11.3 The Kidney Flashcards
What is excretion?
Excretion is the removal from the body of the waste products of metabolic activity
Is defecation considered part of excretion?
Defecation is not considered part of excretion as faeces are undigested food remnants and not metabolic waste products
What are the two key functions of an excretory system?
Removes nitrogenous wastes that may be toxic to the body in large concentrations
Removes excess water to maintain a suitable osmolarity within the tissues and cells
What produces nitrogenous wastes?
Nitrogenous wastes are produced from the breakdown of nitrogen-containing compounds like amino acids and nucleotides
Why must nitrogenous wastes be excreted?
Nitrogenous wastes are toxic to the organism and hence excess levels must be eliminated from the body
What is the type of nitrogenous waste correlated with?
The type of nitrogenous waste in animals is correlated with the evolutionary history of the animal and the habitat
In what form do most aquatic organisms excrete nitrogenous waste?
Most aquatic animals eliminate their nitrogenous wastes as ammonia (NH3)
Why is ammonia the most common form of nitrogenous waste in aquatic environments?
Ammonia is highly toxic but also very water soluble and hence can be effectively flushed by animals in aquatic habitats
Why does nitrogenous waste differ for terrestrial animals?
Terrestrial animals have less access to water and hence must package nitrogenous waste in less toxic forms
In what ways do mammals excrete nitrogenous wastes and why?
Mammals eliminate their nitrogenous wastes as urea, which is less toxic and hence can be stored at higher concentrations
In what ways do reptiles and birds excrete nitrogenous wastes and why?
Reptiles and birds eliminate wastes as uric acid, which requires more energy to make but is relatively non-toxic and requires even less water to flush (it is eliminated as a semi-solid paste)
Why do water levels within an organism change?
Water levels within an organism are constantly changing as a result of metabolic activity
In general, what reactions produce and consume water?
Water is produced via condensation reactions (anabolism) and is consumed during hydrolysis reactions (catabolism)
What impacts tissue viability?
The concentration of water within cells (osmolarity) will impact tissue viability (i.e. governs osmotic pressure within cells)
What are the two terms for animals depending on how they manage their internal osmotic conditions?
Animals may be either osmoconformers or osmoregulators according to how they manage their internal osmotic conditions:
What are osmoconformers?
Osmoconformers maintain internal conditions that are equal to the osmolarity of their environment
What are osmoregulators?
Osmoregulators keep their body’s osmolarity constant, regardless of environmental conditions
How do osmoconformers minimise water movement?
By matching internal osmotic conditions to the environment, osmoconformers minimise water movement in and out of cells
What is the advantage of osmoconformers?
Less energy is used to maintain internal osmotic conditions within an osmoconformer
What is an advantage of osmoregulators?
While osmoregulation is a more energy-intensive process, it ensures internal osmotic conditions are always tightly controlled
What is the key difference between osmoconformers and regulators?
Osmoregulators can maintain optimal internal conditions whereas osmoconformers are affected by environmental conditions
What specialised systems do animals have in terms of excretion?
All animals possess a specialised excretory system for osmoregulation and the removal of nitrogenous wastes
What is the difference between the excretory system in mammals and insects?
In mammals, the excretory system (kidneys) is separate from the digestive system of the animal
In insects, the excretory system (Malpighian tubules) connects to the digestive system of the animal
- What circulatory system do insects have
Malpighian Tubules
Insects have a circulating fluid system called hemolymph that is analogous to the blood system in mammals
- What is the role of these tubules?
Malpighian Tubules
Malpighian tubules branch off from the intestinal tract and actively uptake nitrogenous wastes and water from the hemolymph
- Where are these materials passed from the tubules?
Malpighian Tubules
The tubules pass these materials into the gut to combine with the digested food products
- What is the ultimate product of the excretory system?
Malpighian Tubules
Solutes, water and salts are reabsorbed into the hemolymph at the hindgut,
whereas nitrogenous wastes (as uric acid) and undigested food materials are excreted via the anus
What is the role of the kidneys?
The kidney functions as the blood’s filtration and water balancing system – it removes metabolic wastes for excretion
Describe the blood flow through the kidney?
Blood enters the kidneys via the renal artery and exits the kidneys via the renal vein
What filters blood in the kidney?
Blood is filtered by specialised structures called nephrons which produce urine
Where is urine transported to from the kidneys? What is it transported through?
The urine is transported from the kidneys via the ureter, where it is stored by the bladder prior to excretion
What is the role of the nephrons?
The kidney contains specialised structures called nephrons which function to filter the blood and eliminate wastes
What does the filtering function of the kidney say about the composition of the blood entering and leaving the kidney?
