Chapter Note: Excretion in Humans | Biology for GCSE/IGCSE - Year 11 PDF Download

 Page 1


Excretion
? Excretion is the removal of the waste substance of metabolic reactions (chemical reactions that take
place inside cells), toxic materials and substances in excess of requirement
? Egestion and excretion are different, egestion is the removal of undigested food in our body
? Carbon dioxide must be excreted as it dissolves in water easily to form an acidic solution which can
lower pH of cells
? This can reduce activity of enzymes in the body which are essential for controlling the rate of metabolic
reactions therefore too much carbon dioxide in the body is toxic
? Urea is also toxic to the body in higher concentrations and so must be excreted
? Liver
? Many digested food molecules absorbed into the blood in the small intestine are carried to the
liver for assimilation
? These include amino acids which are used to build proteins such as fibrinogen, a protein found
in blood plasma that is important in blood clotting
? Excess amino acids absorbed in the blood that are not needed to make proteins cannot be
stored, so they are broken down in a process called deamination
? Functions of liver
? Converts excess amino acids into urea
? Synthesises plasma protein such as fibrinogen
? Controls the amount of glucose in the blood with the help of insulin and glucagon
? Makes bile
? Breaks down old RBC’s
? Stores vitamin A, B, D, E and K
? Makes cholesterol which is needed for the repair cell membrane
Page 2


Excretion
? Excretion is the removal of the waste substance of metabolic reactions (chemical reactions that take
place inside cells), toxic materials and substances in excess of requirement
? Egestion and excretion are different, egestion is the removal of undigested food in our body
? Carbon dioxide must be excreted as it dissolves in water easily to form an acidic solution which can
lower pH of cells
? This can reduce activity of enzymes in the body which are essential for controlling the rate of metabolic
reactions therefore too much carbon dioxide in the body is toxic
? Urea is also toxic to the body in higher concentrations and so must be excreted
? Liver
? Many digested food molecules absorbed into the blood in the small intestine are carried to the
liver for assimilation
? These include amino acids which are used to build proteins such as fibrinogen, a protein found
in blood plasma that is important in blood clotting
? Excess amino acids absorbed in the blood that are not needed to make proteins cannot be
stored, so they are broken down in a process called deamination
? Functions of liver
? Converts excess amino acids into urea
? Synthesises plasma protein such as fibrinogen
? Controls the amount of glucose in the blood with the help of insulin and glucagon
? Makes bile
? Breaks down old RBC’s
? Stores vitamin A, B, D, E and K
? Makes cholesterol which is needed for the repair cell membrane
? How urea is made
? Protein in food is taken into the alimentary canal
? The protein molecules are broken down to amino acids during digestion
? The amino acids are absorbed into the blood and taken into the liver by hepatic portal vein
? Amino acids that are not needed are deaminated into ammonia
? The ammonia is converted into urea
? The urea is excreted by the kidney
? Changes in urine
? The colour and quantity of urine produced in the body can change quickly
? Large quantities of urine are usually pale yellow in colour because it contains a lot of water and
so the urea is less concentrated
? Small quantities of urine are usually darker yellow/orange in colour because it contains little
water and so the urea is more concentrated
? There are various reasons why the concentration of urine will change
¦ Water intake - the more fluids drunk, the more water will be removed from the body and
so a large quantity of pale yellow, dilute urine will be produced
¦ Temperature - the higher the temperature the more water is lost in sweat and less will
appear in urine meaning a smaller quantity of dark yellow concentrated urine will be
produced
¦ Exercise - the more exercise done, the more water is lost in sweat and so less appear in
urine meaning smaller quantity of dark yellow concentrated urine will be produced
? Kidney
? The kidneys are located in the back of the abdomen and have 2 important function in the body
¦ They regulate the water content of the blood
¦ They excrete the toxic waste products of metabolism
? Each kidney contains around a million tiny structures called
nephrons also known as kidney tubules or renal tubules
? The nephrons start in the cortex of the kidney, loop down into
the medulla and back up to the cortex
? The contents of the nephrons drain into the innermost part of
