Excretion Flashcards
What are the 2 roles of the kidneys?
Filter nitrogenous waste products (urea) out of the blood
Maintain water potential of the blood/tissue fluid (osmoregulation)
How much blood passes through the kidneys per minute?
90-120cm cubed
How much urine do people produce per day?
1-2 dm cubed
What are the regions of the kidneys?
Capsule Cortex Medulla Pelvis Ureter
What blood vessel transports blood to the kidneys?
Renal artery
What blood vessel transports blood away from the kidneys?
Renal vein
What is the medulla of the kidneys made of?
Renal pyramids
Where does each nephron start?
In the Cortex of the kidney
What is vasa recta?
The blood that enters in the artery, branches to become the capillary network around the nephron
Compare the blood in the renal vein and renal artery?
Renal vein has much reduced urea concentration
Renal vein has almost the same concentration of glucose and amino acids
The renal vein has a restored ion balance (back to ideal levels)
Why is there slightly less glucose in the renal vein than the renal artery?
Some of the kidney cells will use the glucose in respiration
What parts of a nephron are in the cortex?
Afferent arteriole Efferent arteriole Glomerulus Bowman's capsule Proximal convoluted tubule Distal convoluted tubule
What parts of a nephron are in the medulla?
Descending limb
Loop of Henle
Ascending limb
What is ultrafiltration?
The filtering of substances out of the blood at the molecular level
Why is the blood pressure in the glomerulus high?
Due to the decrease in diameter from afferent arteriole to efferent arteriole
Why is there a lower water potential in the blood compared to the fluid in the Bowman’s capsule?
Blood contains plasma proteins that are too large to be filtered so stay in the blood and lower water potential
Why does the fluid enter the Bowman’s capsule?
The hydrostatic pressure is greater than the force of osmosis so the fluid is forced out of the blood
What is the fluid in the Bowman’s capsule called?
Filtrate (it has been filtered)
What are the 3 layers that molecules have to pass through to move from the blood to the renal filtrate?
The endothelium of the capillary
Basement membrane of capillary
Special cells called podocytes
What about the endothelium of the capillary allows molecules to pass into the Bowman’s capsule
There are small gaps so only small molecules like glucose can get through while large molecules like red blood cells can’t
What is the basement membrane of the Bowman’s capsule made of?
It is like a fine mesh made of collagen and glycoproteins
What does the basement membrane of the Bowman’s capsule do?
It acts as a filter and prevents the passage of molecules with a relative molecular mass greater than 69000. Most proteins and all blood cells are too large to leave the glomerulus
What do podocytes do?
They are an additional filter, they have extensions called pedicels that wrap around the capillaries forming slits. Fluid from the blood in the glomerulus can pass between these cells into the lumen of the Bowman’s capsule.
What is the glomerular filtration rate (GFR)?
The volume of blood filtered through the kidneys in a given time
What does the glomerular filtration rate (GFR) give us an idea of?
How well the kidneys are working
What happens to the useful substances that were filtered into the Bowman’s capsule?
Selective reabsorption
Where does most selective reabsorption occur?
85% occurs in the proximal convoluted tubule
How are the cells lining the proximal convoluted tubule adapted for selective reabsorption.
Microvilli - increase SA, more reabsorption
Gaps between microvilli - useful substances absorbed into cell easily - small diffusion pathway
Mitochondria - provide energy for active transport
Capillary close by to take away substances that are reabsorbed.
How does glucose/amino acids get reabsorbed?
Na+ - K+ pump removed Na+ from the cells lining the proximal convoluted tubule.
This lowers the [Na+] in the cell so they transported into the cell with glucose/amino acids by facilitated diffusion.
[Glucose/amino acids] increases in the cell so they diffuse into the tissue fluid.
What extra thing can aid the reabsorption of glucose/amino acids?
Active transport into the cell (alongside the facilitated diffusion)
How is water reabsorbed in the proximal convoluted tubule?
Glucose/amino acids moving into the cell by facilitated diffusion lowers the water potential of the cell so water moves in by osmosis and then gets reabsorbed into the blood.
Why isn’t all glucose reabsorbed when there is a high concentration?
All glucose carriers will become saturated so excess glucose is excreted
What is the role of the loop of Henle?
Create a low water potential in the tissue fluid around the nephron.
What happens to ions in the descending limb?
They do not leave as it is impermeable to ions. However, Na+ and Cl- ions diffuse into it.
What happens to water at the descending limb?
Moves out by osmosis and moves into the capillaries
What happens at the lower part of the ascending limb?
It is permeable to ions but not to water so Na+ and Cl- ions diffuse out but the water remains in
What happens in the upper part of the ascending limb?
Active removal of Na+ and Cl- ions creates a low water potential in surrounding tissue
What does the low water potential in the tissue around the ascending limb cause?
Water to move out of the collecting duct by osmosis and enter the capillary. Any water that stays in the collecting duct passes to pelvis as urine.
How does the collecting duct change the concentration of urine?
It’s permeability to water can change. Lower water potential in blood, more permeable, more water reabsorbed into the blood.
Higher water potential in blood is opposite
How do the 2 limbs of the loop of Henle act as a counter current multiplier?
The ascending limb uses ATP to create the concentration gradient needed for water to be removed from the descending limb - moving back into the blood
Definition of counter-current system?
This is where fluid flows in opposite directions in two vessels close to one another
Why is a counter-current system important for the two limbs of the loop of Henle?
It maximises the concentration gradient that can be built up between the two, all the way along the length
What hormone mediates Na+ ions?
Aldosterone
What mediates Calcium ions?
PTH
After the ascending limb where does further balancing of water and salt content occur?
Distal convoluted tubule
What do the cells of the distal convoluted tubule have to maximise the effectiveness of transporting ions?
Microvilli - large SA
Lots of mitochondria for active transport
If the body lacks salt what occurs at the distal convoluted tubule?
Sodium ions are actively pumped out, then chloride ions follow because of the electrochemical gradient and move back into the blood
What is the distal convoluted tubule also permeable to apart from ions?
Water
What hormone does the distal convoluted tubule and the collecting ducts respond to in order to save more water?
ADH (Anti-diuretic hormone)
Where is the final volume and concentration of urine determined?
Collecting ducts
What does osmoregulation mean?
Maintenance of water potential in the blood
Why is osmoregulation important?
Affects osmosis into cells
Too high in blood, cells burst
Too low in blood, cells shrivel, dehydrate, low metabolism
How does the body gain water?
Food, drink, metabolism
How does the body lose water?
Breathing, sweat, urine
Where is ADH made?
Hypothalamus
Where is ADH stored/secreted?
Posterior pituitary gland
What monitors the water potential of the blood flowing through the brain?
Osmoreceptors
What type of hormone is ADH?
Peptide/protein
When is ADH released into the bloodstream?
When the water potential of the blood falls
What part of the nephron does ADH act on?
The cells of the collecting duct (and DCT)
Have specific receptors that are a complementary shape to ADH
What cells produce ADH?
ADH-synthesising nerve cells
Neurosecretory cells
What happens when the water potential of the blood decreases?
Detected by osmoreceptors in hypothalamus
Posterior pituitary gland stimulated to release more ADH into the blood
ADH increases permeability of DCT and collecting duct so more water is reabsorbed into blood
A small amount of highly concentrated urine is produced and less water is lost