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EXCRETION IN
HUMANS
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EXCRETION IN HUMANS
The metabolic reactions that take place in the body produce waste products which would be
toxic to the body if not removed.

Excretion is the removal of metabolic waste products in living organisms.

Excretion plays a vital role in homeostasis, which is the maintenance of a constant internal
environment. If waste products are not removed, they could lead to changes in the composition
of the tissue fluid surrounding cells or the cytoplasm of cells. Changes to the pH, temperature
and water potential of cells could disrupt metabolism, leading to the death of cells and
subsequently the death of the organism.

Role of the main excretory organs

The major excretory organs are the lungs,

kidneys and skin. The liver and alimentary
canal playa small role in excretion.

Lungs skin

The lungs excrete carbon dioxide formed as a lungs

waste product of cellular respiration. Carbon
dioxide is transported to the lungs from every
respiring cell via the circulatory system. The
lungs also excrete water vapour and heat. liver
kidney
Kidneys

The kidneys and bladder form the urinary

system. The urinary system removes waste
material from the bloodstream and regulates colon
the amount of water and salts in the body. The
kidneys produce urine, which is liquid that
contains water and nitrogenous waste products
such as urea, uric acid and ammonia.
Excretory organs of the human body
Skin

Sweat is the salty fluid secreted by the sweat glands in the skin. Skin excretes a small amount
of urea, water and salts. As sweat evaporates, heat is lost from the body.

Liver

The liver breaks down extra amino acids in the bloodstream into glucose and urea. The urea is
carried in the bloodstream to the kidneys, where it is excreted in urine. The liver also breaks
down haemoglobin into bile pigments. The bile pigments are carried in bile to the duodenum of
the alimentary canal.

Alimentary canal

Bile pigments (from the liver) and cholesterol are excreted from the colon in the faeces.
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The urinary system consists of the following organs:

• two kidneys
• two ureters
• one urinary bladder
• one urethra.

The two kidneys are found in

the abdominal cavity just
below the ribs towards the
back of the body. One
kidney lies on each side of
the vertebral column.

The renal arteries branch off

the aorta and supply the
kidneys with oxygenated
blood.

The renal veins carry blood

from the kidneys to the
inferior vena cava, which
passes blood back to the
heart.

The concave indentation of a

kidney is called the hilus.
This is the point at which the
blood vessels as well as the
ureter connect to the kidney.
Organs of the urinary system

Urine flows through the ureters and into the bladder. It is stored in the bladder before it is
excreted out of the body through the urethra. Both the ureters and bladder have strong
muscular walls.

Summary of the functions of the kidneys

• control the water and salt concentration in the body.
• remove nitrogenous waste from the blood so that it can be excreted.
• regulate the pH of the blood plasma by controlling the acid-base equilibrium of the
blood.
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External structure of the kidneys

A tough, semi-transparent membrane

called the renal capsule covers the surface
of the kidneys. The capsule is surrounded
by layers of fat that protect the kidney
against mechanical injury. The kidneys are
kept in place in the abdominal cavity by
connective tissue and blood vessels.

Human kidneys are bean-shaped and have

a smooth surface. The concave indentation
is called the hilus. This is the point at which
blood vessels and nerves enter and exit the
kidney.

Internal structure of the kidneys

The longitudinal section of the kidney below shows the three main parts: the cortex, medulla
and renal pelvis.

Cortex and medulla

The darker outer region of the

kidney below the capsule is called
renal capsule
the cortex.
calyces
The medulla is found below the renal pelvis
cortex. The medulla is divided into
triangular structures called
pyramids, which point towards the
renal artery
hilus.

The cortex and medulla contain

thousands of tiny tubules called medulla
nephrons
renal vein
• Nephrons are responsible for
the filtration of blood in the
kidneys.
ureter
• Nephrons are the functional unit
of the kidneys and enlarge the cortex
surface area of the kidneys for
filtration. Longitudinal section of a kidney

• The pyramids in the medulla are made up of the collecting ducts of the nephrons.

