8 — excretion in humans

Question 1

Chemical reactions in cells

Answer 1

• Anabolic reactions → build up complex molecules )simple→ complex)
  
  • Photosynthesis — synthesis of glucose usin CO2 n H2O
  • Formation of glycogen from glucose molecules
• Catabolic reactions → break down complex molecules (complex → simple)
  
  • Oxidation of glucose during respiration to form CO2 n water
  • Deamination of excess amino acids to form urea.

Question 2

Metabolism

Answer 2

The sum of all the chemical reactions within the body of an organism

Question 3

Metabolic waste products:

Answer 3

• CO2 → abnormally high lvl can cause hypercarbia (expires from body)
• Urea → excess amino acids deaminated to form Urea
  
  • Abnormally high conc can cause abdominal pain, nausea, vomiting, irregular heartbeat, muscular cramps
  • Produced in liver, excreted by kidneys
  • Aka waste product of amino acid metabolism in TYS 2014
• Mineral salts or ions
  
  • High lvl → lower wp in blood plasma → water exits tissue cells into blood by osmosis → cells crenated
• Water
  
  • Produced during aerobic respiration
  • High lvl in blood plasma → wp^ →water enters tissue cells, cells swell n burst
  • Metabolic water is water produced by metabolic reactions such as aerobic respiration carried out by cells in the body

Question 4

Excretion

Answer 4

Excretion is the process of removal of metabolic waste products, toxic substances and substances in excess of the body’s requirements [1] that would be toxic to the body when accumulation of metabolic waste products & toxic substances in excess occurs. [1]

Question 5

Ultrafiltration

Answer 5

Ultrafiltration is the process where most of the blood plasma and dissolved substances r forced out of the glomerulus into the bowman’s capsule by high hydrostatic blood pressure.

Question 6

Selective reabsorption

Answer 6

Selective reabsorption is the process where certain substances are reabsorbed from the filtrate back into the blood as they pass through nephrons.

Question 7

Egestion

Answer 7

Egestion is the removal of undigested material from the alimentary canal

Question 8

Journey of urea

Answer 8

Proteins in food → excess amino acids → liver deaminates aa to form urea → diffuse from liver cells into tissue fluid down cg → dissolves into blood plasma → hepatic vein to heart → aorta → renal artery → kidney → being small molecules, r forced out of blood by high hydrostatic blood pressure during ultrafiltration at glomerulus across partially permeable basement membrane → BCap → urea in glomerular filtrate flows along nephron to collecting duct → ureter → urinary bladder → excreted in urine out of body thru urethra.

Question 9

Describe the function of a Nephron + structure of nephron adapted for its function [6]

Answer 9

Function of kidney nephron is to excrete metabolic waste products such as urea from the body through the formation of urine. [1]

Ultrafiltration takes place at the glomerulus of the nephron whereby small soluble molecules like urea, glucose, amino acids, water and dissolved mineral salts, but not large molecules like proteins, red blood cells and fats, [1] are forced out from the glomerulus into the Bowman’s capsule of the nephron across the partially permeable basement membrane surrounding the glomerulus [1] to form glomerular filtrate, by high hydrostatic blood pressure, [1] due to the difference in diameter between the wider efferent arteriole and the narrower efferent arteriole; [1]

Selective reabsorption takes place along the LONG tubules of the nephron. At the proximal convoluted tubule, all glucose and amino acid molecules are selectively reabsorbed into the network of blood capillaries surrounding PCT, by diffusion and active transport, due to the presence of abundant mitochondria in the cells at the PCT to release large amount of energy for active transport. The network of blood capillaries maintain a steep concentration gradient to transport substances to the heart to be pumped to all parts of the body by blood; [1]

Most of the water molecules are reabsorbed by osmosis and dissolved mineral salts by diffusion, [1] into the blood capillaries surrounding the proximal convoluted tubule, along the LONG tubule which is the Loop of Henle, distal convoluted tubule and collecting duct; [1]

