Excretion as an example of homeostatic control Flashcards
Define excretion
Removal of potentially toxic products of metabolism
Outline the main metabolic waste products in mammals
Carbon dioxide
- Waste product of cellular respiration
- Excreted from lungs
Bile pigments
- Formed from breakdown of haemoglobin in liver
- Excreted in bile from liver into small intestine
- Egested with faeces
Nitrogenous waste (urea)
- Formed from breakdown of amino acids by liver
- Excreted by kidneys in urine
Which blood vessel brings oxygenated blood to the liver?
Hepatic artery
Which blood vessel takes deoxygenated blood from the liver to the heart?
Hepatic vein
What is the role of the hepatic portal vein?
Carry products of digestion from intestine to liver
How can the hepatic portal vein and hepatic artery be differentiated?
- Hepatic artery is narrow
- Hepatic portal vein separates into many branched vessels
Which blood vessel carries blood from the small intestine to the liver?
Hepatic portal vein
What are hepatocytes?
Liver cells
How are hepatocytes adapted for their function?
- Metabolically active cells
- Constantly dividing by mitosis
- Lots of mitochondria
- Large surface area in contact with the blood to maximise the exchange of substances
What are the roles of hepatocytes?
- Absorb products of digestion, oxygen and toxins from blood
- Break down toxins
- Convert glucose to glycogen
- Deaminate amino acids
- Break down red blood cells to produce bile
- Secrete bile into canaliculi
What are canaliculi?
- Spaces in the liver
- Bile secreted into canaliculi drains into bile ductules then to gall bladder
Outline the role of Kupffer cells
- Line sinusoids
- Act as macrophages
- Ingest foreign particles
Explain how the structure of the liver is adapted for its functions in the body
- Blood from hepatic portal vein (HPV) brings products of digestion and cell metabolism to
liver - Blood from hepatic artery (HA) brings oxygenated blood to liver
- Blood from the HPV and HA combine in the sinusoids
- Provides raw materials and oxygen for hepatocytes
- Hepatocytes line sinusoids and absorb digested food, toxins, and oxygen from blood
- Also break down toxins, convert glucose to glycogen, deaminate amino acids
- Kupffer cells act as macrophages
- Engulf and digest foreign cells and debris
- Bile secreted into canaliculi as haemoglobin broken down in hepatocytes
- Hepatic vein removes deoxygenated blood carrying products of detoxification
- e.g. urea away from the liver
Define transamination
- Conversion of one amino acid to another
- Carried out by hepatocytes
- Necessary as diet may not contain correct balance of amino acids
Define deamination
Removal of an amine group (NH2) from a molecule
What is the removed amine group converted to?
- Amine group → ammonia → urea
- Converted in the ornithine cycle
Why is it necessary for deamination to occur in the liver?
- Body cannot store excess proteins or amino acids
- Excess protein would be egested
- Hepatocytes process excess protein to produce organic compounds - Can be used in respiration or converted into lipids
- Toxic urea also produced - Excreted in urine
Why would a high intake of protein likely result in a high concentration of urea in urine?
- High intake of protein leads to a large amount of amino acids
- Excess amino acids cannot be stored
- Amino acids deaminated
- NH2 group converted to ammonia (NH3)
- Large amount of ammonia enters ornithine cycle
- Converted to urea
- Increased blood concentration of urea leads to more urea in urine
Where is the enzyme catalase found?
In hepatocytes
What is the function of catalase?
Hydrolyses hydrogen peroxide (H2O2) into water and oxygen
Why is catalase important?
- H2O2 produced naturally in body
- H2O2 is toxic
- Catalase converts H2O2 into non-toxic water and oxygen
Outline another method by which hepatocytes detoxify the body
- Alcohol dehydrogenase enzyme breaks down ethanol to ethanal
- Ethanol found in alcoholic drinks
- Ethanal converted to ethanoate
- Ethanoate used to build up fatty acids or in cellular respiration
Why does excess drinking particularly affect the liver?
- Ethanol from alcoholic drinks absorbed and concentrated in liver
- Hepatocytes detoxify ethanol using alcohol dehydrogenase enzyme
- Forms ethanal
- Ethanal further modified to form ethanoate
- An excess of toxic ethanol affects cells of the liver first
Why is fatty tissue build-up a common symptom of excess drinking?
- Ethanoate produced as ethanol detoxified in the liver
- Ethanoate fed into pathway synthesising fatty acids
- Fatty acids built up into lipids
- If excess alcohol consumed, fat likely to build up in hepatocytes as a result of detoxification
Define osmoregulation
Control of internal solute concentration in a living organism
Outline the function of the kidneys
- Excretion of urea
- Osmoregulation - Maintain water balance and pH of the blood
What is the function of the cortex?