the kidney and the urine collects there before it flows into the
ureter to be carried to the bladder for storage
? Regions of the kidney
? The kidney has 2 portions. The outer portion is known as the
cortex and the inner portion is called the medulla
? On the inner curve of the kidney is a space called the renal
pelvis. Urine collects in the renal pelvis and exits the kidney
via the ureter. The ureter from each kidney drains into the
urinary bladder located in the pelvis. The bladder drains via
the urethra
Page 3


Excretion
? Excretion is the removal of the waste substance of metabolic reactions (chemical reactions that take
place inside cells), toxic materials and substances in excess of requirement
? Egestion and excretion are different, egestion is the removal of undigested food in our body
? Carbon dioxide must be excreted as it dissolves in water easily to form an acidic solution which can
lower pH of cells
? This can reduce activity of enzymes in the body which are essential for controlling the rate of metabolic
reactions therefore too much carbon dioxide in the body is toxic
? Urea is also toxic to the body in higher concentrations and so must be excreted
? Liver
? Many digested food molecules absorbed into the blood in the small intestine are carried to the
liver for assimilation
? These include amino acids which are used to build proteins such as fibrinogen, a protein found
in blood plasma that is important in blood clotting
? Excess amino acids absorbed in the blood that are not needed to make proteins cannot be
stored, so they are broken down in a process called deamination
? Functions of liver
? Converts excess amino acids into urea
? Synthesises plasma protein such as fibrinogen
? Controls the amount of glucose in the blood with the help of insulin and glucagon
? Makes bile
? Breaks down old RBC’s
? Stores vitamin A, B, D, E and K
? Makes cholesterol which is needed for the repair cell membrane
? How urea is made
? Protein in food is taken into the alimentary canal
? The protein molecules are broken down to amino acids during digestion
? The amino acids are absorbed into the blood and taken into the liver by hepatic portal vein
? Amino acids that are not needed are deaminated into ammonia
? The ammonia is converted into urea
? The urea is excreted by the kidney
? Changes in urine
? The colour and quantity of urine produced in the body can change quickly
? Large quantities of urine are usually pale yellow in colour because it contains a lot of water and
so the urea is less concentrated
? Small quantities of urine are usually darker yellow/orange in colour because it contains little
water and so the urea is more concentrated
? There are various reasons why the concentration of urine will change
¦ Water intake - the more fluids drunk, the more water will be removed from the body and
so a large quantity of pale yellow, dilute urine will be produced
¦ Temperature - the higher the temperature the more water is lost in sweat and less will
appear in urine meaning a smaller quantity of dark yellow concentrated urine will be
produced
¦ Exercise - the more exercise done, the more water is lost in sweat and so less appear in
urine meaning smaller quantity of dark yellow concentrated urine will be produced
? Kidney
? The kidneys are located in the back of the abdomen and have 2 important function in the body
¦ They regulate the water content of the blood
¦ They excrete the toxic waste products of metabolism
? Each kidney contains around a million tiny structures called
nephrons also known as kidney tubules or renal tubules
? The nephrons start in the cortex of the kidney, loop down into
the medulla and back up to the cortex
? The contents of the nephrons drain into the innermost part of
the kidney and the urine collects there before it flows into the
ureter to be carried to the bladder for storage
? Regions of the kidney
? The kidney has 2 portions. The outer portion is known as the
cortex and the inner portion is called the medulla
? On the inner curve of the kidney is a space called the renal
pelvis. Urine collects in the renal pelvis and exits the kidney
via the ureter. The ureter from each kidney drains into the
urinary bladder located in the pelvis. The bladder drains via
the urethra
? Nephrons
? The nephron is the functional unit of the kidney. There are about 1 million nephrons in each
kidney. The nephron has a long collecting tube that terminates in a hollow, cup-like structure
called a bowman's capsule. Inside the bowman’s capsule is a network is very fine arteries called
glomerulus
? Each tubule of nephron begins in the cortex, loops down to medulla, back into the cortex and
? then goes down to the pelvis. In the pelvis, the tubule.
