(i) Excretion

What is excretion?

Excretion is the removal of toxic waste products of metabolism from an organism.

What are metabolic waste products?

Substances produced by chemical reactions in cells that must be removed because they may be harmful.

Why must waste products be removed?

They can build up to toxic levels and damage cells.

What is the difference between excretion and egestion?

Excretion removes waste products made by metabolism, while egestion removes undigested food from the digestive system.

What are the main excretory organs in humans?

The lungs, kidneys and skin.

What are the excretory products of the lungs?

Carbon dioxide and water vapour.

What are the excretory products of the kidneys?

Urea, excess water and excess ions.

What are the excretory products of the skin?

Sweat containing water, ions and small amounts of urea.

Where does carbon dioxide come from in humans?

Carbon dioxide is produced during aerobic respiration as a waste product.

Where is carbon dioxide removed from the body?

It is removed by the lungs during exhalation.

Where does urea come from?

Urea is produced in the liver when excess amino acids are broken down.

Why is urea removed from the body?

Urea is toxic and high concentrations can damage cells.

Where do plants produce carbon dioxide?

Carbon dioxide is produced during respiration in plant cells.

Where do plants produce oxygen?

Oxygen is produced during photosynthesis.

Why can oxygen become a waste product in plants?

Plants may produce more oxygen during photosynthesis than they need for respiration.

How do plants remove carbon dioxide and oxygen?

Carbon dioxide and oxygen diffuse out through stomata in the leaves.

What are stomata?

Small pores in the leaf that allow gas exchange.

Why do gases leave through stomata?

They move by diffusion down a concentration gradient.

What are the two main functions of the kidney?

Excretion of waste products and osmoregulation of water content in the blood.

What is osmoregulation?

The control of the water content of the body.

Why is osmoregulation important?

It maintains suitable conditions for cells and enzymes.

What waste product does the kidney remove?

Urea.

Where does urea come from before reaching the kidney?

The liver.

How does urea reach the kidney?

It is transported dissolved in the blood plasma.

What is the urinary system?

The organ system responsible for producing, storing and removing urine.

What are the main parts of the urinary system?

Kidneys, ureters, bladder and urethra.

What is the function of the kidneys?

They filter the blood, remove urea and regulate water and ion levels.

What is the function of the ureters?

They transport urine from the kidneys to the bladder.

What is the function of the bladder?

It stores urine before removal.

What is the function of the urethra?

It carries urine out of the body.

What is a nephron?

The functional unit of the kidney where filtration and selective reabsorption occur.

Where are nephrons found?

In the kidneys.

What are the main parts of a nephron?

Bowman's capsule, glomerulus, convoluted tubules, loop of Henle and collecting duct.

What is the glomerulus?

A network of capillaries where ultrafiltration occurs.

What is Bowman's capsule?

A cup-shaped structure surrounding the glomerulus that collects filtrate.

What are convoluted tubules?

Tubes where selective reabsorption of useful substances occurs.

What is the function of the loop of Henle?

It helps control water reabsorption and urine concentration.

What is the function of the collecting duct?

It controls how much water is removed from the filtrate.

What is ultrafiltration?

The filtration of small molecules from the blood into Bowman's capsule.

Where does ultrafiltration occur?

In the glomerulus and Bowman's capsule.

Why is pressure high in the glomerulus?

The incoming arteriole is wider than the outgoing arteriole, increasing blood pressure.

How does high pressure help ultrafiltration?

It forces small molecules through the filtration membrane into Bowman's capsule.

What substances enter Bowman's capsule during ultrafiltration?

Water, glucose, amino acids, ions and urea.

What is the glomerular filtrate?

The fluid formed after ultrafiltration containing small molecules from the blood.

What substances remain in the blood after ultrafiltration?

Blood cells and large plasma proteins.

Why do proteins and blood cells stay in the blood?

They are too large to pass through the filtration membrane.

What is selective reabsorption?

The movement of useful substances from the filtrate back into the blood.

Where does selective reabsorption mainly occur?

The proximal convoluted tubule.

Why is glucose selectively reabsorbed?

Glucose is needed for respiration and energy release.

Why should glucose not be lost in urine?

It is a useful molecule needed by cells.

What happens to amino acids after filtration?

They are selectively reabsorbed into the blood.

What happens to useful ions after filtration?

They are selectively reabsorbed as needed.

How is water reabsorbed into the blood?

Water moves from the collecting duct into surrounding capillaries by osmosis.

Why is water reabsorbed?

To maintain the correct water concentration of the blood.

What hormone controls water reabsorption?

ADH (antidiuretic hormone).

Where is ADH produced?

The hypothalamus.

Where is ADH released from?

The pituitary gland.

What is the role of ADH?

ADH controls the permeability of the collecting duct and regulates water content of the blood.

What happens when blood water content is too low?

More ADH is released, causing more water reabsorption and concentrated urine.

What happens when blood water content is too high?

Less ADH is released, causing less water reabsorption and dilute urine.

How does ADH affect the collecting duct?

It makes the collecting duct more permeable to water.

Why does ADH increase water reabsorption?

More water can leave the collecting duct and return to the blood.

What is urine made of?

Water, urea and ions.

Why does urine contain urea?

Urea is a waste product that must be removed.

Why does urine contain ions?

Excess ions are removed to maintain balance.

Does healthy urine contain glucose?

No, glucose is selectively reabsorbed into the blood.

Does healthy urine contain protein or blood cells?

No, they are too large to pass through ultrafiltration.

What happens to urine after it is produced?

It moves through the ureters, is stored in the bladder and leaves through the urethra.

What is the difference between filtration and reabsorption?

Filtration moves substances from blood into the nephron; reabsorption moves useful substances back into the blood.

What are common A* mistakes about kidneys?

Thinking kidneys produce urea; urea is produced in the liver.

What are common A* mistakes about urine?

Thinking urine contains glucose; healthy urine does not contain glucose.

What are common A* mistakes about ADH?

Thinking ADH controls glucose; ADH controls water balance.

What are common A* mistakes about ultrafiltration?

Forgetting that proteins and blood cells remain in the blood.