Excretion

Question 1

Define excretion

Answer 1

The removal of metabolic waste from the body

Question 2

Why must carbon dioxide be excreted?

Answer 2

Too much CO2 in blood reduces ability of haemoglobin to carry oxygen, as hydrogen ions produced compete for space with oxygen
Carbon dioxide combines directly with haemoglobin to form carbaminohaemoglobin, which has a lower affinity for oxygen
Too much carbon dioxide can cause respiratory acidosis: where blood pH falls below 7.35, causing difficult breathing, headache, drowsiness, restlessness, tremor and confusion

Question 3

Why must nitrogenous waste be excreted?

Answer 3

Amino acids can’t be stored, but they contain a lot of energy. The amino group is toxic, and so must be removed, before the rest of the acid can be respired. The amino group forms ammonia, which is soluble and toxic, which is then converted to less soluble and toxic urea.

Question 4

What are liver cells called?

Answer 4

Hepatocytes

Question 5

How is the liver supplied with blood?

Answer 5

2 sources: hepatic artery and hepatic portal vein
Hepatic artery supplies liver with oxygenated blood for respiration (liver carries out lots of metabolic processes, needs ATP)
Hepatic portal vein supplies liver with deoxygenated blood from digestive system. Blood contains uncontrolled concentrations of compounds, some of which could be toxic.

Question 6

How is blood removed from the liver?

Answer 6

Via the hepatic vein. This rejoins the vena cava

Question 7

How are cells arranged inside the liver?

Answer 7

Cells are arranged to ensure best possible contact with blood. Liver divided into lobes which are further divided into lobules.

Question 8

What is the arrangement of liver cells within a liver lobule?

Answer 8

Blood supply vessels split into smaller and smaller branches, running between the lobules. At points, vessels open and blood runs into the lobules, with blood from artery and portal vein mixing. Runs along sinusoids, lined with liver cells for max. contact with cells. Empty into a branch of the hepatic vein in lobule centre.

Question 9

What are the arrangement of the vessels within a liver lobule

Answer 9

Hepatic artery and portal vein run around the edge
Hepatic vein runs through middle
Bile canaliculi run into bile duct at edge

Question 10

How are liver cells specialised?

Answer 10

Hepatocytes have many microvilli so large SA for exchange. Dense cytoplasm as have many organelles for all the metabolic processes they carry out.

Question 11

What are Kupffer cells?

Answer 11

Specialised macrophages. Exist in the sinusoids of the liver, and are involved in breaking down and recycling red blood cells. Bilirubin is produced when haemoglobin breaks down, which is the brown stuff in poo.

Question 12

What are the 2 reactions that occur in the production of urea?

Answer 12

Deamination and the ornithine cycle

Question 13

Describe the process of deamination

Answer 13

The amino group in an amino acid is removed, along with the hydrogen attached to the central carbon atom. Requires oxygen: add an oxygen to the central carbon with a double bond to form a keto acid and ammonia.

Question 14

Describe the ornithine cycle

Answer 14

A way of converting ammonia (which is very toxic and soluble) to urea (which isn’t quite so toxic and soluble). Ammonia bonds to citrulline, producing water, making arginine which bonds to a water, producing urea (yay) and ornithine, which bonds to ammonia and CO2, releasing a water, making citrulline.
2NH3 + CO2 -> CO(NH2)2 + H2O

Question 15

How may the liver get rid of toxic molecules?

Answer 15

Oxidation, reduction, methylation or combining it with another molecule

Question 16

Describe how the liver detoxifies alcohol

Answer 16

Enzyme ethanol dehydrogenase oxidises ethanol to ethanal, releasing 2 hydrogens which reduce NAD. Ethanal dehydrogenase oxidises ethanal to ethanoic acid, again reducing NAD. This gives ethanoic acid, which then combines with coenzyme A to form acetyl coenzyme A, which can be respired.

Question 17

How is NAD used in the liver?

Answer 17

In breaking down alcohol and fatty acids for use in respiration. If too much alcohol, not enough NAD to break down fatty acids, so liver converts fatty acids to lipids which are stored in hepatocytes, causing liver to enlarge. Called fatty liver, and can lead to cirrhosis or alcohol-related hepatitis.

Question 18

Describe the gross structure of the kidney

Answer 18

Surrounded by a tough capsule
Outer region is the cortex
Then the medulla
Then the pelvis, which leads to the ureter
Blood supplied through renal artery, removed through renal vein.

Question 19

Describe the structure of a nephron tubule

Answer 19

Bowman’s capsule surrounds glomerulus and leads to proximal convoluted tubule. This leads to descending and ascending limb of loop of Henle, which leads to they distil convoluted tubule. This empties into the collecting duct.

Question 20

Describe the structure of the capillaries asscociated with a nephron tubule

Answer 20

Renal artery divides a lot to afferent arteriole which leads into the glomerulus, which leads to the efferent arteriole. This then forms the vasa recta which is a network of capillaries surrounding the tubule, and eventually leads back into the renal vein.

Question 21

Describe the process of ultrafiltration

Answer 21

Afferent arteriole is bigger than efferent arteriole, so blood is under pressure. This pushes fluid into the Bowman’s capsule through 3 layers.
The endothelial wall of the capillary has gaps in it that allows fluids to pass through
The basement membrane consists of a fine mesh of collagen fibres that any molecule with Mr below 69000 can pass through
The epithelial cells of the Bowman’s capsule, podocytes have finger-like projections that fluid can flow around.

