Excretion

Question 1

Define excretion

Answer 1

The removal of toxic materials and metabolic waste products of metabolism from the body, via excretory organs

Question 2

What are the toxic materials and metabolic waste products of metabolism to be removed from the body?

Answer 2

Nitrogenous and other compounds need to be removed (eg ammonia, urea, and uric acid)

Question 3

What are the kidneys?

Answer 3

Specialised organs that remove the toxic waste product urea from the circulating blood and carry out excretion
Regulate the water content of the blood and carry out osmoregulation

Question 4

Where do the kidneys receive and return blood to and from? What do they do to the blood?

Answer 4

Receive blood through the renal artery, return blood through the renal vein
The kidneys remove urea and some water and mineral salts from the blood

Question 5

What produces urine and where does it go?

Answer 5

The kidneys produce urine constantly and passes to the urinary bladder via the ureter

Question 6

Describe the structure of the kidney

Answer 6

Renal cortex: outer portion of the kidney
Renal medulla: inner portion of the kidney which consists of a number of renal pyramids
Renal pelvis: funnel shaped chamber where urine flows from collecting ducts to the ureter

Question 7

What are the structural features in the renal artery and renal vein?

Answer 7

Renal artery has thicker and more muscular walls of vessel and semi-lunar valves are absent

Renal vein has thinner and less muscular walls of the vessel and semi-lunar valves are present

Question 8

Difference between the blood in the renal artery and renal vein

Answer 8

Blood in the renal vein (after passing through kidneys) has

Less oxygen and glucose: used up for aerobic respiration
More carbon dioxide: end product of aerobic respiration
Less urea: metabolic waste product removed from blood during the formation of urine

Question 9

What is the functioning unit of the kidney and what is its function

Answer 9

A nephron

Ultrafiltration, selective reabsorption, secretion

Question 10

Describe the path of blood through the kidneys

Answer 10

Blood from the renal artery enters the afferent arteriole, containing water, useful molecules and waste materials
Is filtered under high pressure at the Bowman’s capsule though ultrafiltration, allowing waste and small molecules like glucose amino acids, mineral ions and water are filtered into the nephron
At the proximal convoluted tubule, useful molecules are selectively reabsorbed
Enters the efferent arteriole, with a smaller diameter than the afferent arteriole, creating high hydrostatic pressure in the glomerulus
Passes out to the renal venule containing useful molecules and water but no waste

Question 11

Describe the path of urine through the kidneys

Answer 11

Through ultrafiltration at the Bowman’s capsule, waste and small molecules are filtered out
Useful molecules like glucose and amino acids are selectively reabsorbed
At the loop of Henle, water is reabsorbed
At the distal convoluted tubule, secretion occurs which allows potassium, hydrogen and ammonium ions to be secreted in the filtrate to regulate its pH
Under the influence of anti-diuretic hormone (ADH), the walls of the collecting duct can become more and less permeable, controlling the amount of water reabsorbed
Urine containing waste dissolved in a small amount of water

Question 12

What processes help with the formation of urine?

Answer 12

Ultrafiltration: Bowman’s capsule and glomerulus
Selective reabsorption: proximal convoluted tubule, loop of Henle, distal convoluted tubule, collecting duct
Secretion: distal convoluted tubule

Question 13

Define ultrafiltration

Answer 13

It is the passage of solutes and water out of the glomerulus driven by the pressure exerted by blood

Question 14

What is the renal corpuscle and what is its function?

Answer 14

It is also known as the Malpighian corpuscle and consists of the Bowman’s capsule and the glomerulus, where ultrafiltration occurs

Question 15

Why is the diameter of the afferent arteriole larger than the efferent arteriole?

Answer 15

It creates a high hydrostatic blood pressure in the glomerulus, driving ultrafiltration

Question 16

What is the glomerulus and its function?

