B5.1.2

What is the eye?

A sensory organ containing receptors sensitive to light intensity and colour

What type of action does the eye allow in response to light levels?

Unconscious reflex action

What parts make up the eye?

Retina, optic nerve, sclera, cornea, iris, pupil, lens, ciliary muscles and suspensory ligaments

What does the retina contain?

Rod and cone cells that are sensitive to light

What are rod cells sensitive to?

Low levels of light

What do cone cells allow us to see?

Colour

What does the optic nerve do?

Transmits electrical impulses from the eye to the brain

What is the sclera?

The tough, white outer layer of the eye that protects it from injury

What does the cornea do?

Covers the eye and causes light to refract as it enters

What is the iris?

The coloured part of the eye containing circular and radial muscles

What do the muscles in the iris do?

Control the size of the pupil

What is the function of the pupil?

Allows light to enter the eye

What does the lens do?

Refracts light rays so they focus onto the retina

What type of lens is the eye’s lens?

Convex

What are the ciliary muscles connected to?

Suspensory ligaments

What do the ciliary muscles do?

Contract or relax to change the shape of the lens

What do the suspensory ligaments do?

Loosen or tighten to change the shape of the lens

What is accommodation?

The process that allows the eye to focus on near or distant objects and adapt to dim light

What happens when viewing a near object?

Ciliary muscles contract, suspensory ligaments loosen, lens thickens and becomes more convergent

What happens when viewing a distant object?

Ciliary muscles relax, suspensory ligaments tighten, lens becomes thinner and less convergent

What happens when adapting to dim light?

Circular muscles relax, radial muscles contract, pupil dilates

What happens when adapting to bright light?

Circular muscles contract, radial muscles relax, pupil constricts

Why does the pupil constrict in bright light?

To protect the retina from damage

What causes blurred vision?

Light rays not focusing on the retina

What is myopia?

Short-sightedness where distant objects appear out of focus

What is hyperopia?

Long-sightedness where close objects appear out of focus

What can spectacle lenses do?

Refract light rays so they focus on the retina

What does laser surgery do for vision?

Changes the shape of the cornea to remove the need for glasses

What can contact lenses do?

Produce a clear image on the retina

What temperature does the human body maintain for enzymes to work best?

Around 37 ºC

What monitors and controls body temperature?

The thermoregulatory centre in the brain

How is body temperature regulated?

By a negative feedback mechanism

What is the thermoregulatory centre sensitive to?

The temperature of the blood

What does the skin contain?

Temperature receptors that send impulses to the thermoregulatory centre

What happens when body temperature is too high?

Blood vessels dilate and sweat is produced

What is vasodilation?

The dilation of blood vessels to help cool the body

How does sweating reduce body temperature?

By transferring energy from the skin to the environment

What happens when body temperature is too low?

Blood vessels constrict, sweating stops and muscles shiver

What is vasoconstriction?

The constriction of blood vessels to retain body heat

What causes shivering?

Contraction of skeletal muscles to increase body temperature

What role do skin hairs play in temperature control?

They lie flat when warm and rise when cold

What causes hairs to rise when cold?

Nerve impulses to hair erector muscles

How do raised hairs help keep the body warm?

They trap a layer of insulating air next to the skin

What happens to cells when water concentration is equal inside and outside?

They remain in their normal state

What happens when water concentration is higher outside the cell?

Water enters the cell by osmosis and may cause it to burst

What happens when water concentration is lower outside the cell?

Water leaves the cell by osmosis and may cause it to shrivel

How does water leave the body during breathing?

Via the lungs during exhalation

What is lost from the skin in sweat?

Water, ions and urea

Is the loss of water, ions or urea by the lungs or skin controlled?

No

How are excess water, ions and urea removed from the body?

Via the kidneys in the urine

What happens to excess amino acids from protein digestion?

They are excreted because the body cannot store them

What happens to amino acids in the liver?

They are deaminated to form ammonia

Why is ammonia converted to urea?

Because ammonia is toxic

How does urea reach the kidneys?

It is released into the bloodstream from the liver and transported to the kidneys

What system are the kidneys part of?

The urinary or excretory system

How is blood transported to the kidneys?

Via the renal artery

How do kidneys produce urine?

By filtration of blood and selective reabsorption

What substances are reabsorbed by the kidneys?

Glucose, some ions and water

What does urine contain?

Water, urea and salts

Where is urea produced?

In the liver when excess amino acids are broken down

Is urea reabsorbed by the kidneys?

No

What is the main waste product in urine?

Urea

Where does blood go after purification in the kidneys?

Back to the circulatory system via the renal vein

How is urine transported from the kidneys to the bladder?

By the ureters

Where is urine stored?

In the bladder

How is urine released from the body?

Through the urethra

How many filtering units are in each kidney?

Over one million

What is each filtering unit in the kidney called?

A nephron

What does each nephron contain?

A tubule

What are the three stages in kidney function?

Filtration, selective reabsorption and formation of urine

What happens during ultrafiltration in the nephron?

Blood is filtered under high pressure and small molecules pass into the tubule while large molecules remain in the blood

What occurs during selective reabsorption in the kidneys?

Useful molecules are reabsorbed while useless ones pass out in urine

Which molecules are completely reabsorbed by the kidneys?

Glucose

How is water reabsorbed in the kidneys?

At a rate that maintains constant blood water levels

How are mineral ions reabsorbed in the kidneys?

At a rate that maintains blood ion levels

What happens to molecules not reabsorbed during urine formation?

They continue through the tubule as urine and pass to the bladder

Which hormone controls water level in the body?

ADH

Where is ADH released from?

The pituitary gland

What effect does ADH have on the kidney tubules?

It increases their permeability causing more water to be reabsorbed

How does the body respond when blood water levels are too low?

ADH is released causing more water reabsorption and producing a smaller volume of concentrated urine

What happens when blood water levels are too high?

Less ADH is produced causing less water reabsorption and more dilute urine

What are the kidneys responsible for?

Removing waste products from the blood

What can happen if the kidneys are damaged or diseased?

Poisonous waste products can build up in the blood

Can humans survive with only one kidney?

Yes

How can kidney failure be treated?

By organ transplant or kidney dialysis

What does dialysis do?

Acts as an artificial kidney to remove urea and waste products and restore water and ion balance

How is blood prepared before entering a dialysis machine?

It is mixed with blood thinners to prevent clotting

How does dialysis fluid flow relative to the blood?

In the opposite direction

Why does urea leave the blood during dialysis?

Because dialysis fluid contains no urea creating a concentration gradient

Why does glucose not leave the blood during dialysis?

Dialysis fluid contains glucose at similar concentration to blood

How do ions move during dialysis?

Ions flow into the blood if lacking or have no net movement if levels are normal

What are disadvantages of dialysis?

It is expensive, time-consuming, requires a rigid diet, and is temporary

What are advantages of dialysis?

Allows kidney failure patients to remain healthy and maintains blood balance

What is another treatment for kidney failure besides dialysis?

Kidney transplant

What is the main advantage of a kidney transplant?

It allows a normal lifestyle without frequent hospital visits

What risk is involved with kidney transplants?

Risk of organ rejection by the immune system

How can the risk of organ rejection be reduced?

By tissue-typing for similar antigens

What medication is taken after a transplant to prevent rejection?

Immuno-suppressant drugs

What risk do immuno-suppressant drugs pose?

Increased vulnerability to pathogens

What do plant hormones coordinate and control?

Growth and responses to light and gravity