Excretion in humans

Question 1

What is excretion?

Answer 1

theremoval of the waste substances of metabolic reactions (the chemical reactions that take place inside cells), toxic materials and substances in excess of requirements

Question 2

What do lungs excrete and why must it be excreted?

Answer 2

• Lungs excrete carbon dioxide during exhalation because it is a waste product of aerobic respiration
• Carbon dioxidemust be excreted as it dissolves in water easily to form an acidic solution which canlower the pH of cells. This canreduce the activity of enzymesin the body which are essential for controlling the rate of metabolic reactions

Question 3

What do kidneys excrete and why?

Answer 3

• Kidneys excrete urea and excess water and ions by producing urine
• Urea is a toxic waste product and must be excreted before it reaches high concentrations in the blood

Question 4

What are kidneys?

Answer 4

Two bean shaped organs that filter the blood

Question 5

What is a ureter?

Answer 5

Tube that transports urine from kidneys to bladder

Question 6

What is a bladder?

Answer 6

Organ that stores urine(excess water, salts and urea) before urination

Question 7

What is the urethrea?

Answer 7

Tube that connects the bladder to the exterior, facilitates the removal of urine from the body

Question 8

What are the two main functions of the kidneys?

Answer 8

• Maintaining water and ion balance in the body (vital for maintaining blood pressure)
• Excrete toxic waste products of metabolism(filter urea out of blood) and substances in excess of requirements (such as salts)

Question 9

How do kidneys adjust water in the blood?

Answer 9

Too little water in blood

1. Hypothalamus detects and pituitary gland releases ADH
2. More ADH enters kidney (meaning more water is reabsorbed)
3. So less urine which is more concentrated is produced

Too much water in blood

1. Hypothalamus detects and pituitary gland releases less ADH
2. Less ADH enters kidney (meaning less water is reabsorbed)
3. So more urine which is less concentrated is produced

Question 10

What are the main roles of the kidney?

Answer 10

• Removal of urea from the blood
  Proteins can’t be stored by the body so excess amino acids are broken down by the liver (process called deamination), the waste product is urea and it is filtered out by kidneys.
• Adjustment of ions (salts) in the blood
  Ions such as sodium are taken into the body in food. Excess ions are removed by the kidneys
• Adjustment of water in the blood

Question 11

What is the cortex?

Answer 11

Blood is filtered here by nephrons

Question 12

What is the renal vein?

Answer 12

Transports filtered blood back to heart

Question 13

What is the renal artery?

Answer 13

Transports unfiltered blood from aorta to kidney

Question 14

What is the pelvis function?

Answer 14

Urine collects here before passing to ureter

Question 15

What is the medulla function?

Answer 15

Reabsorption of water to maintain water balance

Question 16

What are nephrons?

Answer 16

• Each kidney contains around a million tiny structures callednephrons, also known askidney tubulesorrenal tubules. The nephrons start in thecortexof the kidney, loop down into themedullaand back up to the cortex
• The contents of the nephrons drain into the innermost part of the kidney and theurine collectsthere before it flows into theureterto be carried to thebladderfor storage

Question 17

What are the functions of the nephron?

Answer 17

1. Ultrafiltration
2. Selective reabsorption

Question 18

How does ultrafiltration work?

Answer 18

The glomerulus is a knot of blood vessels surrounded by the renal capsule. The capillaries getnarroweras they get further into the glomerulus whichincreases the pressureon the blood moving through them (which is already at high pressure because it is coming directly fromthe renal artery which is connected to theaorta). This forces water, ions, urea and glucose to beforced out of the capillaries and into the Bowman’s capsule, where they form what is known as the filtrate (protein and RBC are too large to be filtered)

Question 19

How does selective absorption work?

Answer 19

After the glomerular filtrate enters the Bowman’s Capsule,glucoseis the first substance to be reabsorbed at therenal tubule. This takes place byactive transport and the nephron is adapted for this by havingmany mitochondriato provide energy for the active transport of glucose molecules. Reabsorption of glucosecannot take place anywhere else in the nephronas the gates that facilitate the active transport of glucose are only found in the renal tubule. As the filtrate drips through theLoop of Henlenecessary salts are reabsorbed back into the blood bydiffusionandactive transport and as salts are reabsorbed back into the blood,waterfollows byosmosis. Water is also reabsorbed from the collecting duct in different amounts depending on how much water the body needs at that time. The final urine contains urea, excess water and excess ions The collecting ducts gather urine from multiple nephrons and deliver it to the renal pelvis, leading to the ureter.

Question 20

Why do doctors check for glucose in people with diabetes urine?

Answer 20

In a person with a normal blood glucose level, there are enough gates present to remove all of the glucose from the filtrate back into the blood. People withdiabetescannot control their blood glucose levels and they are often very high, meaning that not all of the glucose filtered out can be reabsorbed into the blood in the proximal convoluted tubule. As there is nowhere else for the glucose to be reabsorbed, it continues in the filtrate andends up in the urine. This is why one of the first tests a doctor may do to check if someone is diabetic is to test their urine for the presence of glucose

Question 21

What is absorbed back from urine in normal people?

Answer 21

In normal people all of the glucose is reabsorbed, some ions are reabsorbed and most water is absorbed back into the bloodstream

Question 22

What is deamination?

Answer 22

Proteins can’t be stored by the body so excess amino acids are broken down by the liver in a process called deamination. Deamination is the removal of the nitrogen-containing part of amino acids to form urea
- Enzymes in the liver split up the amino acid molecules. The part of the molecule which containscarbonis turned intoglycogenand stored. The other part, which containsnitrogen, is turned intoammonia, which is highly toxic, and so is immediately converted intourea, which is less toxic. The urea dissolves in the blood and is taken to thekidneyto be excreted. A small amount is also excreted insweat

Question 23

What is the importance of excretion?

Answer 23

Excretion is important becauseit helps to maintain the body’s composition of the fluids and prevent toxic substances from building up and killing the body.

The toxic consequences of high urea levels, if it is not excreted effectively, are very serious:
- Cell death
- Reduced response to insulin, leading to diabetes
- Deposits inside blood vessels