EXCRETION IN HUMANS
Excretion is the removal of toxic materials,
the waste products of metabolism and substances in excess of requirements from
organisms. Metabolism is chemical reactions taking place inside cells,
including respiration.
The body excretes four main waste materials.
There are carbon dioxide, urea, water and salts.
The Excretory system of humans is made up of 4
structures: Two kidneys, two ureters, a bladder, and the urethra. The kidneys
act as a filter to filter the waste products from the blood, the ureters
are tubes that transport the main waste products (urine) from the kidneys to
the bladder, where it is stored until it is excreted out of the body
through the urethra.
(Urea is formed in the liver from excess amino
acids)
1.
When you eat a food high in protein, it is digested
in the small intestine into amino acids.
2.
The villi on the walls of the small intestine
absorb the amino acids into the hepatic portal vein.
3.
The hepatic portal vein is a special vein that transports
digested material from the small intestine to the liver.
4.
The liver plays a big role in maintaining the level
of protein in our body. It absorbs all amino acids from the hepatic portal
vein.
- If the body
needs proteins, they will pass through the liver into the blood stream to be
used by the body cells to make protein.
- If the body does
not need proteins. The liver will absorb excess amino acids and break them down
into carbohydrates and nitrogen. The formula of amino acids is CHON; here we
remove the nitrogen from the molecule, to get a carbohydrate. This is called deamination.
Nitrogen is made into urea, which is
a nitrogenous waste product.
5.
The products are then released to the blood stream.
Kidneys Structure
- A kidney consists of two main structures:
•
Cortex (outer layer)
•
Medulla
- Between the cortex and the Medulla, there is a structure called the nephron.
The nephron is the where filtration of toxic materials from the blood
takes place. We have many of them in each kidney.
- In the centre of the kidney there is a cavity called the pelvis,
which leads to the ureter.
The nephron starts with a cup shaped structure called Bowman’s capsule.
Inside the Bowman’s capsule there is a very dense network of blood
capillaries entering as capillaries from the renal artery and exiting as
capillaries from the renal vein. This dense network of capillaries is called
the Glomerulus. The rest of the nephron is a long coiled tube where
materials filtered from the blood flow called the tubule. At some point the tubule becomes straight and is bent in a
U shape tube, this part is called loop of Henle and it is surrounded by
a network of capillaries from the renal vein, it is where reabsorption takes
place. All tubules end at a large tube called the collecting duct where the
content of the nephrons are transported to the pelvis, to be secreted in the
ureter.
Ultrafiltration
The blood in the renal artery contains large amounts of urea, glucose,
water, mineral ions (salts) and some amino acids. When it reaches the
glomerulus, the high pressure of the blood and the concentration gradient of
these materials between the blood and the nephron cause most of these
substances to diffuse from the blood to the bowman’s capsule and become content
of the tubule, which is called glomerular filtrate (glomerular filtrate is a
mixture of urea, water, glucose and mineral ions that diffused from the blood
to the nephron).
Reabsorption
The glomerular filtrate moves in the tubule till it reaches the loop of
Henle, which is surrounded by a dense network of blood capillaries of the renal
vein. Here there is a concentration gradient of the content of the nephron
between the tubule and the blood. Both diffusion and active transport occur to
ensure the complete reabsorption of valuable substances from the glomerular
filtrate back to the blood; these substances are glucose and amino acids. Some
water also moves by osmosis to the blood, as well as some salts.
That leaves urea, excess water and minerals to continue in the tubule
till it reaches the collecting duct and the pelvis. This mixture is called urine. Urine is transported from the
pelvis to the urinary bladder by the ureters. It is then secreted out of the body
through the urethra.
Treating kidney failure
Sometimes a
person’s kidneys may stop working. If this happens, the kidneys can no longer
remove urea or excess water from the blood, and so may begin to feel very ill.
- The best method of treatment in a kidney transplant. A kidney is taken
from a donor and placed in the recipient’s body. It must be a good match or
else the recipient’s immune system will attack and destroy it (This is called
transplant rejection). The recipient will need to take immunosuppressant drugs
for the rest of his or her life to stop the immune system from rejecting the
kidney.
- The alternative to a transplant rejection
is dialysis. This involves filtering
the patient’s blood on a regular basis with a machine, in a similar way to the
kidney. During the process a tube is attached from the patient’s vein to the
dialysis machine. Blood is sucked from the patient’s vein, it goes through the
machine, and out from the other side back to the patient’s vein.
When
the blood enters the dialysis machine, it is very rich in waste materials
(urea, excess water and minerals). The
tubes inside the dialysis machine are made of a partially permeable membrane to
allow diffusion. The tubes are surrounded with dialysis fluid, which is the
same as blood plasma. The concentration of waste products in the blood is much
higher in the blood than in the dialysis fluid. This creates a concentration
gradient, diffusion occurs and waste products leave the blood to the dialysis
fluid, which then exits the machine and gets disposed of. The dialysis fluid
has to be renewed continuously to keep the concentration gradient of waste
products higher in the blood, thus ensuring that all waste products leave the
blood. The clean blood is then returned to the patient’s vein.
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Advantages
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Disadvantages
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Kidney transplant
|
- patients can
lead a more normal life
- cheaper for
the NHS overall
|
- must take
immunosuppressant drugs
- shortage or
organ donors
- kidney only
lasts 8-9 years
- can be
rejected
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Kidney dialysis
|
- available to
all
- no need for
immunosuppressant drugs
|
- patients must
limit their salt and protein intake
- expensive for
the NHS
- regular
treatment so patient is unable to carry a normal life.
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PSB
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