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Human Physiology:
Digestion
Topic 6.1, 11.3
Option H2, H3, H4
3 Dietary Categories

Herbivores
– Cattle, gorillas, snails, and sea urchins,
– eat autotrophs (plants and algae)

Carnivores
– Lions, hawks, spiders, and snakes
– Ingest other animals

Omnivores
– Crows, cockroaches, raccoons, and humans
– Ingest both plants and animals
How do animals obtain and ingest their
food?
 Suspension feeders
– Extract food particles suspended in the surrounding
water
– Ex. Clams and oysters

Substrate feeders
– Live on or in their food source and eat their way
through it.
 Ex. Caterpillars and earthworms

Fluid feeders
– Obtain food by sucking nutrient nutrient-rich fluids from
a living host, either a plant or an animal.
 Ex. Mosquitoes and ticks

Bulk feeders
– Ingest relatively large pieces of food
– Ex. most animals
Overview: Food processing

1.
Four stages
Ingestion
2.
Digestion
3.
Absorption
4.
Elimination
undigested material passes out of the digestive tract
The act of eating
The breaking down of food into molecules small enough for the body to
absorb.
Two phases:
1. Breaking food down mechanically (teethchewing) into smaller
pieces
2. hydrolysis, chemical breakdown, catalyzed by enzymes
Cells lining the digestive tract take up (absorb) the products of
digestion—small molecules such as amino acids and simple sugars
Nutrients travel through blood to cells, where they are made into
macromolecules or further broken down for energy
General compartments for digestion

Food vacuoles are the simplest digestive
compartments.
– Phagocytosis: cell engulfs food particle, which
then fuses with a lysosome.
Most animals have an alimentary canal, a
digestive tube with two openings, a mouth
and an anus.
– Allows food to move in one direction, with
specialized regions in the digestive tube that
carry out digestion and absorption of nutrients
in sequence
General compartments for digestion

Food entering the mouth usually passes
into:
– A pharynx, or throat
– Then passes into the esophagus
– Passed to stomach, muscular and churns
and grinds food
– Chemical digestion and nutrient absorption
occur mainly in the intestine
– Undigested materials are expelled though the
anus.
Human Digestive Tract

Main parts of human alimentary canal:
– Mouth, oral cavity, tongue, pharynx,
esophagus, stomach, small intestine, large
intestine, rectum, and anus.

Main digestive glands
– Salivary glands, pancreas, and liver
– Secrete digestive juices that enter the
alimentary canal through ducts.
– Secretions from liver are stored in gallbladder
before they are released into the intestine.
Human Digestive Tract

Peristalsis
– Rhythmic waves of contractions of smooth muscles in
the walls of the digestive tract
– Once food is swallowed, peristalsis propels it through
the alimentary canal.
– In only 5-10 seconds, food passes from the pharynx
down the esophagus and into the stomach.
– Pyloric sphincter, a muscular ringlike valve, keeps
food in the stomach by regulating the pass of food
into the small intestive.
 Works like a drawstring, closing off the tube and keeping
food in the stomach long enough for stomach acids and
enzymes to begin digestion.
 http://nutrition.jbpub.com/resources/animations.cfm?id=1&d
ebug=0
Human Digestive Tract
Final steps of digestion and nutrient
absorption occur in the small intestine
over a period of 5-6 hours.
 Undigested material moves slowly through
the large intestine (taking 12-24 hours),
and feces are expelled through the anus.

Digestion Begins in the Oral Cavity
Saliva is produced by the salivary glands
through ducts to the oral cavity even
before you eat; it’s a response to presence
of food
 Sight or smell of food causes nerve
stimulation
 In a typical day, salivary glands secrete
more than a liter of saliva

Digestion Begins in the Oral Cavity

Saliva contains several substances necessary for
food processing
– Slippery glycoproteins
 Protects the soft lining of the mouth and lubricates food for
easier swallowing
– Buffers
 Neutralize food acids, helping prevent tooth decay.
– Antibacterial agents
 Kill many of the bacteria that enter the mouth with food.
– Salivary amylase
 Digestive enzyme that begins hydrolyzing food
Digestion Begins in the Oral Cavity