Answer 16

It is a network of capillaries with a selectively permeable basement membrane that only allows molecules small enough to pass through the Bowman’s capsule (eg. Water, glucose, amino acids, urea, ions), forming the glomerular filtrate
Large molecules cannot pass through

Question 17

Define selective reabsorption

Answer 17

Selective reabsorption is the selective uptake of solutes and water through processes such as osmosis, active transport and diffusion

Question 18

Where does selective reabsorption take place?

Answer 18

Proximal convoluted tube
Loop of Henle
Distal convoluted tube
Collecting duct

Question 19

What is selectively reabsorbed and where?

Answer 19

Some salts
All glucose and amino acids at the proximal convoluted tubule (except for diabetes mellitus patients)
Some water
Urea is not reabsorbed in large amounts
Ions like sodium, chloride, bicarbonate, potassium along the kidney tubule

Question 20

Why is selective reabsorption important?

Answer 20

Useful small molecules like glucose and amino acids are part of the filtrate and are important, so must be reabsorbed to not be excreted in urine

Question 21

What is secretion and where does it occur?

Answer 21

It occurs mainly in the distal convoluted tubule, where ions are secreted into the filtrate to regulate the pH of blood

Question 22

Describe the composition of urine in a healthy person

Answer 22

Water, urea, some mineral salts

Question 23

Define osmoregulation

Answer 23

Osmoregulation is the control of water and solute concentrations in the blood to maintain a constant water potential in the blood plasma

Question 24

What can kidneys get damaged due to?

Answer 24

Slowly: diabetes or urinary tract problems

Suddenly: illegal drug use and abuse, insufficient blood flow to kidneys, severe accidents that physically damage the kidney (eg. Car crashes)

Question 25

What is dialysis and when do people need it?

Answer 25

People with 2 failed kidneys require dialysis to remove the toxic metabolic wastes in their body

Question 26

What is haemodialysis?

Answer 26

Where blood from the patient is removed and pumped into a dialysis machine where toxins like urea, and excess salts and water can be removed from the blood and blood is returned to the patient

Question 27

How is a patient prepared for haemodialysis?

Answer 27

An arteriovenus fistula is created (surgically joining an artery and a vein together), for easier access for the needle to remove blood during dialysis and has blood flow and blood pressure high enough to remove blood from the arm during haemodialysis

Question 28

What is the haemodialysis membrane and fluid and tubing like?

Answer 28

Haemodialysis membrane is partially permeable membrane, allows small molecules to pass through
Dialysate contains the same concentration of essential substances as healthy blood, so useful substances like glucose, amino acids, mineral salts and water do not diffuse out. If the patient lacks these essential substances, they will diffuse into the patient’s blood
The fluid does not contain metabolic waste products
The tubing is long, narrow, coiled to ensure maximal amount of urea can diffuse out

Question 29

Direction of blood and dialysate flow in haemodialysis

Answer 29

Opposite, maintains concentration gradient for removal of waste products

Question 30

What is peritoneal dialysis and how does it occur?

Answer 30

The diffusion of small molecules through the selectively permeable peritoneal membrane into the dialysate in the abdominal cavity
A soft plastic tube (catheter) is surgically inserted into the patient, and dialysis fluid enter the abdomen cavity and is left there for waste products to diffuse into the fluid, removed at the end of the treatment into the drainage bag

Question 31

Similarities of haemodialysis and peritoneal dialysis

Answer 31

Both treatments are based off the process of diffusion of substances
Patient’s blood is always separated from the dialysis fluid by a partially permeable membrane (dialysis tubing / peritoneum)
Both treatments require surgery

Question 32

Differences between haemodialysis and peritoneal dialysis

Answer 32

Haemodialysis cannot be done at home but is more efficient due to counter current flow, and is less frequent but takes longer each time

Question 33

Why is urea considered a metabolic waste product?

Answer 33

Urea is the end product of deamination of amino acids

It is a toxic substance and can be fatal when it accumulates inside the human body

Question 34

Identify constituents of the blood that do not pass into the dialysis fluid

Answer 34

Blood cells and protein molecules

Glucose and amino acids

Question 35

Identify substances that pass into the dialysis fluid

Answer 35

Urea and excess water