Biology CT-1

4.13 Excretion

All dav long, even while you sleep, your body produces waste

substances. Lllnese wastes include carbon dioxide and urea.

Your body must get rid of thesæ things as they are poisonous. Waste

removal is called excretion, Your lungs excrete carbon dioxide. Your

kidneys excrete urea, a waste substance produced by protein metabolism

in lhe liver, Your kidneys are your main excretory organs —

they are towards the back of your body just above your waist.

What kidneys do

Your kidneys remove urea, water, and Other unwanted substances from

your blood.

The renal vein carries 'clean'

blood away from the kidneys.

The kidneys remove urea and

other wastes from blood and

excrete it in a liquid called

urine.

Ureters are tubes which carry

urine to the bladder.

The bladder is a bag which

stores urine until you go to the

toilet.

How kidneys Clean blood

Kidneys clean blood by filtering it. They filter all your blood 300 times a

day. The filtering is done by over a million tiny tubes packed into each

kidney, These tubes are called nephrons.

Some people's kidneys are not very good at filtering blood. Kidnev

machines help by filtering their blood for them. The little girl in the

photograph is attached to a kidney machine.

Surgeons can sometimes replace faulty kidneys with healthy ones. This

operation is called a kidney transplant.

110

Where vour kidnevs are .

The renal artery carries 'dirty'

blood to the kidneys.

The sphincter is a ring of muscle

which keeps the bladder closed

until you go to the toilet.

The urethra is a tube which

carries urine out of your body-

The parts of a kidney

'Ihis diagram shows a kidney cut

in half, with one nephron

magnified.

A nephron begins With a cup-

shaped bag containing a bunch

of capillaries called a

glomerulus. 'Dirty' blood enters

a glomerulus and is filtered.

Filtered liquid enters this tube-

shaped part of a nephron. Here

it is turned into urine.

The urine from many nephrons

drains into a collecting duct.

The urine drains into the ureter.

How a nephron works

Gomerulus

'Dirt/' blood

he diagram below shows one nephron straightened out.

'Cleaned'

Renal a'tery

Fenal veiq

unne to

bladder)

Thc liquid left in thc nephron is

urine. It contains urea and water

plus other unwanted substances.

It goes through the collecting ducts

and ureters to the bladder.

lood is filtered by the

lomerulus. Nearlv all the blood

x.xpt red cells filters through into

he nephron.

'Dirty' blood

other useful so

Nephron

Normallv, all the glucose, and as

much water and other useful

substances as the body needs, pass

back into the blood. This process is

called selective reabsorption,

because only certain substances are

selected in this way.

tJse u substances ere

rearso•hed into the 100•d

— 'C'eaned'

Ulooc

unre goes to the

hlAdder

urine

(urea and water'














5M Digestion

Your blood carries food to all parts of your body but after a meal, you

would not expect to have pieces of bacon or egg floating in it. This does

not happen because digestion breaks food down into soluble molecules

which dissolve in your blood before traveling around your body.

Food is digested inside a tube in your body called the alimentary canal,

or gut. Your alimentary canal is over seven meters long and is coiled

inside your abdominal cavity.

Where your food is digested

Digestion starts when food is

taken into the mouth. This is

called ingestion, The teeth are

used to break the food into

small pieces. The pieces are

mixed with a liquid called

saliva and swallowed.

The walls of the gut produce

chemicals called digestive

enzymes. Enzymes break down

food into liquids.

Liquid food passes through the

gut wall into the bloodstream.

This is called absorption, Blood

carries food to all parts of the

body.

Food is taken in by cells, and

used for energy, growth, and

repair. This is called

assimilation.

Dietary fiber and other things in

food which cannot be. digested

pass out through the anus when

you visit the toilet. This is called

defecation,

Substances which cannot be

digested are called feces.

The digestible parts to what you

eat are turned into soluble

molecules which can dissolve in

Food

your blood.

6

Digestive

enzymes

Digested

food

Mouth

Gut

Anus

More about digestive enzymes

Digestive enzymes change the food you eat by breaking down large food

molecules into smaller ones which can be absorbed into the blood. Each

type Of food needs a different enzyme to break it down.

Carbohydrase enzymes break down carbohydrates like starch into

glucose.

Lipase enzymes break down fats and oils into fatty acids and glycerol,

Protease enzymes break down proteins into amino acids.

This is what happens when starch is broken down by carbohyd rase

enzyme and absorbed.

Starch is a carbohydrate found

in rice, bread, and potatoes,

Starch molecules are made up of

many glucose molecules joined

together, like a string Of beads.

Carbohydrase enzymes break

down the starch by cutting it up

into separate glucose molecules,

like you might cut up a string of

beads with scissors,

Glucose molecules are so small

that they can pass through the

cells which form the gut wall,

and then through blood vessel

walls into the bloodstream.

Blood carries the glucose to the

cells of the body for

assimilation.

Other kinds of food are broken

down by other enzymes in the

same way

ENZYME

ENZYME

Starch mo.tLle

gluccse molecules

joneo together;

Carbohydrase enzymes out stard•

u. into glucose molecules

Glucose molecules

Cells Of gut

Glucose in bloodstream














5.08 Absorption, and the liver

Digestion breaks down food so it can be absorbed into your blood and

carried to all parts of your body. Carbohydrates are digested into glucose,

proteins into amino acids, and fats and oils into fatty acids and glycerol.

But vitamins, minerals. and water in food don't have to be digested since

they are already easily absorbed,

Digested foods are absoli•ed through your gut wall into the blood and

carried away. Most absorption on in your small intestine.

Inside the small intestine

Your small intestine is over six

metres long. Inside, millions of

tiny fingers called villi stick out

of its walls. One villus is about a

millimetre long,

Absorption takes place through the

villi. They provide a large surface

for absorbing digested food,

Inside a villus

A villus has a surface layer only

one cell thick. This lets digested

food pass through easily.

It has a network of capillaries

which carry blood.

It has a lymph vessel called a

lacteal which carries lymph-

here your small intestine is .

It extends down from vour waist.

Glucose, amino acids, and some

fatty acids and glycerol pass into

the blood in the capillaries.

Blood carries them to the liver,

which is described on the next

page.

Most Of the fatty acids and

glycerol pass into the lacteal,

They are carried in the lymph to

a vein in the neck. From there

they flow into the blood and are.

carried to all parts of the body.

Blood vessel carrying digested

food to the liver.

Lymph vessel carrying digested

food to the bloodstream.

What happens to digested food in the liver

Your liver is the largest organ in your body. It weighs over one kilogram

and stretches all the way across your body, just above the waist.

The liver is a chemical factory, a food store, and a central heating system.

Here are just a few of the jobs it does:

I It stores glucose as glycogen.

It changes this back to glucose

when the body needs it.

2 It stores the minerals copper and

potassium, as well as iron

needed to make red blood cells,

3 It stores vitamins A, B, and D.

4 It takes the goodness out of

unwanted amino acids, and

changes What is left into a waste

called urea. Urea is removed

from your body by your

kidneys.

5 It takes some poisons from the

blood and makes them

harmless. These poisons come

from germs, alcohol, and drugs.

6 It makes bile, which is needed

for digestion.

7 It makes fibrinogen, which is

needed for blood to clot in

wounds.

8 These and many other iObs done

in the liver produce heat, which

the blood carries around your

body to keep it warm.

Glucose, amino acids, vilamins,

minerals, and some fatty acids

and glycerol are absorbed into

the villi.

Villus

to the qut

B!ood full ct digested feod

to the liver

The liver

Food storage

MINFNA'.S 7;

Chemical

factory

Heat

production

FOOD

CHEMICALS