Oral Cavity
– Mechanical and chemical digestion begins in the oral
cavity.
– Chewing cuts, smashes, and grinds food, making it
easier to swallow and exposing more food surface to
digestive enzymes
– Teeth and tongue are prominent
 Teeth grind and crush food
 Tongue, muscular organ with taste buds, allows you to taste
your meal and manipulates food and helps shape it into a
ball called a bolus.
– In swallowing, it pushed the bolus to the back of the oral cavity
and into the pharynx.
PharynxSwallowing
 Pharynx has openings for both the esophagus and the
trachea (wind-pipe)
 Most of the time, esophageal opening is closed and air
enters the trachea and proceeds to the lungs.

When you eat:
–
a bolus of food enters the pharynx, triggering the swallowing reflex
– The esophageal sphincter relaxes and allows the bolus to enter the
esophagus
– Larynx (voice box) moves upwards and tips the epliglottis over the
tracheal opening.
– Epiglottis prevents food from passing into the trachea.
– After the bolus enters the esophagus, the larynx moves downward, the
epiglottis moves up again, and breathing passage reopens
– Esphogas sphincter contracts above the bolus.
Esophagus: Peristalsis
Esophagus is a muscular tube that conveys
food boluses from the pharynx to the
stomach.
 Muscles at the very top of esophagus are
under voluntary control; thus, the act of
swallowing begins voluntarily.
 Then, Involuntary waves of contraction by
smooth muscles in the rest of the
esophagus take over.

Esophagus: Peristalsis
As food is swallowed, muscles above the
bolus contract, pushing the bolus
downward
 Simultaneously, muscles around the bolus
relax, allowing the passageway to open.
 Muscle contractions continue in waves until
the bolus enters the stomach.
 Waves of smooth muscle contraction also
move materials through small and large
intestine

Stomach: stores and breaks
down
food
 Stomachs are the main reason we do not have to eat



constantly
Highly elastic and can stretch to accommodate about 2
Liters of food and drink, usually enough to satisfy our
body’s needs for many hours.
Some digestion occurs in the stomach.
The stomach secrete gastric juice:
– made up of mucus, enzymes, and strong acid.
– Functions to break apart the cells in food
– Also kills most bacteria and other microbes that are swallowed
with food.
Stomach: stores and breaks
down food
Stomach wall is highly folded, and has pits
that lead to tubular gastric glands.
 Three types of cells that secrete different
components of the gastric juice:

– Mucous cells: secrete mucous, which
lubricates and protects the cells lining the
stomach
– Parietal cells: secrete HCl acid
– Chief cells: secrete pepsinogen, an inactive
form of the enzyme pepsin
Stomach: stores and breaks
down food

Pepsinogen, HCl, and pepsin:
– 1. Pepsinogen and HCl are secreted into the
lumen (cavity) of the stomach.
– 2. HCl converts pepsinogen to pepsin.
– 3.Pepsin then activates more pepsinogen,
starting a chain reaction. Pepsin begins the
chemical digestion of proteins. Proteins will be
further digested in small intestine.
Stomach: stores and breaks
down food

Prevention of gastric juice from digesting
away stomach lining:
– Secreting pepsin in the inactive form of
pepsinogen helps protect the cells of the
gastric glands
– mucus helps protect the stomach lining from
both pepsin and acid.
– Still, epithelium is constantly eroded; enough
new cells are generated by mitosis to replace
the stomach lining completely about every
three days.
Stomach: stores and breaks
down food

Gastric glands are regulated by a combination of
nerve signals and hormones:
– When you see, smell, or taste food, a signal from your brain to
your stomach stimulates your gastric glands to secrete gastric
juice.
– Once food is in your stomach, substances in the food stimulate
cells in the stomach wall to release the hormone gastrin in the
circulatory system.
– Gastrin circulates in the blood stream, returning to the stomach
wall, stimulating further secretion of gastric juice.
– As much as 3L of gastric juice may be secreted a day.
– A negative-feedback mechanism inhibits secretion of gastric
juice when the stomach contents become too acidic.
 Acid inhibits the release of gastrin, and with less gastrin in the blood, the
gastric glands secrete less gastric juice.
Stomach: stores and breaks
down food