Urea is not reabsorbed into the blood capillaries and discharged as urine via the ureter; [1]

Question 10

IF: walls of glomerulus leaky, predict 2 ways in which this might alter urine composition. [2]

Answer 10

1. Presence of blood proteins in urine [1]
2. Presence of RBCs or WBCs in urine [1]

Question 11

Ultrafiltration

Answer 11

1. Afferent arteriole lumen wider than efferent arteriole → high hydrostatic blood pressure by contraction of left ventricle in heart and due to higher resistance to blood flow at the efferent arteriole (narrow lumen) as compared to in glomerulus
2. Blood plasma forced out of glomerular blood capillaries into the Bowman’s Capsule → partially permeable basement membrane around glomerular capillaries filters small soluble molecules → small soluble molecules enter Bowman’s Capsule while large molecules r carried away by efferent arteriole

Filtrate: water and small molecules - (glucose, AA, mineral salts, urea)
- Not filtered out:
- Large molecules (plasma proteins, red/ white blood cells)
- Plasma proteins: fibrinogen, thrombin etc (needed in blood clotting)

Question 12

Adaptations of glomerulus:

Answer 12

• a network of blood capillaries → large SA for filtration process
• BC (blood capillaries) r 1 cell thick + hv tiny pores in capillary wall
• BC covered by thin partially permeable membrane impermeable to large molecules such as blood cells, platelets and proteins.

Question 13

Describe the role of glomerulus and blood capillary. [3]

Answer 13

Structure A is the glomerulus is the site of ultrafiltration where high hydrostatic blood pressure forces small molecules such as urea out of the glomerulus across the partially permeable basement membrane into the Bowman’s capsule;[1]

Structure B is the blood capillary which transports blood containing glucose, amino acids, water and mineral salts which are reabsorbed by selective reabsorption at the kidney tubules through diffusion and active transport back to the heart via the renal vein; [1]

To ensure that useful substances are not excreted in the urine; [1]

Question 14

What happens if the length of the afferent arteriole lumen is reduced?

Answer 14

• Reduced or absence of high hydrostatic blood pressure in the glomerulus
• Ultrafiltration is less efficient or reduced
• Rate of urine production is reduced

Question 15

Water reabsorption

Answer 15

1. At Loop of Henle, some water is reabsorbed from the filtrate back into blood vessel via osmosis
2. At distal convoluted tubule (fine-tuning process), some salts r reabsorbed back into blood vessel via AT.
3. Blood pH regulated by selectively reabsorbing H+ ions.
4. At collecting duct, some water is reabsorbed back into blood vessel via osmosis
5. Remaining fluid in tubule passes out of collecting duct into renal pelvis to form urine

Question 16

Osmoregulation Answ tech

Answer 16

Loss/ intake of water (thru sweating to remove excess heat from body/drink water) → WP of blood decreases/ ^ -> stimulates hypothalamus in brain → hypothalamus produces more/less ADH and stimulates pituitary gland to release more/ less ADH into bloodstream → cells lining the walls of collecting duct become more/ less permeable to water → more/ less water reabsorbed from collecting duct into surrounding blood capillaries → Smaller/ larger volume of water lost as urine & more/ less conc urine produced → WP of plasma returns to normal

Question 17

Kidney failure

Answer 17

Refers to the inability of the kidney to function properly, causing the accumulation of excretory products in the blood

Question 18

Causes and treatments for kidney failure

Answer 18

• Causes:
  
  • High blood pressure (hypertension)
  • Diabetes
  • Alcohol abuse
  • Severe accidents that cause physical dmg to kidneys
  • Complications from major surgeries
• Treatments:
  
  • Kidney transplant
  • Dialysis

Question 19

Diabetes mellitus

Answer 19

1. Islets of Langerhans in pancreas X secrete enuf insulin
2. Inability to convert excess glucose into glycogen to be stored in liver n muscle tissues
3. When glucose in blood is filtered into glomerular filtrate, not all the glucose can be reabsorbed from the kidney tubules
4. Glucose excreted in urine