Question 22

Describe the process of selective reabsorption of glucose or amino acids

Answer 22

ATP is used to actively transport sodium ions out of the cell (and into blood)
Low sodium ion concentration inside the cell; high concentration of sodium ions outside the cell in the PCT
Sodium ions move into the cell via diffusion
Go in via co-transporter proteins, so bringing in another molecule into the cell, such as glucose or amino acids
High concentration of glucose in the cell, low glucose conc. in blood
Glucose diffuses into blood, down conc. gradient via facilitated diffusion
Blood flow maintains concentration gradient

Question 23

How are small proteins reabsorbed in the proximal convoluted tubule in the kidney?

Answer 23

In selective reabsorption, via pinocytosis. Vesicle then fuses with a lysosome that breaks it down into amino acids which then go into the blood.

Question 24

How are the proximal convoluted tubule, and the cells that compose it, specialised, and why?

Answer 24

Long convoluted tubule and microvilli to increase SA for facilitated diffusion
Tight junctions between cells to prevent filtrate exchanging substances directly with blood
Single layer of cuboidal epithelium to decrease diffusion distance
Many mitochondria to provide energy for active transport
Lots of rough ER to synthesise proteins
Infolding of basal membrane

Question 25

What is reabsorbed in the proximal convoluted tubule?

Answer 25

Most of the water, all of the glucose, most of the ions, some urea, most small proteins and all amino acids

Question 26

Which is which: afferent arteriole and efferent arteriole?

Answer 26

Afferent comes first, goes in, is bigger; efferent comes second, goes away, is smaller.

Question 27

How does the loop of Henle create a concentration gradient in the medulla?

Answer 27

Sodium and chloride ions are actively transported out of the ascending limb
Ascending limb is impermeable to water, and so water can’t follow it
Therefore, there is a high concentration of ions in the medulla
In the descending limb, the ions diffuse in down their concentration gradient
Water moves out of descending limb and into medulla/capillaries via osmosis, as low water potential in medulla and high in loop
Low water potential in medulla that gets lower as descend created.

Question 28

Why is the loop of Henle described as a countercurrent multiplier?

Answer 28

Countercurrent: things flowing in opposite directions
Multiplier: long loop multiplies effect of concentration difference

Question 29

Which is which: hypertonic, hypotonic, isotonic?

Answer 29

Hypertonic is more concentrated (think hypermobile)
Hypotonic is less concentrated (think hypothermia)
Isotonic is same concentration (just remember this one)

Question 30

What does ADH stand for?

Answer 30

Antidiuretic hormone

Question 31

What does diuresis mean?

Answer 31

Production of large volumes of water

Question 32

Where is water reabsorbed in the kidneys?

Answer 32

Most in the PCT and descending limb of loop of Henle, ‘fine tuning’ occurs in DCT and collecting duct

Question 33

What sequence of events occurs that results in the reabsorption of more water from urine?

Answer 33

Dehydration, decreased water potential of blood (stimulus), osmoreceptors in hypothalamus detect this, neurosecretory cells stimulated, more ADH released from posterior pituitary gland, ADH in blood (protein hormone), ADH arrives at collecting duct, binds to receptor on cell surface membrane making up wall of collecting duct, more aquaporins are put into the cell surface membrane allowing more water to be reabsorbed.

Question 34

What substances are present/absent in the glomerular filtrate?

Answer 34

Glucose, amino acids, vitamins, some hormones, urea, water, inorganinc ions. Not plasma proteins, erythrocytes

Question 35

What substances are present/absent in the renal fluid?

Answer 35

Some vitamins, hormones, urea, water, inorganic ions. Not glucose, amino acids, plasma proteins or erythrocytes

Question 36

What substances are present/absent in urine?

Answer 36

Urea, some vitamins, some hormones, some water, some inorganic ions. Not glucose, amino acids, plasma proteins, erythrocytes

Question 37

How is ADH produced and stored?

Answer 37

Produced in the hypothalamus, transported along axons to posterior pituitary gland where it is stored until needed.

Question 38

What is haemodialysis?

Answer 38

Involves ‘cleaning’ the blood outside the body. Not needed regularly: 3x a week. Long sessions in a dialysis clinic, can’t operate during them. Blood is removed from the body via a shunt from artery to vein (removed from vein) and passed over an artificial dialysis membrane where solutes can diffuse across a partially permeable membrane, into/out of the dialysis fluid.

Question 39

What is peritoneal dialysis?

Answer 39

Involves ‘cleaning’ the blood inside the body. Needed several times a day, but can walk about in the process. Permanent tube inserted into the abdomen, dialysis fluid poured in to abdominal cavity between organs. Solutes are exchanged across the abdominal cavity wall.

Question 40

What are the dangers of kidney failure?

Answer 40

Waste products build up in the blood that are potentially toxic.

Question 41

What are the symptoms of kidney failure?

Answer 41

weight loss and poor appetite, swollen ankles feet or hands (due to water retention) , shortness of breath, blood or protein in your urine (protein in your urine is not something you will notice as it can only be detected during a urine test), an increased need to urinate, particularly at night, itchy skin, muscle cramps, high blood pressure (hypertension), nausea

Question 42

How can kidney failure be treated?

Answer 42

Haemo/peritoneal dialysis or kidney transplant

Question 43

How can urine samples be used for diagnostics?

Answer 43

Any substance with Mr of less than 69000 will pass into glomerular filtrate. If there is too little/too much/a presence of an abnormal substance, kidneys won’t reabsorb all of it, so will be present in urine. Can test urine for these substances

Question 44

How does a pregnancy test work?

Answer 44

Test for ‘human chorionic gonadotrophin’ (HCG) which is produced when the embryo implants into the uterus lining. Monoclonal antibodies are identical and will bind to HCG. As urine rises up the membrane that makes up the test strip, HCG attaches to antibodies that are attached to a coloured marker/enzyme. This moves up the strip and attaches to an immobilised band of antibodies and forms a coloured line. Second line is formed of antibodies not bound to HCG: a control line.