About every 20 seconds, the stomach contents are
mixed by the churning action of muscle in the stomach
wall and result in acid chyme.
Opening between the esophagus and the stomach is
usually closed until a bolus arrives.
Backflow of acid chyme causes heartburn (which should
be called esophagus-burn)
– Can also cause acid-reflux (gastroesophageal reflux
disease; GERD)
Stomach: stores and breaks
down food




Pyloric sphincter helps regulate the passage of acid
chyme from the stomach into the small intestine.
The stomach takes about 2-6 hours to empty after a
meal; acid chyme leaves stomach only a squirt at a time.
Acid chyme rich in fats stimulates the small intestine to
release a hormone that slows the emptying of the
stomach, providing more time for digestion.
Other hormones secreted by the small intestine influence
the release of digestive juices from the pancreas and gall
bladder.

Gastric Ulcers
Gastric Ulcers:
– Open sores that form when mucus, which normally
protects the stomach wall from the corrosive effect of
digestive juice, fails to protect it.
 Small intestine and esophagus are also susceptible to ulcers
– Symptoms are usually gnawing pain in the upper
abdomen, which may occur a few hours after eating.
– Were formerly thought to result from the production of
too much pepsin/and or acid or too little mucus:
 For years, the blame was put on factors that cause
these effects, such as aspirin, ibuprofen, smoking,
alcohol, coffee, and stress
 However, strong evidence now points to…

H. pylori
–
–
–
–
–
–
–
–
–
Gastric Ulcers
A spiral-shaped bacteria
Low pH of the stomach kills most microbes, but not this one!
Burrows beneath mucus and releases harmful chemicals
Growth seems to result in a localized loss of protective mucus
and damage to the cells lining the stomach
WBC fight infection, causing mild inflammation of the stomach,
called gastritis.
Gastric ulcers form when pepsin and HCl destroy cells faster than
the cell can regenerate from the H. pylori attack.
Eventually, stomach will erode to the pint where it actually has a
hole in it, which can lead to a life threatening infection in
abdomen or internal bleeding.
70-90% of ulcer and gastritis sufferers have this bacterial
infection
Also found in 30% of healthy people. linked to certain kinds of
stomach cancer
Gastric Ulcers

Treatment
– Usually respond to a combination of antibiotics and bismuth (the active ingredients of
Pepto-Bismol) which eliminates bacteria and
promotes healing.
– Drugs that reduce stomach acidity may also
help, and researchers are working on
preventitive treatment for H.Pylori.
Small Intestine
Once at the S.I., food has been mechanically reduced to
smaller pieces and mixed with juices; it now resembles a
thick soup.
 Starch digestion began in the mouth, and protein
breakdown began in the stomach.
 All other chemical digestion occurs in the s.i.
 Nutrients are also absorbed into the blood from the s.i.
 Length of over 6m, making it the longest organ of the
alimentary canal. Diameter is only about 2cm, which is
why it’s called the “small” intestine.

Small Intestine
Contributing to digestion in s.i. are two
large glandular organs: pancreas and liver.
 Pancreas:

– Produces pancreatic juice
 a mixture of digestive enzymes and an alkaline
solution rich in bicarbonate
 Alkaline solution neutralizes acid chyme as it
enters the small intestine
Small Intestine

Liver
– Performs a wide variety of functions, including
the production of bile:
 Contains bile salts that emulsify fats, making them
more susceptible to attack by digestive enzymes.
 Gall bladder stores bile until it is needed in the
small intestine.
Small Intestine

First 25 cm or so of the s.i. is called the
duodenum.
– Where acid chyme squirted from the stomach
mixes with bile from the gall bladder,
pancreatic juice from the pancreas, and
digestive enzymes from gland cells in the in
the intestinal wall.
Small Intestine
All four types of macromolecules
(carbohydrates, proteins, nucleic acids,
and fats) are digested.
 Refer to table 21.11 on pg. 438