Question 20

Dialysis fluid vs blood plasma [3]

Answer 20

Similarities:

Blood plasma and dialysis fluid contain similar
concentration of glucose, amino acids and dissolved mineral salts. [1]

Difference:

Blood plasma contains urea but dialysis fluid does not contain urea. [1]
Blood plasma contain blood cells and platelets but dialysis fluid does not contains blood cells and platelets. [1]

Dialysis fluid: X
Blood plasma: Contains metabolic waste products

Dialysis fluid: Direction of flow opposite to blood plasma

Question 21

Describe and explain the changes in concentration of urea along the nephron as it passes from the renal arteriole into the urine. [6] + Describe how excess amino acid is removed from the body [4]

Answer 21

• Urea is transported in blood by the renal arteriole to the kidney to be excreted as metabolic waste products in urine [1]
• Glomerulus – urea molecule is small enough to pass through the partially permeable basement membrane to form part of the filtrate in the Bowman’s capsule through the process of ultrafiltration. [1]
• Concentration of urea in the filtrate increases as it moves through the nephron[1]
• Proximal convoluted tubule - Most of the water in the filtrate is selectively reabsorbed back into blood via osmosis but not urea, thus the filtrate becomes more concentrated in urea and lesser in volume. [1]
• Loop of Henle and distal convoluted tubule – some water and mineral salts are reabsorbed back into the blood, the filtrate becomes more concentrated with urea [1]
• Collecting duct – the effect of ADH can stimulate more / less water being reabsorbed into the blood, causing the final urine formed to be more / less concentrated in urea. [1]

[4]:

Excess amino acids goes through deamination in the liver.[1]
The carbon residue will be converted into glucose and then into glycogen to be stored in the liver. [1]
The amino group will be converted to urea to be excreted out of the body [1] via ultrafiltration in kidney nephron to form urine. [1] (high hydrostatic blood pressure due to wider afferent arteriole compared to narrower efferent arteriole and across a partially permeable basement membrane)

Question 22

Explain why there is no glucose and protein present in the urine of a normal individual. [4]

Answer 22

• Protein molecules are too large [1] to pass through the partially permeable
  basement membrane at the glomerulus during ultrafiltration and are not
  filtered from the blood [1]
• Glucose molecules are small enough to pass through the partially
  permeable basement membrane at the glomerulus to be filtered during
  ultrafiltration into the Bowman’s capsule. [1]
• Selective reabsorption took place and all glucose molecules are
  selectively reabsorbed into the blood capillaries at the proximal
  convoluted tubule via active transport. [1]

Question 23

Explain how eating salty food would affect the composition of the urine.[6]

Answer 23

• Increase in salt concentration decreases water potential in blood plasma [1]
  Detected by hypothalamus which produce more ADH and stimulate
  pituitary gland to release more ADH. [1]
• Anti-diuretic hormone (ADH) increases the permeability of the collecting
  duct to water [1] and increases water reabsorption into the blood
  capillaries. [1]
• Hence decreasing the volume of water in urine produced and the urine is
  more concentrated with urea. [1]
• Excess salts which are not reabsorbed will also make up the concentrated
  urine. [1]

Question 24

Describe how the dialysis machine functions in the removal of metabolic waste products from the patient. [4] + [6]

Answer 24

1. Blood is drawn from a vein in the patient’s arm and pumped through a long dialysis tubing in the dialysis machine which separates the blood and the dialysis fluid, to increase surface area for faster diffusion to take place.
2. The dialysis tubing is bathed in dialysis fluid which contains similar concentrations of glucose, amino acids and other essential substances as blood to prevent net movement of useful substances out of blood via diffusion. [1]
3. Urea and other metabolic waste products diffuse out of the tubing down the steep concentration gradient from blood into the dialysis fluid as there are no metabolic waste products in dialysis fluid. [1]
4. Blood cells, platelets and big molecules (plasma proteins) stay in the tubing as they are too big to pass through the pores of the partially permeable membrane. [1]
5. Dialysis fluid flows in the opposite direction of blood to maintain constant steep diffusion / concentration gradient. [1]