? Ultrafiltration
? Arterioles branch off the renal artery and lead to each nephron where they form a knot of
capillaries (the glomerulus) sitting inside the cup shaped Bowman’s capsule
? The capillaries get narrower as they get further into the glomerulus which increases the
pressure on the blood moving through them
? This eventually causes the smaller molecules being carried in the blood to be forced out of the
capillaries and into the Bowman’s capsule where they form what is known as filtrate
? This process is known as ultrafiltration
? The substances forced out of the capillaries are glucose, water, urea and salts
? The glomerulus filters water, glucose, urea and salts from the blood
? Selective reabsorption
? After the glomerular filtrate enters the Bowman’s capsule, glucose is the first substance to be
reabsorbed at the first convoluted tubule
? This takes place by active transport
? The nephron is adapted by this by having many mitochondria to provide energy for active
transport of glucose molecules
? Reabsorption of glucose cannot take place anywhere else in the nephron as the gates that
facilitate the active transport of glucose are only found in the proximal convoluted tubule
? People with diabetes cannot control their blood glucose levels; they are often very high, meaning that
not all glucose filtered out can be reabsorbed into the blood. As there is nowhere for glucose to go it
ends up in the urine
Page 4


Excretion
? Excretion is the removal of the waste substance of metabolic reactions (chemical reactions that take
place inside cells), toxic materials and substances in excess of requirement
? Egestion and excretion are different, egestion is the removal of undigested food in our body
? Carbon dioxide must be excreted as it dissolves in water easily to form an acidic solution which can
lower pH of cells
? This can reduce activity of enzymes in the body which are essential for controlling the rate of metabolic
reactions therefore too much carbon dioxide in the body is toxic
? Urea is also toxic to the body in higher concentrations and so must be excreted
? Liver
? Many digested food molecules absorbed into the blood in the small intestine are carried to the
liver for assimilation
? These include amino acids which are used to build proteins such as fibrinogen, a protein found
in blood plasma that is important in blood clotting
? Excess amino acids absorbed in the blood that are not needed to make proteins cannot be
stored, so they are broken down in a process called deamination
? Functions of liver
? Converts excess amino acids into urea
? Synthesises plasma protein such as fibrinogen
? Controls the amount of glucose in the blood with the help of insulin and glucagon
? Makes bile
? Breaks down old RBC’s
? Stores vitamin A, B, D, E and K
? Makes cholesterol which is needed for the repair cell membrane
? How urea is made
? Protein in food is taken into the alimentary canal
? The protein molecules are broken down to amino acids during digestion
? The amino acids are absorbed into the blood and taken into the liver by hepatic portal vein
? Amino acids that are not needed are deaminated into ammonia
? The ammonia is converted into urea
? The urea is excreted by the kidney
? Changes in urine
? The colour and quantity of urine produced in the body can change quickly
? Large quantities of urine are usually pale yellow in colour because it contains a lot of water and
so the urea is less concentrated
? Small quantities of urine are usually darker yellow/orange in colour because it contains little
water and so the urea is more concentrated
? There are various reasons why the concentration of urine will change
¦ Water intake - the more fluids drunk, the more water will be removed from the body and
so a large quantity of pale yellow, dilute urine will be produced
¦ Temperature - the higher the temperature the more water is lost in sweat and less will
appear in urine meaning a smaller quantity of dark yellow concentrated urine will be
produced
¦ Exercise - the more exercise done, the more water is lost in sweat and so less appear in
urine meaning smaller quantity of dark yellow concentrated urine will be produced
? Kidney
? The kidneys are located in the back of the abdomen and have 2 important function in the body
¦ They regulate the water content of the blood
¦ They excrete the toxic waste products of metabolism
? Each kidney contains around a million tiny structures called
nephrons also known as kidney tubules or renal tubules
? The nephrons start in the cortex of the kidney, loop down into
the medulla and back up to the cortex
? The contents of the nephrons drain into the innermost part of
the kidney and the urine collects there before it flows into the
ureter to be carried to the bladder for storage
? Regions of the kidney
? The kidney has 2 portions. The outer portion is known as the
cortex and the inner portion is called the medulla
? On the inner curve of the kidney is a space called the renal
pelvis. Urine collects in the renal pelvis and exits the kidney
via the ureter. The ureter from each kidney drains into the
urinary bladder located in the pelvis. The bladder drains via
the urethra
? Nephrons
? The nephron is the functional unit of the kidney. There are about 1 million nephrons in each
kidney. The nephron has a long collecting tube that terminates in a hollow, cup-like structure
called a bowman's capsule. Inside the bowman’s capsule is a network is very fine arteries called
glomerulus
? Each tubule of nephron begins in the cortex, loops down to medulla, back into the cortex and
? then goes down to the pelvis. In the pelvis, the tubule.