- Filtering of the blood - Dense capillary network carries blood from renal artery to nephrons
What is the function of the medulla?
- To regulate concentration of the urine
- Contains tubules of nephrons and collecting ducts
What is the function of the pelvis?
Collects urine before passing down the urethra
What is the function of the glomerulus and Bowman’s capsule?
Ultrafiltration
What is the function of the proximal convoluted tubule?
Selective reabsorbtion
What is the function of the loop of Henle?
Produces high solute concentrations in medulla
What is the function of the distal convoluted tubule?
Adjusts individual solute concentrations and pH of blood
What is the function of the collecting duct?
Controls volume and concentration of urine
Explain why ultrafiltration occurs in the glomerulus
- Pores in capillary wall - Allow small molecules to leave blood and enter Bowman’s capsule
- High hydrostatic pressure in glomerulus - Produced by the afferent arteriole being wider than efferent arteriole
Explain why plasma proteins can’t enter the Bowman’s capsule
Too large to pass through pores in capillary endothelium
Describe the structure and function of podocytes
- Cells in wall of Bowman’s capsule
- Act as additional filter
- Extensions (pedicels) wrap around capillaries
- Prevent cells, platelets and proteins that have passed through epithelial cells and basement membrane from entering Bowman’s capsule
Where are microvilli located in the nephron?
Proximal convoluted tubule
What is reabsorbed by selective reabsorption in the proximal convoluted tubule?
- Some ions
- All glucose, amino acids
- Lots of water
- No urea
Explain how selective reabsorption occurs in the proximal convoluted tubule
- Walls are a single layer of cells thick
- Microvilli provide large surface area for absorption
- Protein pumps allow reabsorption of glucose and ions by active transport
- Water then reabsorbed by osmosis
- ATP produced by many mitochondria
What is the role of the loop of Henle?
- Increases solute (Na+ and Cl-) concentration in the medulla
- Raises solute concentration in medulla higher than that of normal body fluids (hypertonic)
Describe the passage of fluid through the loop of Henle
- Glomerular filtrate flows deep into the medulla in descending limb of loop of Henle
- Flows back out to cortex in ascending limb
- Descending limb very permeable to water
- Ascending limb very permeable to sodium ions, impermeable to water
How does the loop of Henle create a high solute concentration in the medulla?
- Descending limb is permeable to water
- Some water removed from descending limb by osmosis
- Ascending limb pumps sodium and chloride ions from the filtrate into medulla by active
transport - Creating high solute concentration in medulla
- Water potential of filtrate leaving loop of Henle is higher than surrounding tissue
- More water removed from filtrate in collecting duct
What is the role of the distal convoluted tubule?
- Ions exchanged between the filtrate and blood
- Blood solute levels and pH are adjusted
What is the role of the collecting duct?
- Determines concentration and volume of filtrate
- Water diffuses out of collecting duct as it passes through medulla
How does the collecting duct control the solute levels of the blood?
By osmoregulation
How is the body’s water content monitored?
- Osmoreceptors in hypothalamus
- Sensitive to concentration of inorganic ions in blood
Describe the role of ADH (anti-diuretic hormone) in human osmoregulation
- Produced by osmoreceptors in hypothalamus
- Passes along axons to posterior pituitary gland where it is stored
- Secreted when blood is hypertonic (water content too low)
- Makes walls of collecting duct and distal convoluted tubule permeable to water
- More aquaporins inserted into membranes of collecting duct cells
- More water reabsorbed from filtrate
- Small volume of concentrated urine excreted
Define secondary messenger
Molecule that relays signals received at a cell surface to molecules inside cells
How does ADH cause an increase in permeability of the collecting duct?
- ADH binds to receptors on cell membrane of collecting duct cells
- Triggers formation of cyclic AMP (cAMP) - cAMP is a secondary messenger
- cAMP causes a cascade of events :
- Vesicles in cells lining collecting duct fuse with cell surface membrane
- Membranes of vesicles contain aquaporin channels
- Aquaporins make membrane permeable to water
Explain what happens when water content of the blood is too low
- ADH released from pituitary gland
- Travels in blood
- Increases permeability of collecting duct
- Aquaporins (protein channels) inserted into membrane of collecting duct cells
- More water in filtrate reabsorbed by osmosis
- Urine becomes more concentrated
- Blood plasma becomes more dilute
Explain what happens when water content of the blood is too high
- ADH no longer released from the pituitary gland
- Decreases permeability of the collecting duct
- Aquaporins removed from cell membranes
- Urine becomes more dilute
- Blood plasma becomes less dilute
What type of molecule is ADH?