Consequently, the composition of blood entering the kidney (via renal artery) differs to that exiting the kidney (via renal vein)
How will the urea content of blood in the renal vein differ from the blood in the renal artery?
Less urea (large amounts of urea is removed via the nephrons to form urine)
How will the water and solute/ion content of blood in the renal vein differ from the blood in the renal artery?
Less water and solutes / ions (amount removed will depend on the hydration status of the individual)
How will the glucose and oxygen content of blood in the renal vein differ from the blood in the renal artery?
Less glucose and oxygen (not eliminated, but used by the kidney to generate energy and fuel metabolic reactions)
How will the carbon dioxide content of blood in the renal vein differ from the blood in the renal artery?
More carbon dioxide (produced by the kidneys as a by-product of metabolic reactions)
What is a nephron?
The nephron is the functional unit of the kidney
What are the 4 components of a nephron?
Bowman’s capsule
Proximal convoluted tubule
Loop of Henle
Distal Convoluted Tubule
What is the Bowman’s Capsule?
first part of the nephron where blood is initially filtered (to form filtrate)
What is the Loop of Henle?
a selectively permeable loop that descends into the medulla and establishes a salt gradient
What is the proximal convoluted tubule?
folded structure connected to the Bowman’s capsule where selective reabsorption occurs
What is the distal convoluted tubule?
a folded structure connected to the loop of Henle where further selective reabsorption occurs
How does blood enter and leave the Bowman’s capsule?
The blood to be filtered enters the Bowman’s capsule via an afferent arteriole and leaves the capsule via an efferent arteriole
Where is blood filtered within the glomerulus?
Within the Bowman’s capsule, the blood is filtered at a capillary tuft called the glomerulus
What does the efferent arteriole form in the Bowman’s Capsule and what is its purpose?
The efferent arteriole forms a blood network called the vasa recta that reabsorbs components of the filtrate from the nephron
What does each nephron connect to?
Each nephron connects to a collecting duct (via the distal convoluted tubule), which feed into the renal pelvis
Why are collecting ducts not considered to be a part of the nephron?
The collecting ducts are shared by nephrons and hence are not technically considered to be part of a single nephron
What is the role of nephrons?
Nephrons filter blood and then reabsorb useful materials from the filtrate before eliminating the remainder as urine
What are the 3 processes that occur in a nephron?
ultrafiltration
selective reansorption
osmoregulation
What is ultrafiltration?
Blood is filtered out of the glomerulus at the Bowman’s capsule to form filtrate
What is selective reabsorption?
Usable materials are reabsorbed in convoluted tubules (both proximal and distal)
What is osmoregulation?
The loop of Henle establishes a salt gradient, which draws water out of the collecting duct
Where does ultrafiltration occur and give a definition?
It is the non-specific filtration of the blood under high pressure and occurs in the Bowman’s capsule of the nephron
What is the glomerulus encapsulated by?
This glomerulus is encapsulated by the Bowman’s capsule
, which is comprised of an inner surface of cells called podocytes
What do podocytes have?
Podocytes have cellular extensions called pedicels that wrap around the blood vessels of the glomerulus
What is between the podocytes and glomerulus?
Between the podocytes and the glomerulus is a glycoprotein matrix called the basement membrane that filters the blood
What is the role of the basement membrane?
Blood is filtered by a mesh called the basement membrane, which lies between the glomerulus and Bowman’s capsule
Why can blood freely leave the glomerulus?
Glomerular blood vessels are fenestrated (have pores) which means blood can freely exit the glomerulus
How do podocytes allow for fluid to move freely?
The podocytes of the Bowman’s capsule have gaps between their pedicels, allowing for fluid to move freely into the nephron
What is the sole filtration barrier in the nephron?
Consequently, the basement membrane functions as the sole filtration barrier within the nephron
What does the basement membrane restrict?
The basement membrane is size-selective and restricts the passage of blood cells and large proteins
Due to the basement membrane being size-selective, what is not in the filtrate?
Hence when the blood is filtered, the filtrate formed does not contain any blood cells, platelets or plasma proteins
What is the role of ultrafiltration?
Ultrafiltration involves blood being forced at high pressure against the basement membrane, optimising filtration
- What creates the high hydrostatic pressure?
Ultrafiltration
This high hydrostatic pressure is created in the glomerulus by having a wide afferent arteriole and a narrow efferent arteriole
- what is the role of the high hydrostatic pressure?
Ultrafiltration
This means it is easy for blood to enter the glomerulus, but difficult for it to exit – increasing pressure within the glomerulus
- What other adaption does the glomerulus have?
