The human excretory system includes the kidneys, ureters, bladder, and urethra. The kidneys filter waste from the blood to produce urine via tiny tubules called nephrons. The urine travels from the kidneys down the ureters and into the bladder, where it is stored until excretion through the urethra. For those with kidney failure, dialysis provides an artificial filtration of the blood to remove waste, while transplantation involves surgically implanting a donor kidney to replace non-functioning ones.

1. Human Excretory System

2. What are the kidneys?
• 2 Kidneys – supplying oxygenated blood from the aorta, via renal arties.
Removing deoxygenated blood by the vena cava, via renal veins.
• Ureter is a tube leading from the kidneys to the bladder.
• Bladder (muscular membranous sac in the abdomen which receives urine
and stores it for excretion). Stretchy to hold large quantities.
• Sphincter muscles – when contract, urethra is sealed shut --> no urine out
• Urethra – tube from the bladder to the outside

3. Kidney
3 main parts
Cortex – outer part
Medulla – inner part
Pelvis – broadened top part of the ureter
Kidneys are made up of tiny tubules (nephrons) thousands of them
"thousands of tiny tubes in the kidney that produce urine from filtered
blood"

4. Filtration
• Blood --> in renal artery --> renal capsule (in nephron)
[cup shaped with mass of capillaries in the centre (glomerulus)]
• High pressure at these capillaries squeezes small molecules (like water,salt,glucose and
urea) out and into the renal capsule.
• This high blood pressure is because the vessels supplying the blood are wider than the
ones removing blood, meaning blood can't easily escape
(Filter – only allow certain materials to pass through) Proteins/ RBC too big to pass through
Glomerulus: a tangle of capillaries in the 'cup' of a renal capsule.

5. Reabsorption
• Now, in the renal capsule, is a solution of glucose, salt, urea in water
• Glucose, salt and water are useful for the body, taken back into the
blood. Loop of Henle
• At the collecting duct, mainly water, urea and salts (urine)
• Sent to bladder

6. Kidney Dialysis
If a person's kidneys stop working properly (infection, tumor, damaged, obstructions), allows urea to
build up in the blood.
• Dialysis = exchange of substances between two solutions through a partially permeable
membrane.
• Dialysis machines, for treating people with kidney failure.
• Blood flows from a person --> through the machine --> back into the person
• In the machine = dialysis fluid contains glucose, water, salts (like normal blood)
• As blood flows through the tubes, substances diffuse along concentration gradients eg. There is
no urea in the dialysis fluid, so urea diffuses out of the blood
• Treatment last several hours and is need 2-3 times a week!

7. Kidney Transplant
• 2-3 times a week
• Donor gives kidney to recipient.
• The donor and recipient need to be a close match. Their tissue-types need
to match or the immune system will reject it.
- attack it recognising it as foreign
• But even after transplant, patient needs to take immunosuppressants for
life
• Stop the immune system (WBC) working efficiently
• Problems?
• More likely to catch infections