Nutrition, Digestion, Absorption, and Excretion

© Dr. Regis FerriereDepartment of Ecology & Evolutionary Biology

University of Arizona

Lecture 2

ECOL 182 - Spring 2010

Our main questions in this lecture…

• Why do animals eat?• How do animal digest food?• How is food uptake regulated?• How does the animal body deal with wastes?• How does the animal body maintain water and salt

balance in spite of excretion?• What is the structure of the mammalian kidney?

How does it function? How is its functionregulated?

Animals cannotsynthesize the

acetyl group, butthey ingest it in their

food and use it tosynthesize many

molecules.

What do animals require from food?

• Animals are heterotrophs: derive energy &molecular building blocks, directly orindirectly, from _______.

• For many animals, food provides essentialcarbon skeletons that they cannotsynthesize themselves.Humans require 8 essential _______

from food.

What do animals require from food?

• Animals need mineral elements for a variety of functions:Macronutrients, like Ca for bone growth, blood

clotting, nerve and muscle action, enzyme activation(1.2kg in a 70kg body).

Micronutrients, like Fe for enzymatic reactions,oxygen binding in blood (4g in a 70kg body).

• Animals must obtain vitamins from food.Carbon compounds functioning as _______ for growth

and metabolism.Humans require 13 vitamins.

Quiz check

Food provides humans with the eight _____ andthirteen _____ that they need. ____ like ____ areneeded in small or trace amounts. ____ is a ____ thatis required for bone growth.A. MacronutrientB. CalciumC. VitaminsD. MicronutrientsE. Essential amino acidsF. Iron

How do animals digest food?

• Animals digest their foodextracellularly.Food is ingested into a body cavity.

Digestive _______ are secretedinto that cavity. They break downthe food into nutrient molecules thatcan be absorbed by the cells liningthe cavity.

• Absorptive areas of the gut arecharacterized by a large surface areaproduced by extensive folding andnumerous villi (= folds) and microvilli(= cell projections).

The lining of the small intestine has extensive folds

Three-dimensional view of fold

Cross sectionof smallintestine

Muscle

FoldVilli

Fold

Villi

Bloodvessels

Muscle

Microvilli are extensions of epithelial cells in villi.Villus

Microvilli of epithelialcellsEpithelial

cells

Bloodvessels

Lacteal(lymphsystem)

F3: fig. 43.11

F2: fig. 43.14

Tissue layers of the vertebrate gut.Mucosa is secretory and absorptive.

Submucosa contains blood vessels andnerves. Muscle layers are separated by

a nerve net that controls gut movements.

How do animals avoid digesting themselves?!

• Most digestive enzymes areproduced in inactive forms orzymogens.

• When secreted in the gut, azymogen becomes activatedby an other enzyme.

• Cells lining the gut areprotected by ______ .

How does the gastrointestinal system function?

• Chemical digestion begins in the ______, where amylase issecreted with saliva and starts digestion of carbohydrates.

• The human ______ stores and breaks down ingested food.Alcohol, caffeine, aspirin... can be absorbed through the

stomach wall.• Parietal cells secrete HCl, and chief cells secrete

pepsinogen activated into pepsin by very low pH.Pepsin begins the digestion of protein.

How does the gastrointestinal system function?

• In the ___________ (3 sections), digestion of proteins andcarbohydrates continues; digestion of lipids and absorptionof nutrients begin.

• Most digestion occurs in duodenum (section 1).Entering the duodenum from the stomach, the acidic

chyme is neutralized by pancreatic bicarbonate ions.Liver and pancreas provide many enzymes and other

secretions. Bile aids in digesting lipids.• 90% nutrient absorption occur in sections 2-3.• All of the blood leaving the digestive tract flows to the liver.

Liver cells absorb the nutrients and either store them orconvert them to molecules the body needs.