- Hormone
- Made from protein
Which organ removes ADH from the blood?
The liver
Explain the role of the medulla and the collecting duct of the kidney in the maintenance of the water balance in blood
- Collecting duct has aquaporins
- Medulla is hypertonic (high solute concentration)
- Reabsorption of water allows excretion of concentrated urine
- Secretion of ADH increases permeability of collecting duct to water
Explain how the structure of the nephron and its associated blood vessels enable the kidney to carry out its functions
- Osmoregulation and excretion of urea are the functions of the kidney
- Ultrafiltration occurs in the glomerulus
- Basement membrane and podocytes prevent loss of large proteins and blood cells
- High hydrostatic pressure in glomerulus due to larger afferent than efferent arteriole
- Selective reabsorption of glucose and other useful substances in proximal convoluted
tubule - Microvilli give large surface area
- Protein pumps reabsorb specific solutes e.g. ions, glucose, amino acids
- Loop of Henle creates high solute concentration in medulla
- Water reabsorbed in descending limb of loop of Henle
- Active transport of sodium ions out of ascending limb from filtrate to medulla
- Ascending limb is impermeable to water
- Distal convoluted tubule adjusts pH and concentration of Na+/K+/H+ ions
- Water reabsorbed in collecting duct
- Collecting duct permeability to water varies due to number of aquaporins
- Number of aquaporins controlled by presence of ADH
- Osmoregulation by varying the amount of water reabsorbed
- More ADH = more aquaporins = more water reabsorbed
Explain how a longer loop of Henle assists desert animals prevent excessive water loss
- More sodium and chloride ions pumped out of ascending limb into medulla
- Builds up greater water potential gradient
- Allows reabsorption of more water from collecting duct
Explain the differences in fluid composition of blood, glomerular filtrate and urine
- Large molecules (e.g. proteins, blood cells) present in blood only
- Endothelium, basement membrane and podocyte fenestration prevent large
molecules reaching Bowman’s capsule - Glucose present in blood and glomerular filtrate only
- Glucose completely reabsorbed at the proximal convoluted tubule
- Amino acids present in blood and glomerular filtrate only
- All amino acids reabsorbed at the proximal convoluted tubule
- Ion concentration higher in blood and glomerular filtrate than in urine
- Some ions reabsorbed throughout nephron
- Urea concentration much higher in urine
- Urea concentration increases as water removed from filtrate
Which blood vessel brings blood to the kidney?
Renal artery
Which blood vessel takes blood away from the kidney?
Renal vein
Distinguish between the composition of the blood of the renal artery and the blood of the renal vein
- Less urea in renal vein
- Less oxygen in the renal vein
- More carbon dioxide in renal vein
- Less glucose in renal vein
- Water and concentration of sodium ions at normal level in renal vein but variable in renal
artery
Why does kidney failure lead to health issues?
- Kidneys remove toxins and maintain water and electrolyte balance
- If they fail, toxins not removed and may cause damage
- If water or electrolyte balance is too far away from normal, cells may suffer osmotic damage
What are the symptoms of kidney infection?
- Protein and/or blood in urine
- Basement membrane and podocytes damaged
- No longer act as filter for large plasma proteins and blood cells
What are the effects of kidney failure?
Build up of toxic urea in blood
- Body cannot excrete urea
High blood pressure
- Loss of osmotic balance can lead to increase in blood pressure
- Can causes heart problems and strokes
Weakened bones
- Calcium and phosphate balance in blood altered
How is glomerular filtration rate (GFR) measured?
- Creatinine levels in blood measured
- Creatinine is breakdown product of muscles
- Normally excreted out in urine
- High creatinine blood levels suggest reduced kidney function
- Caused by reduction in filtration rate
Other than kidney damage, what else can affect GFR?
- Age
- GFR naturally decreases with age
- Gender
- Males usually have more creatinine in blood than females
What are the treatments of kidney failure?
- Kidney transplant
- Renal dialysis
State the advantages of a kidney transplant compared to haemodialysis
- Transplant cheaper in long term
- Haemodialysis requires frequent hospital visits
State the problems associated with kidney transplants
- Shortage of donors
- Possibility of organ rejection
- Ongoing need for immunosuppressant drugs
- Needs matching tissue type and blood group
What does glucose in urine indicate?
Diabetes