[6]:

During kidney dialysis, blood is drawn from the vein in the patient’s arm and pumped through a partially permeable dialysis tubing in
the dialysis machine to ensure that large molecules such as blood cells, platelets and proteins remain in the blood in the tubing; [1]
The dialysis tubing is narrow, long and coiled to increase surface area to volume ratio to speed up rate of exchange of substances between patient’s blood and dialysis fluid; [1]
The tubing is bathed in dialysis fluid which contains the same concentration of essential substances such as glucose, amino acids and mineral salts as healthy blood to ensure they do not diffuse out of the blood into the dialysis fluid; [1]
Glucose, amino acids and mineral salts can diffuse into the blood if the person’s blood is lacking in them; [1]
The dialysis fluid does not contain metabolic waste products such as urea, uric acid, creatinine, excess water and mineral salts
to set up concentration gradient for urea to diffuse out of the blood into the dialysis fluid; [1]
The direction of blood flow is opposite to the flow of dialysis fluid to maintain concentration gradient for the removal of metabolic waste products such as urea; [1]

Extra:
- Dialysis tubing is immersed in dialysis fluid at. 37 DC to reduce heat loss from patient

Question 25

Compare and contrast between dialysis and kidney function. [6]

Answer 25

Similarities:

• Both involves partially permeable membrane to allow small molecules to filter/ pass through [1]
• Both help in the removal of metabolic waste products such as urea. [1]

Differences:
Movement of substances
D: Involves diffusion only
K: Involves diffusion and active transport

Processes:
D: Does not involve ultrafiltration and selective reabsorption.
K: Involves ultrafiltration and selective reabsorption.

Pressure
D: Pressure provided by dialysis machine.
K: Pressure provided by the left ventricle of heart & difference in diameter of lumen of efferent
arteriole and afferent
arteriole.

Hormones
D: Does not involve any hormones.
K: Involves ADH regulation

Fluid
D: Dialysis fluid is needed.
K: Dialysis fluid not required. Only
involved glomerular filtrate.

Urine concentration
D: Cannot produce more concentrated/diluted urine
K: Can produce more concentrated/
diluted urine

Question 26

Explain the change in the concentration of proteins between the blood plasma in the renal artery and the fluid in the nephron. [1]

Answer 26

High protein concentration in blood plasma in renal artery and no proteins in the
fluid of nephron and collecting duct as proteins are large molecules, which
cannot pass through the partially basement permeable membrane. [1]

Question 27

Compare the concentration of sodium ions at collecting duct and at loop of Henle in Figure 6.2 and give a reason for your answer [1]

Answer 27

Concentration of sodium ions at Z is higher than at Y because water has been
reabsorbed by selective absorption from the filtrate using osmosis (same or slightly reduced amount of sodium ions in a smaller volume of water). [1]
Or
Concentration of sodium ions at Z is lower than at Y because sodium ions needed by
the body are reabsorbed by selective reabsorption from the filtrate using active
transport and diffusion. [1]

Question 28

Suggest what would happen if the dialysis solution is not changed after five hours. [1]

Answer 28

No concentration gradient between blood and
dialysis solution, no net movement of nitrogenous
waste from the body to the cavity after 5 hours. [1]

Question 29

Define metabolic water

Answer 29

Metabolic water is the water produced as a result of chemical reactions in the body. [1] such as respiration and condensation in the body. (2 examples) [1]

Question 30

Describe the function of dialysis fluid in dialysis. [2]

Answer 30

Dialysis fluid does not contain any metabolic wastes like urea so as to create a steep concentration gradient between blood
plasma and dialysis fluid to remove urea from the blood plasma faster by diffusion. [1]
Glucose could also diffuse from the dialysis fluid to the blood plasma to restore a normal level in the blood plasma. [1]