all directions, from an area of greater concentration to lesser concentration

Facilitated diffusion – simple diffusion with the help of a protein channel for carrier assisted movement of larger items across the mucosal cell wall

42

Absorption and TransportActive transport – Force by which

particles move from area of greater concentration to lesser concentration with help from a carrier or ferry

Pinocytosis – Penetration of larger materials by attaching to the thicker cell membrane and being engulfed by the cell

43

Absorption in the Large IntestineMain absorptive task: to take up waterMost water in chyme is absorbed in

the first half of the colonOnly a small amount remains to form and eliminate the feces

Dietary fiberNot digestedContributes bulk to food massHelps form feces

44

Water: good for digestion

45

TransportNutrients must be transported to cellsVascular system (blood, circulation) –

Most of the products of digestion are water-soluble nutrients can be absorbed into the vascular systemdirectly from the intestinal cells

Composed of veins and arteries and is responsible for supplying the entire body with nutrients and oxygen necessary for life.

Transports waste, such as carbon dioxide and nitrogen, to the lungs and kidneys for removal 46

TransportLymphatic system – route for fatty

materials which are not water soluble

Fat molecules pass into the lymph vessels in the villi, flow into the larger lymph vessels of the body and eventually enter the blood stream

47

MetabolismMacronutrients in food

have been broken down through digestion (-> monosaccharides, fatty acids, amino acids) and now must be absorbed into blood stream

They will be converted to energy or stored for later use.

48

MetabolismEnergy for Fuel - the sum of chemical

reactions occurring within a living cell to maintain life

2 types of metabolism: Catabolism – the breaking down of larger

substances into smaller unitsEg. The process of breaking down large

carbohydrate and protein chains into their smaller building blocks – monosaccharides and amino acids- is a catabolic reaction

Anabolism – the process in which cells build large substances from smaller particles

49

Help for Anabolism

50

I

Metabolic pathways

51

MetabolismEnergy for storageIf the amount of food consumed yields more energy than is needed, the “leftover’ energy is stored for later use in the body.E.g. Excess glucose can be stored as glycogen in the liver and muscles for quick energy at a later time

52

Metabolism and fight/flight

53

Documents

Chapter 5 Digestion, Absorption, and Metabolism 1