The kidney and osmoregulation

Question 1

What is the difference between osmoregulators and osmoconformers?

Answer 1

• Osmoregulators maintain a constant body fluid osmolarity regardless of the osmolarity of the external environment
• E.g. land animals, marine mammals
• They balance retention of water with the concentration of sugars, salts, amino acids
• Cells with high osmolarity gain water by osmosis (high solute concentration), vice versa
• Osmoconformers match their osmolarity of their body fluids with its surroundings
• E.g. marine invertebrates (jellyfish)
• They retain ions from the environment in the body fluids

Question 2

What is osmoregulation?

Answer 2

• The process by which the balance of water and solutes in the body fluids is maintained
• Carried out by kidneys, they remove nitrogenous waste, excess sugars and salts
• Amino acids are converted to toxic ammonia and then into urea (less toxic), safely excreted

Question 3

How is osmoregulation carried out in insects?

Answer 3

• Use specialised system called malpighian tubules: series of tubes that extend from body cavity and drain into insects digestive system
• Instead of blood they have haemolymph which surrounds organs
• Malpighian tubule system removes nitrogenous waste, salts and water from haemolymph
• Nitrogenous waste excreted in form of uric acid

Question 4

Explain the steps involved in removing nitrogenous waste from insects through the malpighian system.

Answer 4

1. Ions such as (Na+) and (K+) and salts are actively transported from the haemolymph into the malpighian tubules, increases osmolarity
2. Water moves into the lumen from haemolymph by osmosis
3. Nitrogenous waste enters tubules from the haemolymph along an electrical gradient
4. H2O, ions and nitrogenous waste move from tubules into digestive system, where nitrog. converted into uric acid
5. Useful salts, water reabsorbed into haemolymph
6. Uric acid laves along faeces

Question 5

What are kidneys and what do they do?

Answer 5

• Remove toxic products of metabolism from the blood
• Maintain the balance of water and solutes in the blood

Question 6

What blood vessels are associated with the kidneys?

Answer 6

• Renal artery transports blood into the kidney
• Renal vein transports blood out of the kidney
• Blood takes up waste, adjusts to the solute and water concentration, respiration takes place in the cells of the kidney

Question 7

What is the blood composed of in the renal artery compared to renal vein?

Answer 7

• Renal artery has a higher urea content, the product of breakdown of excess amino acids
• Renal artery has higher water content, product of respiration in cells and of digestion
• Renal artery has higher salt content, product of digestion, excess salt removed from blood in kidneys
• Renal artery has higher O2 content, carried in blood from lungs, used up by the cells in kidney
• Renal artery has lower CO2 content, produced by cells in kidney, released into blood
• Renal artery has slightly higher glucose content, product of digestion, partially used by kidney cells, most sugar reabsorbed into blood

Question 8

Explain the structure and function osmoregulatory system.

Answer 8

• Renal artery: carries oxygenated blood with salts and urea to kidneys
• Renal vein: carries deoxygenated blood (removed urea and excess salts) away from kidneys
• Kidney: regulates water content of blood and filters blood
• Ureter: carries urine from kidneys to bladder
• Bladder: stores urine
• Urethra: releases urine outside of body

Question 9

Explain the structure and function of a nephron.

Answer 9

• Kidney is called fibrous capsule, which contains cortex, medulla, renal pelvis
• Kidney made of many tubules called nephrons
• Nephrons are the functional unit and are responsible for the formation of urine
• Cortex: location of the glomerulus, Bowman’s capsule, proximal convoluted tubule and distal convoluted tubule
• Medulla: location of loop of Henle and collecting duct
• Renal pelvis: part of nephrons, connect to the ureter

Question 10

What are the structure of the glomerulus?

Answer 10

• Inside the Bowman’s capsule of each nephron is a glomerulus
• The glomerulus is supplied with blood by an afferent arteriole (blood from renal artery)
• Afferent arteriole splits into a ball of capillaries that forms the glomerulus
• Capillaries of glomerulus rejoin to form efferent arteriole
• Blood flows from the glomerulus into network of capillaries that run closely alongside the rest of the nephron (see image of neuron), the capillaries surround the medulla
• These networks eventually form the renal vein

Question 11

How do glomerulus and Bowman’s capsule perform ultrafiltration?

Answer 11

• Glomerulus sits within the Bowman’s capsule
• Blood in the glomerulus is under high pressure, afferent arteriole wide, increases blood pressure throughout glomerulus
• The outward pressure in the glomerulus is much higher than in other capillary networks
• High pressure forces small molecules in blood of the glomerulus into Bowman’s capsule
• Molecules include: Cl-, Na+, glucose, urea, amino acids
• The fluid in the Bowman’s capsule: glomerular filtrate (only small molecules allowed)

Question 12

How does the blood composition differ between blood plasma and glomerular filtrate?

