Excretion
The Excretory System
Excretion – The removal of waste products of metabolism
Egestion –The removal of undigested wastes (faeces) from the gut
Three main waste products:
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Urea
Carbon Dioxide
Water
Excretory Organs:
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Lungs
 Excrete carbon dioxide and water
Skin
 Excretes sweat containing water, salts and urea
Kidneys
 Remove urea from the blood and excrete it in urine they produce
Deamination:
Deamination – The breaking down of amino acids, which are not needed to build proteins, in the
liver.
Process of Deamination:
1. The amino group of the amino acid is removed
2. The amino group is combined with hydrogen to form ammonia
3. Urea is produced by the interaction of ammonia produced by respiration in a clynical
reaction known as the ornithine cycle.
4. Urea travels in the bloodstream to the kidneys and is removed from the blood
5. The rest of the amino acid molecule is changed by the liver cells into fat or into carbohydrate
molecules which are used to release energy
Human Urinary System
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Kidneys:
o Receive oxygenated blood from the liver containing urea. This blood is pumped from
the heart through the aorta to the renal artery which leads to the kidney
o Remove excess salts and wastes from the blood to form urine.
o “Purified” deoxygenated blood leaves the kidney through the renal vein and returns
to the heart through the vena cava.
Ureters:
o Thin tubes which carry urine away from the kidneys
Bladder:
o Receives urine from the ureters and stores the urine temporarily
Urethra
o A thin tube through which urine leaves the body.
The Structure of the Kidney
The Nephron
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The nephrons and their associated blood vessels are the structural and functional units of
the kidney.
They are tiny tubules packed tightly together in the kidney
There are millions of nephrons in each kidney
The Nephron consists of:
 Glomerulus – A knot of capillaries in the kidney
 The glomeruli are mostly situated in the cortex of the kidney. This is why the cortex
has a denser blood supply in than the medulla and is much redder.
 The Bowman’s capsule – an expanded cup-like structure which surrounds the glomerulus.
 A very long convoluted tubule leading from the Bowman’s capsule
 Urine made in the nephrons passes from their convoluted tubules into much larger
collecting tubules and then into the pelvis of the kidney
 A capillary network from the glomerulus which surrounds the tubule
 Each nephron has 2 capillary networks: one forming the glomerulus and one
network surrounding the tubule
Urine Production in the nephron:
Urine consists of water, urea and sodium chloride
The following processes are involved in the production of urine:
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Filtration
Reabsorption
Secretion
Filtration:
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Filtration takes place in the Bowman’s capsule and the glomerulus
The diameter of the arteriole bringing blood into the glomerulus is wider than the one taking
blood away from the glomerulus. This difference creates high blood pressure in the
glomerulus
Water and solutes are forced out of the blood, across the walls of the capillaries and
Bowman’s capsule and into the cavity of the tubule.
The glomerulus behaves like a sieve:
 Smaller particles such as ions, glucose and amino acids pass out of the blood and
into the tubule.
 Larger particles such as proteins and blood cells remain in the blood which leaves
the glomerulus
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Filtration is not a selective process: The filtrate in the tubule contains not only waste
products, but useful substances such as glucose and amino acids
Reabsorption:
 Occurs as the filtrate passes along the tubule.
 Cells of the tubules reabsorb useful substances such as glucose and amino acids into blood
capillaries surrounding the tubule.
Secretion:
 Occurs with substances such as urea which are more concentrated in the urine than in the
filtrate
 These substances are actively transported against the concentration gradient from the blood
capillaries
 As a result of secretion, urea is 100 times more concentrated in the urine than in blood.
Treatment of Kidney Failure
Reasons for Kidney Failure:
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Infection
Injury
High blood Pressure
Diabetes
Methods of Kidney Failure Treatment
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Kidney Machine
Kidney Transplant
Kidney Machines:
How they work:
 An artery in the patients arm is connected the kidney machine
 Blood flows from the person’s artery to the machine and then from the machine to a vein in
the person’s arm
 The machine removes urea and excess salts from the blood by dialysis
 Dialysis describes the diffusion of a solute through a partially permeable membrane
 During dialysis, blood is pumped through the dialysis tubing for 6 to 10 hours at a time, 3 to
4 times a week.
Dialysis:
 Dialysis fluid contains most of the constituents of normal blood such as glucose, amino acids
and salts in the same concentrations as blood plasma
 Since there is no concentration gradient, there is no net diffusion of glucose or amino acids
 Excess salts diffuse from the blood to the dialysis fluid
 Urea diffuses out of the blood, across the dialysis membrane and into the dialysis machine
down the concentration gradient.
 The dialysis tubing has very small pores which allow only the solute molecules, such as
glucose, amino acids and urea to diffuse through but not larger molecules such as proteins
or blood cells.
Kidney Transplants:
 The failed kidney is removed and replaced by a healthy kidney donated by another person
 A major problem is that of rejection of the transplanted kidney by the recipient’s immune
system
 Immunosuppressant drugs can be taken to prevent transplant rejection
 To reduce the risk of rejection, the donated kidney must be of a similar tissue type to the
recipient’s. I.e. from a close relative.