Small Intestine

Carbohydrate digestion:
– Begins in the oral cavity and is completed in
the s.i.
– Pancreatic amylase hydrolyzes starch (a
polysaccharide) into the disaccharide maltose
– Maltose is then hydrolyzed into glucose via
maltase.
– Sucrase hydrolyzes table sugar and lactase
digests milk sugar (lactose)

Small Intestine
Protein digestion:
– S.i. completes protein digestion from the stomach
– Pancreas and duodenum secrete hydrolytic enzymes
that completely dismantle polypeptides into amino
acids.
 dipeptidase
– Hydrolyzes fragments only two or three amino acids long.
 trypsin and chymotripsin
– Break polypeptides into smaller polypeptides
 aminopeptidase and carboxypeptidase
– Split off one amino acid at a time, working from both ends of a
polypeptide.
Small Intestine

Nucleic acid digestion:
– Nucleases hydrolyzes the nucleic acids in
food.
 From the pancreas
 Split DNA and RNA (which are present in the cells
of food items) into their component nucleotides,
which are then broken down into nitrogenous
bases, sugars, and phosphates by other enzymes
produced by the duodenal cells.
Small Intestine

Fat digestion:
– Most fat remains undigested until it reaches the
duodenum.
– Hydrolysis of fats is problematic due to fats
insolubility in water. Emulsification= Problem
Solved!!!
 Bile salts in bile cause fat globules to be physically broken up
into smaller fat droplets, a process called emulsification.
 Many small droplets allows for a larger surface area of fat
exposed to lipase, an enzyme that breaks fat molecules
down into fatty acids and glycerol.
Small Intestine
By the time persistalsis has moved the
mixture of chyme and digestive juices
through the duodenum, chemical digestion
of your meal is just about complete.
 Main function of the rest of the small
intestine is the absorption of nutrients and
water.

Small Intestine

Structurally, great for nutrient absorption.
– Lining has a huge surface area– roughly 300 m2,
about the size of a tennis court
– Extensive surface area results from several kinds of
folds and projections.
 Villi: large circular folds with numerous, small fingerlike
projections around the inner wall of the s.i.
 Microvilli: many tiny surface projections found on epithelial
cells lining a villus.
– extend into the lumen of the intestine and greatly increase the
surface area across which nutrients are absorbed.
Small Intestine
Some nutrients are absorbed via simple
diffusion; other nutrients are pumped against
concentration gradients into the epithelial cells
 The core of each villus is penetrated by a small
lymph vessel and a network of capillaries.

– After fatty acids and glycerol are absorbed by an
epithelial cell, these building blocks are recombined
into fats and are transported into the lymph vessel.
– Amino acids and sugars pass out of the intestinal
epithelium and then across the thin walls of the
capillaries into the blood.

Small Intestine
Capillaries that drain nutrients away from the villi
converge into larger veins and eventually into a
main vessel, the hepatic portal vein, that leads
directly to the liver:
– Liver gets first access to nutrients absorbed from a
meal
– Converts many nutrients into new substances that the
body needs.
– One of its main functions is to remove excess glucose
from the blood and convert it to glycogen, which is
stored in liver cells.
– From the liver, blood travels to the heart, which
pumps the blood and the nutrients it contains to all
parts of the body.
Large Intestine
Also known as the colon
 1.5 m long and 5 cm in diameter
 It joins the small intestine at a T-shaped
junction, where a sphincter controls the
passage of unabsorbed food material out
of the small intestine.

– One arm of the T is a blind pouch called the
cecum.
 A small, fingerlike extension of the cecum is the
appendix: contains a mass of w.b.c’s that make a
minor contribution to immunity.
Large Intestine
Main function is to absorb water from the
alimentary canal.
 About 7L of fluid enters the lumen of the
digestive tract each day as the solvent of
the various digestive juices.
 About 90% of this water is absorbed back
into the blood and tissue fluids, with the
small intestine reclaiming most of it and
the colon finishing the job.