? Ultrafiltration
? Arterioles branch off the renal artery and lead to each nephron where they form a knot of
capillaries (the glomerulus) sitting inside the cup shaped Bowman’s capsule
? The capillaries get narrower as they get further into the glomerulus which increases the
pressure on the blood moving through them
? This eventually causes the smaller molecules being carried in the blood to be forced out of the
capillaries and into the Bowman’s capsule where they form what is known as filtrate
? This process is known as ultrafiltration
? The substances forced out of the capillaries are glucose, water, urea and salts
? The glomerulus filters water, glucose, urea and salts from the blood
? Selective reabsorption
? After the glomerular filtrate enters the Bowman’s capsule, glucose is the first substance to be
reabsorbed at the first convoluted tubule
? This takes place by active transport
? The nephron is adapted by this by having many mitochondria to provide energy for active
transport of glucose molecules
? Reabsorption of glucose cannot take place anywhere else in the nephron as the gates that
facilitate the active transport of glucose are only found in the proximal convoluted tubule
? People with diabetes cannot control their blood glucose levels; they are often very high, meaning that
not all glucose filtered out can be reabsorbed into the blood. As there is nowhere for glucose to go it
ends up in the urine
? Reabsorption of water and salts
? As the filtrate drips through the loop of Henle, necessary salts are reabsorbed back into the
blood by diffusion
? As salts are reabsorbed back into the blood, water follows by osmosis
? Water is also reabsorbed from the collecting duct in different amounts depending on how much
water the body needs at that time\
? Kidney failure
? The kidneys might not work properly for several reasons including accidents or disease ]
? Humans can survive with one functioning kidney but if both are damaged then there will quickly
be a build-up of toxic wastes in the body which will be fatal if not removed
? Both kidneys damaged
? No treatment
¦ Death as toxins such as urea build up and salt and water balance is upset
? Kidney dialysis
¦ Artificial kidney machines carry out function of kidney outside the body
? Kidney transplant
¦ One healthy kidney from a donor replaces both diseased kidneys
? Kidney dialysis
? The usual treatment for someone with kidney failure is dialysis
? This is an artificial method of filtering the blood to remove toxins and excess substances
? Patients are connected to a dialysis machine which acts as an artificial kidney to remove most
of the urea and restore/maintain the water and salt balance of blood
? Unfiltered blood is taken from an artery in the arm, pumped into the dialysis machine and then
returned to a vein in the arm
? Inside the machine the blood and dialysis fluid are separated by a partially permeable
membrane, the blood flows in the opposite direction to dialysis fluid, allowing exchange to occur
between the 2 where a concentration gradient exists
? Dialysis fluid contains
¦ A glucose concentration similar to a normal level in blood
¦ A concentration of salts similar to a normal level in blood
¦ No urea
? Dialysis fluid
? As the dialysis fluid has no urea in it, there is a large concentration gradient meaning that urea
diffuses across the partially permeable membrane form the blood to the dialysis fluid
? As the dialysis fluid contains a glucose concentration equal to normal blood sugar level, this
prevents the net movement of glucose across the membrane as no concentration gradient
exists
? As the dialysis fluid contains a salt concentration similar to the ideal blood concentration,
movement of salts across the membrane only occurs where there is an imbalance
? The fluid in the machine is continually refreshed so that concentration gradients are maintained
between the dialysis fluids and the blood
? Dialysis may take 3-4 hours to complete and needs to be done several times a week to prevent
damage to the body from the buildup of toxic substances
Page 5


Excretion