Answer 12

• Blood plasma and glomerular filtrate have equal concentration of urea and glucose
• Blood plasma has slightly more Na+ ions
• Glomerular filtrate has lightly more Cl- ions
• Blood plasma has way more protein

Question 13

How is the structure of the glomerulus adapted to perform ultrafiltration?

Answer 13

• Blood in glomerular capillaries is separated from lumen of Bowman’s capsule by three layers
• First cell layer is endothelium of capillary, which contains fenestrations (gaps) where fluid can pass through
• Second layer is basement membrane, made of collagen protein and glycoproteins, mesh-like structure (sieve), prevents large proteins from passing from blood plasma
• Third layer is endothelium of Bowman’s capsule, foot-like projections called podocytes, gaps between them allow small molecules to pass

Question 14

How does blood pass through the glomerulus?

Answer 14

• Substances dissolved in blood plasma can pass into Bowman’s capsule because of the fenestrations between capillary endothelial cells, mesh-like basement membrane, gaps between projections
• Substances that pass into Bowman’s capsule make up glomerular filtrate
• RBC and platelets remain in blood (too large)
• Basement membrane stops large protein molecules from getting through

Question 15

What does the proximal convoluted tubule do?

Answer 15

• Useful substances in the glomerular filtrate are reabsorbed into the blood, as the filtrate passes along the nephron
• Called selective reabsorption, only certain substances
• Reabsorbed substances: water, salts, glucose & amino acids
• Reabsorption occurs in proximal convoluted tubule
• The loop of Henle and collecting duct are also involved in reabsorption
• The lining of the proximal convoluted tubule is one layer of epithelial cells

Question 16

How is the proximal convoluted tubule adapted to selective reabsorption?

Answer 16

• Contains microvilli on the luminal membrane, increases surface area for reabsorption
• Co-transporter proteins, each type transports a specific solute (e.g. glucose, amino acid) across the luminal membrane
• Many mitochondria, provide energy for K+ and Na+ pumps in basal membrane of cells
• Cells tightly packed together, no fluid can pass between the cells (substances must pass through cells)

Question 17

How does selective reabsorption occur?

Answer 17

• Na+ are transported from proximal convoluted tubule into surrounding tissues by active transport
• Na+ create electrical gradient (positive charge), Cl- follow by diffusion
• Sugars and amino acids transported by co-transporter proteins
• Movement of ions, sugars and amino acids into surrounding tissue raises osmolarity in tissues, water leaves by osmosis from proximal convoluted tubule into tissues
• Urea diffuses out of proximal convoluted tubule
• All substances that leave proximal convoluted tubule into tissues move into capillaries down concentration gradient

Question 18

What does the Loop of Henle do?

Answer 18

• The Loop of Henle creates a high solute (hypertonic) concentration in the tissue fluid of the medulla, to conserve water
• The Loop of Henle uses a countercurrent multiplier system, the filtrate flows in opposite directions in the descending and ascending limbs of the loop
• It also creates a steep concentration gradient across the medulla

Question 19

How do the two limbs of the Loop of Henle act in regard to a changing osmolarity?

Answer 19

• Na+ and Cl- pumped out of ascending limb into the surrounding medulla region, increases osmolarity (gains water)
• As the loop descend into the medulla, the interstitial fluid becomes more hypertonic (high osmolarity)
• Ascending limb is permeable to salts NOT water
• Salts released from ascending limb are drawn down into the medulla, further establishing a salt gradient, osmolarity decreases as it rises back into the cortex (loses water), since solutes are removed
• The descending limb is permeable to water, H2O moves out by osmosis due to high osmolarity of medulla. Medulla now has high ion concentration (high osmolarity). Little water in limb (move up)
• The descending limb has low permeability to ions, osmolarity of filtrate increases (gains water)
• Water and ions the leave the loop into medulla, move into nearby capillaries called vasa recta (provide O2)

Question 20

How does the Loop of Henle conserve water?

Answer 20

• To conserve water for a long period of time, animals have very long loops of Henle
• The longer the loop, the greater an animal’s ability to conserve water (generates steeper concentration gradient, more water can be reabsorbed)
• These animals also have thicker medulla regions, provides additional space for long loops of Henle
• Loop length and medulla thickness help conserve H2O
• Positive correlation

Question 21

What is dehydration and what are its consequences?

Answer 21

• If balance of water and solutes in body are not maintained, water lost from body and not replaced
• Due to excessive sweating, diarrhoea
• Osmolarity of body fluids increases above surrounding cells, cells shrink as water moves out
• Consequences: low volume of dark, concentrated urine, not being able to sweat, reduced ability to regulated body temperature
• Drop in blood pressure (reduced blood volume), elevated heart rate (compensation for drop in blood pressure), fatigue (increased exposure to metabolic waste)
• Metabolic waste usually removed from body (usually dissolved in water)

Question 22

What is overhydration and what are its consequences?