Large Intestine
As water gets absorbed, remains of the
digested food become more solid as they
are moved along the colon via peristalsis.
 Remains make feces, the waste products
of digestion, which consist mainly of
indigestible plant fibers

Large Intestine

Surprise, surprise!
– Bacteria, such as E.coli, live in your colon!
– Produce important vitamins (biotin, folic acid,
B vitamins, and Vitamin K) that are absorbed
into the bloodstream through the colon
Large Intestine
Rectum: terminal portion of the colon
where feces are stored until they can be
eliminated.
 Strong contractions of the colon create the
urge to defecate.
 Two rectal sphincters, one voluntary and
one involuntary, regulate the opening of
the anus.


Diarrhea
Large Intestine
– If the lining of the colon is irritated by a viral
or bacterial infection, the colon is less
effective in reclaiming water

Constipation
– Peristalsis moves the feces along too slowly;
the colon reabsorbs too much water, and the
feces become too compacted.
– Usually caused by a diet with not enough
fiber or from a lack of exercise.
Digestion Animation

http://highered.mcgrawhill.com/sites/0072495855/student_view0/
chapter26/animation__organs_of_digestio
n.html
Liver
Liver performs more functions than any
other organ in the body.
 After breaking down a.a. for energy or for
recycling into other molecules, the liver
prepares nitrogenous wastes (ammonia)
for disposal synthesizing urea.
 It also converts toxins such as alcohol and
other drugs into inactive products that can
be excreted in urine.

Liver

Liver can synthesize bile for digestion of
fats, as well as proteins essential to many
of the body’s functions
– Proteins made in the liver include plasma
proteins important in blood clotting and in
maintaining osmotic balance of the blood, as
well as lipoproteins that transport fats and
cholesterol to body tissues.
Liver

Regulates the amount of glucose in blood.
– Converts glucose into glycogen, which it
stores for later use.
– Plays a key role in body metabolism
Liver
Strategically located between the
intestines and the heart
 Capillaries from the si and li converge into
vessels that lead into the hepatic portal
vein.

– Transports nutrients absorbed by the
intestines directly to the liver.
– Gives the liver the opportunity to modify and
detoxify substances absorbed by the digestive
tract before the heart (and then the rest of
the body)
Liver
Liver

Liver
Alcohol consumption
– Under heavy, frequent exposure to alcohol,
damage may become so severe that abnormal
scar tissue forms in the liver called cirrhosis
 Distorts the liver’s internal structure
 Scar tissue slowly replaces functioning liver tissue,
gradually diminishing blood flow through the liver.
 Usually causes a build-up of toxins in blood,
excessive bruising or bleeding, diabetes, and
cancer.
 About 25,000 Americans die each year from
alcohol-related cirrhosis.
Liver

Cirrhosis treatment
– Stop drinking!
– Liver transplant
– To prevent cirrhosis, U.S. guidelines advise
not more than two alcoholic drinks for men
and 2 drink for women per day.
Kidney
Main processing centers of our excretory
system are the two kidneys.
 Each is a compact organ, about the size of
your fist, nearly filled with about 80 km of
fine tubes (tubules) and an intricate
network of blood capillaries
 Body contains about 5L of blood, which
circulates repeatedly, allowing for about
1,100-2,000 L to pass though the
capillaries in our kidneys every day.

Kidney

From this enormous (1,100-2000 L) traffic
of blood, our kidneys extract daily about
180L of filtrate.
– Filtrate is made of water, urea, and a number
of valuable solutes including glucose, amino
acids, ions, and vitamins.
– If we excreted all the filtrate as urine, we
would lose vital nutrients and dehydrate
rapidly.
– But our kidneys refine the filtrate,
concentrating the urea and returning most of
the water and solutes to the blood.
Kidneys
Kidneys
In a typical day, we excrete only about 1.5
L of urine.
 “Plumbing” plan of kidneys:

– 1. Blood to be filtered enters each kidney via a renal
artery
– 2. Blood that has been filtered leaves the kidney in
the renal vein
– 3. urine leaves each kidney via a duct called a ureter
and passes into the urinary bladder.
– 4. periodically, the bladder empties during urination
via a tube called the urethra, which empties near the
female vagina or through the male penis
Kidney