How does the gastrointestinal system function?The digestive tract:

1. MouthMechanical and chemical processing(chewing reduces size of food; salivadigests carbohydrates)

2. EsophagusTransports food

3. StomachMechanical and chemical processing(digestion of proteins)

4. Small intestineChemical processing and absorption(digestion of proteins, fats, carbohydrates;absorption of nutrients and water)

5. Large intestineWater absorption and fecesformation

6. RectumHolds feces

7. AnusFeces elimination

Accessory organs:

Salivary glandsSecrete enzymes thatdigest carbohydrates;supply lubricatingmucus

LiverSecretes moleculesrequired fordigestion of fats

GallbladderStores secretionsfrom liver; emptiesinto small intestine

PancreasSecretes enzymesand other materialsinto small intestine

F3: fig. 43.5

F2: fig. 43.6

Quiz check

Food is broken down by ____ which are secreted in inactiveforms called ____. For example, ____ results from theactivation of ____ (secreted by ____) by low pH in the stomach.Low pH is caused by the release of HCl by ____. Pepsininitiates the digestion of ____ in the stomach. The digestion oflipids start in the first section of the ____ called ____.A. Chief cells E. ZymogenB. Proteins F. PesinogenC. Small intestine G. DuodenumD. Digestive enzymes H. Pepsin

I. Parietal cells

Insulin causes cells in liver tosynthesize glycogen; fat-storagecells synthesize triglycerides.

Glucoselevel falls

Glucoselevel rises

(normal glucoselevels in blood)

If glucoselevel is too high

If glucoselevel is too low

Glucagon causes cells in liver tocatabolize glycogen; fat-storagecells catabolize fatty acids.

Glucose Glycogen

Homeostasis

Glycogen Glucose

• When food is present in the gut,nutrients are absorbed.

• During the postabsorptiveperiod (stomach and smallintestine empty) energymetabolism and biosynthesismust run on internal reserves.

• The ____ directs the traffic of themolecules that fuel metabolism:glucose and fat.

• Pancreatic hormones insulin andglucagon are key players ofglucose homeostasis.

How is the flow ofnutrients regulated?

Pancreas secretesINSULIN

Pancreas secretesGLUCAGON

F3: fig. 43.16

F2: fig. 43.17

How do animals deal with ingested toxins?

• Toxins in food may come from natural sources,but many come from human activities such as the use of

pesticides and the release of pollutants into theenvironment.

• Toxins such as PCBs (polychlorinated biphenyls, used asinsulating fluid in electrical transformers) accumulate in thebodies of prey and are transferred and further concentratedin the bodies of their predators.This is bioaccumulation: produces high concentrations

of toxins in animals high up the food chain.

What roles do excretory organs play in maintaininghomeostasis?

• Excretory organs control the volume, concentration, andcomposition of the extracellular fluids of animals.

• Water enters or leaves cells by osmosis.Osmosis = movement of water across a membrane from

high water content to low water content.• Excretory organs control extracellular fluid osmolarity by

filtration, secretion, and reabsorption.Osmolarity = concentration of osmotically active particles.

• The ______ is the major excretory organ of vertebrates.• ______ is the output of excretory organs.

How do animals excrete toxic wastes fromnitrogen metabolism?

• The end products of metabolism of carbohydrates andfats are water and CO2:Not difficult to eliminate.

• Metabolism of proteins and nucleic acids also producesnitrogenous wastes.Most common form: Ammonia.Highly toxic, must be excreted, or detoxified =

converted into other molecules: urea or uric acid.

How do mammals maintain salt and waterbalance?

• The kidney is well adapted for water excretion orconservation.

• Mammals and birds have high body temperaturesand high metabolism, hence potential for high rateof water loss. In mammals and birds, kidneys have evolved a

unique adaptation: ability to reabsorb waterfrom urine.

As a result, mammals and birds can produceurine more concentrated than their extracellularfluids.