Renal pelvis

The collecting ducts of the nephrons release urine into the renal pelvis. This basin-like cavity
connects with the ureters, which carry the urine to the bladder.
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Structure and functioning of nephrons

Each nephron consists of the following parts:

 The Bowman’s capsule is the closed end of the nephron. It is curved inwards to from a
cup-like structure.
 The glomerulus is a mass of blood capillaries that fit into the cup-shaped part of the
Bowman’s capsule.
 The malpighian body is the term used for the Bowman’s capsule and glomerulus together
(a Malpighian body is sometimes referred to as a renal corpuscle).
 The renal tubule is a long tubule that leads from the Bowman’s capsule to the collecting
duct. It is divided into the following three regions.

- The proximal convoluted tubule

- The Loop of Henle

- The distal convoluted tubule

 Many distal convoluted tubules from many nephrons join to form a tube called the
collecting duct.
Many collecting ducts lead into a calyx.
Several calyces lead to a renal pelvis that leads to the ureter.
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Blood supply to the nephron

 The renal artery carries blood from the aorta into the kidney and then branches
repeatedly to form smaller and smaller arterioles.

 The smallest arteriole that carries blood to the Malpighian body is called an afferent
arteriole. The afferent arteriole branches to form a mass of capillaries that make up the
glomerulus in the Bowman’s capsule.

 The capillaries of the glomerulus join to form the efferent arteriole that carries blood
away from the malpighian body.

 The efferent arteriole branches repeatedly to form a network of capillaries around the
renal tubule. These capillaries join to form venules that join to form the renal vein.

 The renal vein carries blood away from the kidneys to the vena cave.

Structure of a nephron
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Functioning of the nephrons

The nephrons are responsible

for filtering the toxic products
from the blood and producing
urine, which contains the toxic
products that the body needs to
eliminate. This process takes
place in four main stages.

 Ultrafiltration
 Tubular reabsorption
 Tubular excretion
 Osmoregulation

Ultrafiltration

Blood flows into the glomerulus via the

afferent arteriole. The diameter of the
afferent arteriole is wider than that of
the efferent arteriole and this causes
the pressure of the blood in the
capillaries of the glomerulus to rise as
blood leaving the glomerulus is slowed
down by the narrower efferent
arteriole. This increased pressure
forces some of the blood plasma and
smaller molecules in the blood through
the capillary walls and into the
capsular space of the Bowman’s
capsule. The filtered fluid is called
glomerular filtrate.

The filtrate includes waste substances

such as urea, uric acid and creatinine,
but it also includes important nutrients
such as glucose, vitamins, amino acids
and salts.

Larger molecules such as plasma proteins and blood cells, remain in the blood capillaries as
they are too big to pass through the holes in the walls of the capillaries and the wall of the
Bowman’s capsule. The blood then leaves the glomerulus via the efferent arteriole.

The walls of the Bowman’s capsule are made up from specialised cells called podocytes.
Podocytes are flattened cells with small spaces between them. The spaces are too small to
allow larger molecules to pass through.
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Tubular reabsorption

When the efferent arteriole leaves the glomerulus, it branches to form a network of capillaries
around the renal tubule. The efferent arteriole contains the substances that were not filtered into
the capsular space.

The glomerular filtrate flows from the capsular space into the proximal convoluted tubule. The
glomerular filtrate contains many useful substances such as nutrients and water that the body
still needs. As the filtrate flows through the proximal convoluted tubule, most useful substances
are returned to the capillaries by tubular reabsorption. Waste substances and excess water,
salts and ions remain in the renal tubule and are called the tubular fluid.

Tubular reabsorption can be an active or a passive process depending on the substances that
need to be reabsorbed into the blood

Active reabsorption is when special carrier molecules move substances through a membrane.
The substances are too large to move though the membranes into the tubules by themselves.
This process needs energy. The energy comes from ATP (produced during cellular respiration
by the mitochondria). Molecules of substances such as glucose, amino acids, vitamins and
some mineral salts are actively reabsorbed, so that they are not excreted in the urine.

Passive reabsorption occurs when substances about 65% of the water in the proximal
convoluted tubule moves through the tubule wall by diffusion from a region of high
concentration of water molecules to a region of low concentration of water molecules. This
process does not need energy.
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Tubular excretion

There are some substances that are in the blood or which were passively reabsorbed that are
not needed by the body. As the tubular fluid flows through the distal convoluted tubule, these
substances move actively from the blood into the renal tubule. This process is called tubular
secretion.