Has two main regions:
– Renal cortex (outer layer)
– Renal medulla (inner region)
– Urine flows into a chamber called the renal
pelvis, and from there into the ureter.
Kidney

Nephron
Kidney
– Tiny functional units of the kidney
– About a million
– Consists of a nephron tubule and its
associated blood vessels.
– Extracts a tiny amount of filtrate from the
blood and then refines the filtrate into a
much smaller quantity of urine.
– Each nephron starts and ends in the kidney’s
cortex; some extend into the medulla
Kidney

Nephron (continued):
– Receiving end of the nephron is a cup-shaped
swelling called the Bowman’s capsule.
– At the other end of the nephron is the collecting
duct, which carries urine to the renal pelvis.
– Bowman’s capsule envelops a ball of capillaries
called the glomerulus which together make up the
blood-filtering unit of the nephron.
 1. Here, blood pressure forces water and solutes from the
blood in the glomerular capillaries across the wall of
Bowman’s capsule and into the nephron tubule.
 2. This process creates filtrate, leaving blood cells and large
molecules such as plasma proteins behind in the capillaries.
Kidney
 Nephron (continued)
– Rest of the nephron refines the filtrate. The
tubule has three sections:
 1. proximal tubule
– in the cortex
 2. Loop of Henle
– A hairpin loop carrying filtrate toward –in some cases,
into—the medula and then back toward the cortex
 3. Distal tubule
– A. Called distal because it is the most distant from
Bowman’s capsule
– B. Drains into a collecting duct, which receives filtrate
from many nephrons
– C. in the kidney’s many collecting ducts, the processed
filtrate, urine, passes into the renal pelvis and then into the
ureter.
Nephron
Kidney animation

http://www.sumanasinc.com/webcontent/
animations/content/kidney.html
Kidney

Nephron (continued):
– The intricate association between blood vessels
and tubules is key to nephron function
– Two key networks of capillaries
 1.Glomerulus
– A finely divided portion of an arteriole that branches from the
renal artery.
– Leaving the Glomerulus, the arteriole carries blood to:
 2.Proximal and distal tubules
– This second network functions with the tubules in refining the
filtrate; some of the vessels parallel the loop of Henle, with blood
flowing down in one vessel and back up through another.
Leaving the nephron, the capillaries converge to form
a venule leading toward the renal vein.
Kidney
Kidney

Excretory system produces and disposes
of urine in four major processes:
– Filtration
 Occurs when water and virtually all other
molecules small enough to be forced through the
capillary wall enter the nephron tubule from the
glomerulus.
– Reabsorption
 Refines filtrate; water and valuable solutes,
including glucose, salt, other ions, and amino acids
are returned to the blood from the filtrate.
– Secretion
Kidney
 Also refines the filtrate; substances in the blood are transported into
the filtrate.
– For example, when there is an excess of H+ in the blood, these
ions are secreted into the filtrate, thus keeping the blood from
becoming acidic
 Eliminates certain drugs and other toxic substances from the blood
***both reabsorption and secretion, water and solutes move between
the tubule and capillaries by passing through the interstitial fluid.
– Excretion
 Urine, the product of filtration, reabsorption, and secretion, passes
through the kidneys to the outside via the ureters, urinary bladder,
and urethra.
Kidney

Reabsorption and Secretion in a nephron
Figure 25.11:
– pink arrows show reabsorption , which may
occur via active transport, passive diffusion, or
osmosis.
– Blue arrows show secretion
– Colored area shows the interstitial fluid,
through which solutes and water move
between the tubules and capillaries
 Intensity of color reflects solute conc.; cortex has lowest conc.
and medulla has highest conc.
 Solute conc. of the interstitial fluid exceeds that of the
filtrate water moves by osmosis out of filtrate into interstitial
fluid
Kidney