Quiz check

The main excretory organ in mammals is the ____. Excretory organscontrol the composition and concentration of extracellular fluid by threemechanisms: ______. The composition is controlled so that toxicwastes like ____ are eliminated. The concentration is controlled so that____ is minimized.A. Digestion, absorption, secretionB. Nutrition, filtration, secretionC. Filtration, secretion, reabsorptionD. HClE. Liver I. PancreasF. Ammonia J. Heat gainG. Kidney K. CO2H. Water loss L. Uric acid production

What is the structure of the kidney?

• Nephron: functional unit of the vertebrate kidney.Nephrons form a regular arrangement in the kidney.Each nephron consists of a glomerulus and tubules in

cortex, and loop of Henle in medulla.

Ureter

Kidney

Urinary system Kidney Nephron structure

Cortex

Medulla

Med

ulla

Cor

tex

NephronNephron

Renalartery

Renalvein

Ureter

Bladder

Urethra

In most nephrons,the loop of Henleis relatively shortand is located inthe cortex

In some nephronsthe loop of Henleis long and plungesinto the medulla

F3: fig. 42.10

F2: fig. 42.11

How does the mammalian kidney work?

Blood entersglomerulus.

Pre-urineleaves

GlomerulusBlood leavesglomerulus

Blood is filtered in the glomerulus.

F3: fig. 42.12

F2: fig. 42.13

Three regions Permeability

Thinascendinglimb

Descendinglimb

Thickascendinglimb

Activetransport

Passivetransport

1200

600

900

300

100

300

600

Passivetransport

Descending limb is highly permeable to waterbut impermeable to solutes

Ascending limb is impermeable to waterbut highly permeable to Na+ and Cl–

How does the mammalian kidney work?(b) Urine is concentrated by countercurrent mechanisms

involving the straight portions (limb) of loop of Henle

F3: fig. 42.15

F2: fig. 42.16

Quiz check

Blood is filtered in the ____, which is part of thefunctional unit of the kidney called ____. After filtrationpre-urine moves to the ____ in the _____ that form the____. The ____ is _____ to water and _____ tosolutes, whereas the _____ is _____ to water and_____ to Na+ and Cl-.A. impermeable E. glomerulusB. permeable F. ascending limbC. descending limb G. tubulesD. loop of Henle H. nephron

I. medulla

What mechanisms regulate kidney function?

• The kidney must maintain high glomerular filtration rateeven if blood pressure or osmolarity vary.

• When blood pressure falls, the kidney release theenzyme renin.Renin converts a protein into hormone angiotensin.Angiotensin causes constriction of blood vessels.This triggers release of hormone aldosterone,which enhances water reabsorption,and stimulates thirst.

What mechanisms regulate kidney function?

• Changes in osmolarity influence the release of antidiuretichormone (ADH)

ADH present No ADH presentDistal tubule Cortex

Collectingduct

Loop ofHenle

Solutes MedullaMedullaSolutes

Loop ofHenle

Aquaporins

Distal tubule Cortex

Collectingduct

F3: fig. 42.18

F2: fig. 42.18

ADH stimulates theproduction and controls theintracellular location ofaquaporin proteins.

With ADH, aquaporins canfuse with membrane ofcollecting duct cells andserve as water channels:the cells become morepermeable to water.

Time for a beer…

• Alcohol inhibits ADH release...This is why excessive beer drinking leads to

even more urination and dehydration, whichcontributes to hangover symptoms...

Quiz check

When blood pressure falls or osmolarity varies, ____ arereleased. Lower blood pressure triggers production of____ which causes release of ____, resulting in greater____ and ____ sensation. Change is osmolarity triggerssecretion of ____, causing ____ to fuse with _____ ofcells of ____. Greater ____ also results.

A. water reabsorption E. renin and angiotensinB. aquaporins F. ADH (antidiuretic hormone)C. hormones and enzymes G. collecting ductsD. membrane H. aldosterone

I. thirst