Tubular excretion is needed to:

 get rid of drugs such as penicillin, which did not pass into the glomerular filtrate
 remove unwanted substances that were passively reabsorbed, for example, urea
 remove excess potassium from the blood.
 control the acidity of the blood
If the blood pH is too low (becomes too acid), more hydrogen ions are excreted. If the
blood pH is too high (becomes too alkaline), fewer hydrogen ions are excreted.

In order to maintain the pH of the blood within its normal limits of 7,3 to 7,4, urine may be
as low as pH 4 or as high as pH 9.

When the substances that the body needs to keep have been reabsorbed and the substances
that the body needs to excrete are in the renal tubule, the tubular fluid is called urine. Urine
flows into the collecting duct, through the renal pelvis into the ureter, and from the ureter to the
bladder.

Osmoregulation

Another vital function of the kidney is to control the amount of water in the blood. This process
is called osmoregulation. The amount of water in the blood changes all the time and can be
affected by:
• the amount of water consumed
• the temperature of the environment
• exercise such as running, playing squash or tennis.
The ascending limb of the Loop of Henle does not allow water to pass through it but it allows
sodium salts to pass out of it into the surrounding tissues. The filtrate flows through the Loop
of Henle and moves on to the collecting duct, which runs alongside the area where the sodium
salts have been deposited into the tissue of the kidney. The presence of the sodium salts
alongside the collecting duct causes a strong osmotic pull on the water in the filtrate in the
collecting duct and much of the water present in the collecting duct moves by osmosis back into
the capillary network. This action makes sure that the filtrate is concentrated and no excess
water is lost. The filtrate can now be called urine.
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The hormone ADH (anti-diuretic hormone) controls osmoregulation in the kidneys. ADH is
released by the pituitary gland.

On a hot day considerable

water may be lost through
sweat. This will cause the
blood to becomes more
concentrated as it decreases
in volume ie the water
potential of the blood
decreases and the blood
becomes more salty. The
body will conserve water in
the following way:

• Osmoreceptors in the
hypothalamus of the brain
monitor the water potential of
the blood. When the
decreased water potential is
detected, the pituitary gland
releases more ADH.

• The ADH increases the

permeability of the distal
convoluted tubules and
collecting ducts. This allows
more water to pass out of the
filtrate that is in the collecting
ducts into the medulla of the
kidney. This water then
moves into the blood
capillaries. This causes the
water potential of the blood to
return to normal.

When the water content of the

blood returns to normal, it is detected by the hypothalamus and normal levels of ADH are
released.

If conditions are such that the blood has too much water (ie the water potential increases), the
pituitary gland will release less ADH and as a result the walls of the collecting ducts will be less
permeable to water. This means that less water is reabsorbed and more water remains in the
urine to be excreted.
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Sodium regulation of the blood

The adrenal glands, situated on top of the kidneys, produce another hormone, aldosterone,
that controls the excretion of sodium ions by the kidneys. Aldosterone helps to regulate blood
composition and volume. If levels of sodium in the blood are too high, high blood pressure will
occur. Under normal conditions about 65% of the sodium in the renal filtrate is re-absorbed
from the proximal convoluted tubules and another 25% is re-absorbed from the loop of Henle.

Aldosterone is secreted when blood levels of sodium are low or when blood volume is low
(blood pressure is low). More aldosterone results in more sodium being reabsorbed from the
renal filtrate and returned to the blood. This will also result in more water being reabsorbed into
the blood stream due to osmosis.

If the levels of sodium are too high, less aldosterone will be produced and more sodium will
remain in the renal filtrate.

Formation of urine

In summary, the nephron makes urine by:

• filtering small molecules (water, glucose, amino acids, urea, minerals, and vitamins) out
of the blood during ultrafiltration

• reabsorbing the useful substances from the filtrate back into the blood

• secreting certain molecules and ions during tubular secretion.

The remaining filtrate, including mineral salts, ions and surplus water, is left to flow into the
bladder. The flow of urine from the bladder and out of the body is controlled by a ring of muscle
called the urinary sphincter. It is positioned between the bladder and the urethra.
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Kidney diseases
The kidneys are very sensitive organs that can be damaged easily by:
• substances such as drugs that are taken into the body
• physical damage from the outside such as from a car accident
• diseases such as nephritis which is an infection of the kidneys
• inherited genes.
Kidney damage can cause:
• renal malfunction if the kidneys are not working properly
• renal failure if the kidneys are not working at all.