Answer 22

• Too much water in the body fluids in relation to solutes
• Caused by overconsumption of water, not replacing sugars and salts after excessive sweating, kidney problems
• Osmolarity of body fluids drops below surrounding cells (low solute concentration), water moves into cells
• This can leads to excessive urination, colourless and dilute urine, headaches (swelling of brain cells), high blood pressure (increased blood volume)
• Low heart rate (compensate for increased blood pressure), neurological problems (low concentration of important ions)

Question 23

What is ADH and what does it do?

Answer 23

• Kidneys help in osmoregulation (balance of water and solutes) by altering the amount of water reabsorbed from the glomerular filtrate into the blood
• The volume of water reabsorbed regulated by changing water permeability of the walls of the distal convoluted tubule and collecting duct
• The permeability is regulated by the hormone ADH. It is released by the pituitary gland which is regulated by the hypothalamus
• Hypothalamus monitors the composition of the blood, as it flows past osmoreceptor cells in the brain (receives receptors)

Question 24

How does the pituitary gland react when the hypothalamus detects low blood water content?

Answer 24

• Low water levels also means high blood solute concentration, high blood osmolarity
• Hypothalamus causes pituitary gland to secrete ADH into blood
• Cells in the distal convoluted tubule and collecting duct in kidneys are targeted
• ADH increases permeability of the walls in the kidney to water. This increase in permeability is caused by increasing number of channel proteins called aquaporins in the cell membrane
• More water reabsorbed into blood via distal convoluted tubule
• The loop of Henle generated a concentration gradient across the medulla, osmolarity of medulla increases (water fills into medulla)
• This leaves a concentrated filtrate, the remainder is urine which passes from the renal pelvis to bladder
• Blood water content increases, concentrated urine

Question 25

How does the pituitary gland react when the hypothalamus detects high blood water content?

Answer 25

• Hypothalamus no longer stimulates the pituitary gland to release ADH
• The distal convoluted tubule and collecting duct walls become less permeable to water
• Less water absorbed from the nephron, water instead passes down collecting duct with rest of filtrate
• Blood water content decreases, a lot of dilute urine

Question 26

What is nitrogenous waste and in what three forms can it be removed?

Answer 26

• Comes from the breakdown of excess dietary amino acids and nucleic acids
• Waste converted into ammonia (highly toxic) and converted into urea (or uric acid) for excretion
• Way nitrogenous waste excreted depends on organism requirements
• Ammonia cannot be stored, toxic, sometimes directly excreted by fish and aquatic invertebrates
• Ammonia otherwise converted into less toxic urea, must be excreted (cannot build up). Urea diluted with water to form urine
• Ammonia can also be converted into uric acid, requires more energy, it does not dissolve in water, hence less water needed to excrete
• Uric acid aids in survival of animals that develop inside impermeable egg shell, excreted by birds, athropods and insects

Question 27

What causes kidney failure and what methods can be applied to treat it?

Answer 27

• Physical damage from injury, high blood pressure, diabetes, overuse of drugs, infection
• Human can survive with one functioning kidney
• Damage can lead to build up of urea in blood (toxic at high concentrations)
• Treatments include: dialysis, kidney transplant

Question 28

How does haemodialysis work?

Answer 28

• Processed used to separate small and large molecules with a partially permeable membrane
• Haemodialysis is a treatment needed to be carried out several times a week at a dialysis machine
• Small molecules such as urea and salt can fit through pores in the dialysis membrane, exchange of substances can take place
• Machine contains dialysis fluid that contains ions, salts and no urea, hence urea diffusion gradient
• The salt concentration is similar to ideal blood concen. diffusion only occurs if imbalance
• Glucose concen. equal to normal blood
• Blood and fluid flow in opposite directions, ensure concentration gradient along membrane
• One usage takes 3-4 hours
• Patients given drug that prevents blood clot formation

Question 29

How does a kidney transplant work?

Answer 29

• Single health kidney from a donor and transplanting it into a patient, better for long term solution
• Patient has more freedom, no longer requires dialysis, diet less restrictive, dialysis can lead to nausea
• Risks: immune response to new kidney (different antigens on membrane), usage of immunosuppressant drugs, reduce risk of organ rejection, leave vulnerable to infections, lack of donors

Question 30

What is urinalysis?

Answer 30

• Analysing the composition of urine, shows products of metabolism, molecules present in blood in high concentrations that fit through glomerulus and Bowman’s capsule, drugs
• It shows any deviation from normal urine composition or detects drug use

Question 31

What tests can be carried out during urinalysis?

Answer 31

• pH testing: pH can develop kidney stones, testing strips contain indicator chemicals
• Glucose concentration: high levels sign of diabetes, test strips used to show concentration
• Presence of proteins: proteins usually too large to filter through glomerulus, if they are present sign og high blood pressure, kidney damage or diabetes
• Drug testing: using monoclonal antibodies which bind to specific drugs due to their complementary structure, testing for doping
• White blood cells: sign of infection in urinary tract
• Pregnancy: hormones present in pregnancy can be detected also using monoclonal antibodies (hCG)