? Excretion is the removal of the waste substance of metabolic reactions (chemical reactions that take
place inside cells), toxic materials and substances in excess of requirement
? Egestion and excretion are different, egestion is the removal of undigested food in our body
? Carbon dioxide must be excreted as it dissolves in water easily to form an acidic solution which can
lower pH of cells
? This can reduce activity of enzymes in the body which are essential for controlling the rate of metabolic
reactions therefore too much carbon dioxide in the body is toxic
? Urea is also toxic to the body in higher concentrations and so must be excreted
? Liver
? Many digested food molecules absorbed into the blood in the small intestine are carried to the
liver for assimilation
? These include amino acids which are used to build proteins such as fibrinogen, a protein found
in blood plasma that is important in blood clotting
? Excess amino acids absorbed in the blood that are not needed to make proteins cannot be
stored, so they are broken down in a process called deamination
? Functions of liver
? Converts excess amino acids into urea
? Synthesises plasma protein such as fibrinogen
? Controls the amount of glucose in the blood with the help of insulin and glucagon
? Makes bile
? Breaks down old RBC’s
? Stores vitamin A, B, D, E and K
? Makes cholesterol which is needed for the repair cell membrane
? How urea is made
? Protein in food is taken into the alimentary canal
? The protein molecules are broken down to amino acids during digestion
? The amino acids are absorbed into the blood and taken into the liver by hepatic portal vein
? Amino acids that are not needed are deaminated into ammonia
? The ammonia is converted into urea
? The urea is excreted by the kidney
? Changes in urine
? The colour and quantity of urine produced in the body can change quickly
? Large quantities of urine are usually pale yellow in colour because it contains a lot of water and
so the urea is less concentrated
? Small quantities of urine are usually darker yellow/orange in colour because it contains little
water and so the urea is more concentrated
? There are various reasons why the concentration of urine will change
¦ Water intake - the more fluids drunk, the more water will be removed from the body and
so a large quantity of pale yellow, dilute urine will be produced
¦ Temperature - the higher the temperature the more water is lost in sweat and less will
appear in urine meaning a smaller quantity of dark yellow concentrated urine will be
produced
¦ Exercise - the more exercise done, the more water is lost in sweat and so less appear in
urine meaning smaller quantity of dark yellow concentrated urine will be produced
? Kidney
? The kidneys are located in the back of the abdomen and have 2 important function in the body
¦ They regulate the water content of the blood
¦ They excrete the toxic waste products of metabolism
? Each kidney contains around a million tiny structures called
nephrons also known as kidney tubules or renal tubules
? The nephrons start in the cortex of the kidney, loop down into
the medulla and back up to the cortex
? The contents of the nephrons drain into the innermost part of
the kidney and the urine collects there before it flows into the
ureter to be carried to the bladder for storage
? Regions of the kidney
? The kidney has 2 portions. The outer portion is known as the
cortex and the inner portion is called the medulla
? On the inner curve of the kidney is a space called the renal
pelvis. Urine collects in the renal pelvis and exits the kidney
via the ureter. The ureter from each kidney drains into the
urinary bladder located in the pelvis. The bladder drains via
the urethra
? Nephrons
? The nephron is the functional unit of the kidney. There are about 1 million nephrons in each
kidney. The nephron has a long collecting tube that terminates in a hollow, cup-like structure
called a bowman's capsule. Inside the bowman’s capsule is a network is very fine arteries called
glomerulus
? Each tubule of nephron begins in the cortex, loops down to medulla, back into the cortex and
? then goes down to the pelvis. In the pelvis, the tubule.