A person who has one damaged kidney can lead a normal life. However, if both kidneys are
damaged, the person will need either:

• dialysis, which means that they are attached to an artificial kidney machine or dialysis
machine that does the work of the kidneys

• a kidney transplant, which means that they get a kidney from another person who is
called an organ donor. A kidney transplant does not always work because the person's body
may reject the donated kidney. A kidney transplant is more likely to succeed if the donor is a
blood relative such as a brother, sister or parent. Both dialysis and kidney transplants are very
expensive medical procedures.

Dialysis

When a patient is connected to a dialysis machine, it removes the waste products from the
blood and adjusts the salt and water content of the blood. The dialysis machine consists of long
coiled tubes that are made of semi-permeable membranes. The tubes are in a water bath that is
at body temperature. The water bath contains a dialysis fluid that consists of salts, glucose and
other blood plasma substances. The dialysis fluid contains no waste substances such as urea.

Dialysis takes three to six hours and needs to be done three to four times a week.

The process of renal dialysis is described below:

• A line is inserted into a patient's artery. Blood flows out of the body and is pumped along
numerous narrow tubes, which are made of an artificial semi- permeable dialysis membrane, to
enlarge the surface area for filtration.

• The tube is immersed in a specially prepared dialysis fluid that contains the same
components, such as glucose and sodium, in the same concentration as occur in blood plasma.
Only metabolic waste products such as urea and uric acid are absent from the dialysis fluid.

• The lining of the tube holds back the blood cells and plasma proteins, which are too
large to diffuse through the membrane, but is permeable to other substances.
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• Because glucose and sodium are present in equal concentrations on both sides of the
membrane, no diffusion of these substances occurs. However, diffusion of the waste products
will occur because they are not present in the dialysis fluid.

• Frequent replacement of the dialysis fluid ensures that a concentration gradient is

maintained that favours the diffusion of the waste products out of the tubes.

• The osmoregulatory function of the kidney is also performed during renal dialysis. By
raising the solute concentration of the dialysis fluid by the addition of a solute, water diffuses out
of the tubes by osmosis.

A diagram illustrating the process of dialysis is shown below.

Heparin is an anti-
coagulant which prevents
blood clotting
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Kidney transplants

Kidney transplants are highly successful procedures that are considered routine surgical
practice for people with kidney failure. In a kidney transplant, a kidney is removed from a living
person or from a person who has just died. The kidney is placed inside the transplant patient's
abdomen and is then connected to blood vessels and the ureter. Drugs that suppress the
immune system are given to the transplant patient to prevent rejection of the new kidney. If all
goes well, the transplanted kidney replaces the non-functioning kidney. Kidney transplant
patients can be expected to survive for years. In South Africa, there are not enough kidneys
available to meet the ever-increasing demands for kidney transplants.

Kidney stones

Kidney stones are hard mineral materials formed within the kidney or the ureter. They are
usually formed from the precipitation of the mineral salt calcium oxalate from the urine, but may
be composed of a number of different minerals. Kidney stones often have no single definite
cause. Factors that increase the risk of having kidney stones include inadequate water intake,
obesity and family history of kidney stones. Kidney stones are three times more common in
males than in females. Small kidney stones often pass out of the ureter and into the bladder
with no symptoms. But larger stones can become stuck in the ureter, causing painful muscle
contractions. If the contractions dislodge the stones, the pain will disappear. If they do not, then
further treatment is needed because the stones may cause an infection. Treatment may involve
medication or surgery. Doctors can also use lasers or shock waves to break down the stones
into smaller pieces so that they can be passed in urine.

Cystitis

Cystitis happens when bacteria infect the bladder. When a person has cystitis, they feel pain
and burning discomfort when they urinate. They can also pass only small amounts of urine at a
time. If cystitis is not treated the infection can move up the ureters into the kidneys. Cystitis is
more common in women than it is in men. The opening to the urethra in women is close to the
anus and can easily be infected by bacteria in the faeces. The urethra is very short so when it
becomes infected, the infection can spread very quickly to the bladder.