Ultrafiltration
Additionally, the glomerulus forms extensive narrow branches, which increases the surface area available for filtration
- What is the overall movement of blood in the glomerulus?
Ultrafiltration
The net pressure gradient within the glomerulus forces blood to move into the capsule space (forming filtrate
What is the second process in the nephron?
Selective reabsorption is the second of the three processes by which blood is filtered and urine is formed
What is the role of selective reabsorption?
It involves the reuptake of useful substances from the filtrate and occurs in the convoluted tubules (proximal and distal)
Where does the majority of selective reabsorption occur?
The majority of selective reabsorption occurs in the proximal convoluted tubule, which extends from the Bowman’s capsule
How is the proximal convoluted tubule adapted for selective reabsorption?
The proximal convoluted tubule has a microvilli cell lining to increase the surface area for material absorption from the filtrate
The tubule is a single cell thick and connected by tight junctions, which function to create a thin tubular surface with no gaps
What organelle does the proximal convoluted tubule have a large number of?
There are also a large number of mitochondria within these tubule cells, as reabsorption involves active transport
Where are substances actively transported in the proximal convoluted tubule?
Substances are actively transported across the apical membrane (membrane of tubule cells facing the tubular lumen)
Where do substances passively diffuse in the proximal convoluted tubule?
Substances then passively diffuse across the basolateral membrane (membrane of tubule cells facing the blood)
What do the tubules reabsorb?
The tubules reabsorb all glucose, amino acids, vitamins and hormones, along with most of the mineral ions (~80%) and water
How are mineral ions and vitamins transported in the PCT - (proximal convoluted tubule) ?
Mineral ions and vitamins are actively transported by protein pumps and carrier proteins respectively
How is glucose transported in the PCT?
Glucose and amino acids are co-transported across the apical membrane with sodium (symport)
How is water transported in the PCT?
Water follows the movement of the mineral ions passively via osmosis
What is the third process in the nephron?
Osmoregulation is the third of three processes by which blood is filtered and urine is formed
What is osmoregulation? (definition)
Osmoregulation is the control of the water balance of the blood, tissue or cytoplasm of a living organism
Where does osmoregulation occur?
Osmoregulation occurs in the medulla of the kidney
What is the role of the loop of Henle in osmoregulation?
The loop of Henle establishes a salt gradient (hypertonicity) in the medulla
What is the role of ADH in osmoregulation?
Anti-diuretic hormone (ADH) regulates the level of water reabsorption in the collecting duct
What is the function of the loop of Henle?
The function of the loop of Henle is to create a high solute (hypertonic) concentration in the tissue fluid of the medulla
What is the descending limb permeable to?
The descending limb of the loop of Henle is permeable to water but not salts
What is the ascending limb permeable to?
The ascending limb of the loop of Henle is permeable to salts but not water
What is the solute like in the loop of Henle descending limb?
This means that as the loop descends into the medulla, the interstitial fluid becomes more salty and hypertonic
What surrounds the loop of Henle?
Additionally, the vasa recta blood network that surrounds the loop of Henle flows in the opposite direction (counter-current)
What is the role of the vasa recta?
This means that salts released from the ascending limb are drawn down into the medulla, further establishing a salt gradient
WATER REABSORPTION
1. What draws out water in the collecting duct?
As the collecting duct passes through the medulla, the hypertonic conditions of the medulla will draw water out by osmosis
WATER REABSORPTION
2. What controls water reabsorption?
The amount of water released from the collecting ducts to be retained by the body is controlled by anti-diuretic hormone (ADH)
WATER REABSORPTION
3. When is ADH released and from where?
ADH is released from the posterior pituitary in response to dehydration (detected by osmoreceptors in the hypothalamus)
WATER REABSORPTION
4. What is the role of ADH?
ADH increases the permeability of the collecting duct to water, by upregulating production of aquaporins (water channels)
WATER REABSORPTION
5. What do high levels of ADH do to urine?
This means less water remains in the filtrate, urine becomes concentrated and the individual urinates less (i.e. anti-diuresis)
WATER REABSORPTION
6. What happens when an individual is suitably hydrated?
When an individual is suitably hydrated, ADH levels decrease and less water is reabsorbed (resulting in more dilute urine)
Remember: ADH is produced when you Are DeHydrated
What is critical for the survival of an organism?
Maintaining an appropriate water balance within the body’s tissues and cells is critical to the survival of an organism
What cannot be maintained if water levels are not regulated?
Homeostasis cannot be maintained if water levels drop (dehydration) or are raised (overhydration) without regulation
What is dehydration?
Dehydration is a loss of water from the body such that body fluids become hypertonic
What will individuals experience when dehydrated?