Reabsorption and Secretion in a nephron
Figure 25.11 (continued):
– Proximal and distal tubule function:
 Proximal tubule actively transports nutrients such as glucose
and amino acids from the filtrate into the interstitial fluid, to
be reabsorbed into the capillaries.
 NaCl is reabsorbed from both proximal and distal, and water
follows by osmosis.
 Secretion of H+ and reabsorption of HCO3- also occur in both
proximal and distal, helping to regulate the blood’s pH.
 Potassium conc. in blood is regulated by secretion of excess
K+ into the distal tubule.
 Drugs and poisons processed in the liver are secreted into the
proximal tubule.
Kidney

Reabsorption and Secretion in a nephron
Figure 25.11 (continued):
– Loop of Henle and collecting duct
function:
 Major function is water reabsorption
 Long loop of Henle carries filtrate deep into the medulla and
then back to the cortex
 Presence of NaCl and some urea in interstitial fluid in the
medulla maintains the high conc. gradient that increases
water reabsorption by osmosis.
 As soon as water passes into interstitial fluid, it moves into
nearby blood capillaries and is carried away. This prompt
removal is essential to avoid diluting the interstitial fluid and
destroying the conc. Gradient necessary for water
reabsorption.
Kidney

Reabsorption and Secretion in a nephron
Figure 25.11 (continued):
– Loop of Henle and collecting duct
function:
 water reabsorption stops at hairpin because the
tubule there is impermeable to water.
 As filtrate moves back toward the cortex, NaCl
leaves the filtrate, first passively and then actively as
the cells of the tubule pump NaCl into the interstitial
fluid.
– it is primarily this movement of salt that creates the solute
gradient in the interstitial fluid of the medulla.
Kidney

Reabsorption and Secretion in a nephron
Figure 25.11 (continued):
– Loop of Henle and collecting duct
function:
 Final refining of the filtrate occurs in the collecting duct.
 By actively reabsorbing NaCl, the collecting duct is important
in determining how much salt is excreted in the urine.
 In the inner medulla, the collecting duct becomes permeable
to urea and some leaks out, adding to the high conc. gradient
in the interstitial fluid.
 As filtrate moves through medulla, more water is reabsorbed
before the urine passes into the renal pelvis.
Kidney

Reabsorption and Secretion in a nephron
Figure 25.11 (continued):
– In sum, the nephron returns much of the
water that filters into it from the blood.
– Water conservation is one of the major
functions of the kidney
– Maintain a precise and essential balance btw
water and solutes in our body fluids.
Kidney
– Antidiuretic hormone (ADH)
 Increased in the blood by a control center in the
brain when the solute concentration rises above a set
point.
 Signals nephrons to step up water reabsorption.
 When the solute conc. Is diluted below the set point,
as when we drink a lot of water, blood levels of ADH
drop and water reabsorption is reduced, resulting in
an increased discharge of dilute urine.
 Alcohol inhibits the release of ADH and can cause
excessive urinary water loss and dehydration, which
may account for the symptoms of a hangover.
Kidney

Dialysis
– A person can survive with only one
functioning kidney, but if both kidneys fail,
the build up of toxic wastes and the lack of
regulation of blood pressure, pH, and ion
concentrations will lead to certain death if
untreated.
– Over 60% of kidney disease cases are caused
by hypertension and diabetes, but prolonged
use of pain relievers, alcohol, and other drugs
are also possible causes
Kidney

Dialysis– How it works
– Dialysis means “separation” in Greek
– Like the nephron, the machine sorts small molecules of blood,
keeping some and discarding others.
– Patient’s blood is pumped from an artery through a series of tubes
made of a selectively permeable membrane.
– The tubes are immersed in a dialyzing solution much like the
interstitial fluid that bathes the nephrons.
– As the blood circulates through the tubing, urea and excess ions
diffuse out.
– Needed substances, such as biocarbonate ions, diffuse from the
dialyzing solution into the blood
– Machine continually discards the used dialyzing solution as wastes
build up.
Kidney

Dialysis
– Although life sustaining, it’s costly and time
consuming (three times a week for 4-6 hours
at a time)
– Also requires severe dietary and lifestyle
restrictions
– Waiting list for kidney transplants ,
unfortunately, is quite long.