homeostasis Flashcards
what is the role of the kidneys?
make urine by filtering waste products out of the blood and reabsorbing useful molecules, one of the main functions is to excrete waste products such as urea, regulate the water potential of the blood
how do the kidneys excrete waste and regulate blood water potential?
as the blood passes through capillaries in the cortex, substances are filtered out of the blood and into long tubules that surround the capillaries in ultrafiltration, useful substances such as glucose and water are reabsorbed into the blood in selective reabsorption, the remaining unwanted substances pass through the ureter to the bladder and are excreted as urine
what are nephrons?
the long tubules along with the bundle of capillaries where the blood is filtered, around one million in each kidney
what happens before ultrafiltration?
blood from the renal artery enters arterioles in the cortex, each arteriole splits into a glomerulus, where ultrafiltration takes place
what is a glomerulus?
a bundle of capillaries looped inside a hollow ball called a Bowman’s capsule
what is the process of ultrafiltration?
the blood enters the glomerulus through the afferent arteriole, and leaves through the efferent arteriole, the efferent is smaller in diameter creating high hydrostatic pressure, forcing liquid and small molecules out of the capillary and into the Bowman’s capsule, passing through three layers to get into the bowman’s capsule and enter the nephron tubule, capillary wall, basement membrane and epithelium of the BC, larger molecules like proteins can’t pas through so stay in the blood. The substances that enter the BC are called the glomerular filtrate, which goes through selective reabsorption before travelling through the collecting duct and out via the ureter
what is the afferent arteriole?
how blood enters the glomerulus, larger in diameter
what is the efferent arteriole?
how blood leaves the glomerulus, smaller in diameter
what are the structural features of the kidney?
renal artery, renal vein, cortex, medulla, ureter (leading to bladder and urethra), nephrons
what are the structural features in one nephron?
afferent arteriole, bowman’s capsule, glomerulus, efferent arteriole, proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), collecting duct, renal vein, ureter
what is meant by water potential?
describes the tendency of water to move from one area to another, water will move from an area of higher WP to an area of lower WP - it moves down the water potential gradient
what is urine usually made up of?
water, dissolved salts, urea, other substances such as hormones and excess vitamins (CHLOE REMEMBER WHEN THE YOU HAD A VITAMIN OVERDOSE AND THE DOCTOR TOLD YOU YOUR WEE WOULD BE YELLOW? THIS IS WHY!!)
what does urine usually not contain?
proteins and blood cells - they’re too big to be filtered out of the blood, glucose because it is actively reabsorbed back into the blood
what is selective reabsorbtion?
the process whereby certain molecules (e.g. ions, glucose and amino acids) are reabsorbed from the filtrate as they pass through the nephron
which blood vessel supplies the kidney with blood?
the renal artery which branches into the afferent arteriole before reaching the glomerulus
by which two processes is glucose reabsorbed from the proximal convoluted tubule (PCT)?
active transport and facilitated diffusion
explain how the glomerular filtrate is formed at the glomerulus/bowman’s capsule (2 marks)
The efferent arteriole has a smaller diameter than the afferent arteriole, so the blood in the glomerulus is under high pressure. The high pressure forces liquid and small molecules into the Bowman’s capsule, forming the glomerular filtrate.
would you expect the concentration of glucose to be lower at the proximal convoluted tubule or the loop of Henle (1 mark)
the loop of Henle, because glucose is reabsorbed in the PCT, so by the time it reaches the loop of Henle there will be less glucose remaining
what is the glomerular filtration rate?
the rate at which the kidneys filter the blood, GFR usually around 6300cm^3hour^-1
what is osmoregulation?
the kidneys regulate the water potential of the blood and urine so the body has just the right amount of water
how is water lost from the body?
mammals excrete urea in solution so water is lost in excretion, also through sweat (involved in thermoregulation)
what happens if the water potential of the blood is too low?
the body is dehydrated, more water is reabsorbed by osmosis into the blood from the tubules of the nephrons, urine is more concentrated so less water is lost during excretion
what happens if the water potential of the blood is too high?
the body is too hydrated, less water is reabsorbed by osmosis into the blood from the tubules of the nephrons, urine is more dilute, more water is lost during excretion
where does the reabsorption of water take place?