Individuals will experience thirst and excrete small quantities of heavily concentrated urine (to minimise water loss)
What happens to blood pressure when an individual is dehydrated?
Blood pressure will drop (less water in plasma) and the heart rate will increase to compensate for this
What will happen to an individual temperature-wise when dehydrated?
The individual will become lethargic and experience an inability to lower body temperature (due to lack of sweat)
What may severe cases of dehydration cause?
Severe cases of dehydration may cause seizures, brain damage and eventual death
What is overhydration?
Overhydration is a less common occurrence that results when an over-consumption of water makes body fluids hypotonic
How will overhydration affect urine production?
Individuals will produce excessive quantities of clear urine in an effort to remove water from the body
What can overhydration do to body cells?
The hypotonic body fluids will cause cells to swell (due to osmotic movement), which can lead to cell lysis and tissue damage
What can overhydration lead to in mild cases?
Overhydration can lead to headaches and disrupted nerve functions in mild cases (due to swelling of cells)
What can overhydration lead to in severe cases?
In severe cases, overhydration may lead to blurred vision, delirium, seizures, coma and eventual death
What will the need for water conservation depend on?
All animals need to maintain an appropriate water balance, however the need for water conservation will depend on habitat
What habitats will require more efficient water conservation?
Animals in arid, desert environments will need more efficient water conservation than animals in moist, mesic environments
How can water conservation be improved?
Water conservation can be improved by having a longer loop of Henle, which increase the salt gradient in the medulla
How does an increased salt gradient help water reabsorption?
A greater the salt gradient in the medulla means more water is reabsorbed by the collecting ducts and urine is concentrated
What is the length of the loop of Henle positively correlated with?
Hence, the length of the loop of Henle is positively correlated with the degree of water conservation in animals
What type of nephrons do animals have that live in moist environments?
Animals living in moist environments have short loops of Henle that don’t descend deeply into the medulla (cortical nephrons)
What type of nephrons do animals have that live in arid environments?
Animals living in arid environments have long loops of Henle that descend deeply into the medulla (juxtamedullary nephrons)
What are kidney diseases?
Kidney diseases are conditions which incapacitate the kidney’s ability to filter waste products from the blood
What will individuals with kidney diseases demonstrate?
Individuals with kidney diseases will demonstrate a reduced glomerular filtration rate (GFR)
What can untreated kidney diseases lead to?
If untreated, kidney diseases can lead to kidney failure – which is life threatening
What do kidneys prevent?
Kidneys prevent the excretion of blood cells and proteins (during ultrafiltration), as well as glucose (selective reabsorption)
What substances in urine can be an indicator of kidney diseases? (4)
glucose
proteins
blood cells
drugs/toxina
How is the presence of glucose in urine an indicator of kidney diseases?
The presence of glucose in urine is a common indicator of diabetes (high blood glucose = incomplete reabsorption)
How is the presence of proteins in urine an indicator of kidney diseases?
High quantities of protein in urine may indicate disease (e.g. PKU) or hormonal conditions (e.g. hCG = pregnancy)
How is the presence of blood cells in urine an indicator of kidney diseases?
The presence of blood in urine can indicate a variety of diseases, including certain infections and cancer
How is the presence of drugs/toxins in urine an indicator of kidney diseases?
Many drugs pass through the body into urine and can be detected (e.g. performance enhancing drugs)
What does kidney dialysis involve?
Kidney dialysis involves the external filtering of blood in order to remove metabolic wastes in patients with kidney failure
What is the dialyzer similar to?
Blood is removed and pumped through a dialyzer, which has two key functions that are common to BIOLOGICAL MEMBRANES:
What are the two similarities between a dialyzer and a biological membrane?
It contains a porous membrane that is semi-permeable (restricts passage of certain materials)
It introduces fresh dialysis fluid and removes wastes to maintain an appropriate concentration gradient
How often is kidney dialysis needed?
Kidney dialysis treatments typically last about 4 hours and occur 3 times a week – these treatments can be effective for years
What is a problem of hemodialysis?
Hemodialysis ensures continued blood filtering, but does not address the underlying issue affecting kidney function
What is the best long-term treatment for kidney diseases?
The best long-term treatment for kidney failure is a kidney transplant
Where is the transplanted kidney grafted into?
The transplanted kidney is grafted into the abdomen, with arteries, veins and ureter connected to the recipient’s vessels
What must the transplanted kidney be similar to?
Donors must typically be a close genetic match in order to minimise the potential for graft rejection
How many kidneys are needed to survive?
Donors can survive with one kidney and so may commonly donate the second to relative suffering kidney failure