? Ultrafiltration
? Arterioles branch off the renal artery and lead to each nephron where they form a knot of
capillaries (the glomerulus) sitting inside the cup shaped Bowman’s capsule
? The capillaries get narrower as they get further into the glomerulus which increases the
pressure on the blood moving through them
? This eventually causes the smaller molecules being carried in the blood to be forced out of the
capillaries and into the Bowman’s capsule where they form what is known as filtrate
? This process is known as ultrafiltration
? The substances forced out of the capillaries are glucose, water, urea and salts
? The glomerulus filters water, glucose, urea and salts from the blood
? Selective reabsorption
? After the glomerular filtrate enters the Bowman’s capsule, glucose is the first substance to be
reabsorbed at the first convoluted tubule
? This takes place by active transport
? The nephron is adapted by this by having many mitochondria to provide energy for active
transport of glucose molecules
? Reabsorption of glucose cannot take place anywhere else in the nephron as the gates that
facilitate the active transport of glucose are only found in the proximal convoluted tubule
? People with diabetes cannot control their blood glucose levels; they are often very high, meaning that
not all glucose filtered out can be reabsorbed into the blood. As there is nowhere for glucose to go it
ends up in the urine
? Reabsorption of water and salts
? As the filtrate drips through the loop of Henle, necessary salts are reabsorbed back into the
blood by diffusion
? As salts are reabsorbed back into the blood, water follows by osmosis
? Water is also reabsorbed from the collecting duct in different amounts depending on how much
water the body needs at that time\
? Kidney failure
? The kidneys might not work properly for several reasons including accidents or disease ]
? Humans can survive with one functioning kidney but if both are damaged then there will quickly
be a build-up of toxic wastes in the body which will be fatal if not removed
? Both kidneys damaged
? No treatment
¦ Death as toxins such as urea build up and salt and water balance is upset
? Kidney dialysis
¦ Artificial kidney machines carry out function of kidney outside the body
? Kidney transplant
¦ One healthy kidney from a donor replaces both diseased kidneys
? Kidney dialysis
? The usual treatment for someone with kidney failure is dialysis
? This is an artificial method of filtering the blood to remove toxins and excess substances
? Patients are connected to a dialysis machine which acts as an artificial kidney to remove most
of the urea and restore/maintain the water and salt balance of blood
? Unfiltered blood is taken from an artery in the arm, pumped into the dialysis machine and then
returned to a vein in the arm
? Inside the machine the blood and dialysis fluid are separated by a partially permeable
membrane, the blood flows in the opposite direction to dialysis fluid, allowing exchange to occur
between the 2 where a concentration gradient exists
? Dialysis fluid contains
¦ A glucose concentration similar to a normal level in blood
¦ A concentration of salts similar to a normal level in blood
¦ No urea
? Dialysis fluid
? As the dialysis fluid has no urea in it, there is a large concentration gradient meaning that urea
diffuses across the partially permeable membrane form the blood to the dialysis fluid
? As the dialysis fluid contains a glucose concentration equal to normal blood sugar level, this
prevents the net movement of glucose across the membrane as no concentration gradient
exists
? As the dialysis fluid contains a salt concentration similar to the ideal blood concentration,
movement of salts across the membrane only occurs where there is an imbalance
? The fluid in the machine is continually refreshed so that concentration gradients are maintained
between the dialysis fluids and the blood
? Dialysis may take 3-4 hours to complete and needs to be done several times a week to prevent
damage to the body from the buildup of toxic substances
? Kidney transplants are a better long term solution to kidney failure than dialysis; however, there are
several disadvantages to kidney transplants, including:
? Donors won’t have the same antigens on cell surfaces so there will be some immune response
to the new kidney
? This has to be suppressed by taking immunosuppressant drugs for the rest of their lives –
these can have long term side effects and leave the patient vulnerable to infections
? There are not enough donors to cope with demand
? However, if a healthy, close matched kidney is available, then the benefits of a transplant over dialysis
include:
? the patient has much more freedom as they are not tied to having dialysis several times a
week in one place
? their diets can be much less restrictive than they are when on dialysis
? use of dialysis machines is very expensive and so this cost is removed
? a kidney transplant is a long-term solution whereas dialysis will only work for a limited time