water is reabsorbed into the blood along almost all of the nephron, but regulation of water potential mainly takes place in the loop of Henle, distal convoluted tubule and collecting duct, the volume of water reabsorbed by the DCT and collecting duct is controlled by hormones
what is the loop of Henle?
the loop of Henle is located in the medulla (inner layer) of the kidneys. It’s made from two limbs - the descending limb and the ascending limb. The limbs control the movement of sodium ions so that water can be reabsorbed into the blood
why is ADH called antidiuretic hormone?
because diuresis is when lots of dilute urine is produced, so anti means a small amount of concentrated urine is produced
what is dehydration?
dehydration is what happens when you lose water, e.g. by sweating during exercise, so the water content of the blood needs to be increased
how do blood ADH levels rise when you’re dehydrated?
water content of blood drops so its water potential drops, this is detected by osmoreceptors in the hypothalamus, the posterior pituitary gland is stimulated to release more ADH into the blood, more ADH means the DCT and collecting duct become more permeable so more water is reabsorbed by osmosis, a small amount of highly concentrated urine is produced so less water is lost
how do blood ADH levels fall when you’re hydrated?
water content of blood rises so its water potential rises, detected by osmoreceptors in the hypothalamus, the posterior pituitary gland releases less ADH into the blood, less ADH so DCT and collecting duct become less permeable so less water is reabsorbed, a large amount of dilute urine is produced so more water is lost
describe what happens along the descending limb of the loop of Henle
there’s a lower WP in the medulla than in the descending limb so water moves out of the descending limb into the medulla by osmosis, this makes the filtrate more concentrated (the ions can’t diffuse out as the descending limb isn’t permeable to them). The water in the medulla is reabsorbed through the capillary network
which cells monitor the water content of the blood?
osmoreceptor cells in the hypothalamus
explain the cause of the increase of ADH in the blood during strenuous exercise (4 marks)
strenuous exercise causes more sweating so more water is lost. This decreases the water potential of the blood. This is detected by osmoreceptors in the hypothalamus, which stimulates the posterior pituitary gland to release more ADH
explain the effect that increased ADH levels in the blood have on kidney function (2 marks)
the ADH increases the permeability of the walls of the distal convoluted tubule and collecting duct. This means more water is reabsorbed into the medulla and into the blood by osmosis
Gerbils have longer loops of Henle than mice, suggest and explain how this helps gerbils to produce less urine than mice (4 marks)
A longer descending limb means that more water can be absorbed into the blood from the nephron in the descending limb. A longer ascending limb means more ions are actively transported into the medulla . This means more water moves out of the collecting duct into the capillaries, giving a low volume of urine.
what is meant by homeostasis?
the maintenance of a constant internal environment
what is the internal environment?
made up of tissue fluids that bathe each cell, supplying nutrients and removing wastes, maintaining the features of this fluid at the optimum levels protects the cells from changes in the external environment, giving the organism a degree of independence
what does homeostasis involve?
trying to maintain the chemical make up, volume, and other features of blood and tissue fluid within restricted limits
what does homeostasis ensure?
that the cells are in an environment that meets their requirements and allows them to function normally despite external changes
how is homeostasis not no changes?
continuous fluctuations brought about by variations in internal and external conditions such as temperature, pH and water potential, occurring around an optimum point, homeostasis is the ability to return to that optimum point to maintain a balanced equilibrium
why is homeostasis important for maintaining a constant pH and temperature?
enzymes that control biochemical reactions and proteins such as channel proteins, are sensitive to changes in pH and temperature, which can reduce the rate of enzyme controlled reactions or denature enzymes/proteins, even small fluctuations can impair the function of proteins
why is the control of water potential important?
changes to the WP of the blood and tissue fluid may cause cells to shrink, expand or burst (osmotic lysis) as a result of water entering or leaving by osmosis, so cells cannot operate normally, WP also affects the rate of reactions by changing the concentration of enzymes/substrates
why is maintaining a constant blood glucose concentration important?
ensures a constant water potential, and a reliable source of glucose for respiration
what is the series of stages in control mechanisms?
the optimum point monitored by a receptor which informs coordinator, sending information to the effector, creating a feedback mechanism
what is the optimum point?
the point at which the system operates best, monitored by a receptor