5.3.

Homeostasis in the human body which means the blood in the capillaries is
5.3.1. The structure and function of the human under a lot of pressure.
kidney  Several layers of cells, the wall of the blood
 The kidneys are a pair of bean-shaped capillaries and the wall of the capsule act as a
organs just above the waist. They are filter and the blood cells and the large blood
important organs with many functions in proteins cannot leave the blood vessels as
your body, including producing hormones, they are too big to fit through the gaps.
absorbing minerals, and filtering blood and  However, water, salt, glucose, urea and many
producing urine. other substances are forced out into the start
 Internally, the kidney has three regions: an of the tubule. In fact, the concentration of
outer cortex, a medulla in the middle, and substances in the liquid in the capsule is the
the renal pelvis, which is the expanded end same as that in the blood itself. This process
of the ureter. is known as ultrafiltration – filtration on a
 The renal cortex contains the nephrons, very small scale.
which is the functional unit of the kidney. Glomerulus:
The renal pelvis collects the urine and leads  This is the knot of blood vessels in the
to the ureter on the outside of the kidney. Bowman’s capsule where the pressure builds
The ureters are urine bearing tubes that exit up so that ultrafiltration occurs.
the kidney and empty into the urinary  The volume of the blood leaving the
bladder. glomerulus is about 15% less than the blood
 Blood flows into the kidney along the renal coming in which is a measure of the liquid
artery. The blood is filtered, so fluid which has moved into the capsule as a result
containing water, salt, urea, glucose and of ultrafiltration.
many other substances is forced out into the  Figure 5.52 The kidney filters the blood and
kidney tubules. Then everything the body removes waste materials.
needs is taken back (reabsorbed), including First coiled (convoluted) tubule:
all of the sugar and the mineral ions needed  This is the liquid which enters this first
by the body. tubule is known as the glomerular filtrate.
 The amount of water reabsorbed depends  The first tubule is where much of the
on the needs of the body. The waste product reabsorption takes place. All of the glucose is
urea and excess ions and unwanted water of actively taken back into the blood along with
the body are released as urine. around 67% of the sodium ions and around
 Each kidney has a very rich blood supply 80% of the water.
and is made up of millions of tiny  It has many microvilli to increase the surface
microscopic tubules (nephrons) where all area for absorption.
the filtering and reabsorption takes place. Loop of Henle:
Figure 5.51 The anatomy of human  is part of a kidney where the urine is
kidneys concentrated and more water is conserved.
 The roles of the different areas of a single Second coiled (convoluted) tubule:
kidney tubule in the production of urine are  is part of a kidney where the main water
described below: balancing is done. If the body is short of
Bowman’s capsule: water, more is reabsorbed into the blood in
 is the site of the ultrafiltration of the blood. this tubule under the influence of the anti-
The blood vessel feeding into the capsule is diuretic hormone or ADH. (Diuresis means
wider than the vessel leaving the capsule, passing urine, so antidiuresis means

2017 E.C 4TH QUARTER BIOLOGY NOTE FOR GRADE 12 BASED ON THE NEW CURRICULUM PREPARED BY GEZAHEGN SEBSIBE, FEB. 2017 E.C., ADAMA Page 1
 preventing or reducing urine flow.) Also the kidney tubules and so a more
ammonium ions and some drugs (if they concentrated urine is formed.
have been taken into the body) are secreted  At the same time the amount of water in the
from the blood into this tubule to get rid of blood increases so that the concentration of
them. salts in the blood returns to normal.
 By the end of this second coiled tubule all of  If the water content of the blood is too high,
the salt which is needed by you body has the pituitary gland releases much less ADH
been reabsorbed, leaving the excess in the into the blood. The kidney then reabsorbs
filtrate along with most of the urea. less water back into the blood, and there by
Collecting duct: producing a large volume of dilute urine.
 is also part of a kidney where the liquid  Water is effectively lost from the blood and
(essentially urine) is collected. It contains concentration of salts returns to normal.This
about 1% of the original water, with no system of osmoregulation is an example of
glucose at all. negative feedback.
 The level of salt in the urine will depend on  As the water concentration of the blood falls,
the amount of salt in your diet and the water the level of ADH produced rises. Then as the
content of the urine. water concentration of the blood rises again,
 There is also a much higher concentration of the level of ADH released falls. On an average
urea (about 60 times more) in the urine than day the kidneys will produce around 180 l
in the blood. But, if your body badly needs (that’s about 50 gallons) of liquid filtered out
more water, more may be reabsorbed along of the blood in the glomerulus (glomerular
the collecting duct again under the influence filtrate) – but only about 1.5 l (just over 2.5
of ADH – until the urine passes into the pints) of urine.
pyramid of the kidney and on into your  So more than 99% of the liquid filtered out of
bladder. the blood is eventually returned to it.
 Urine is formed constantly in your kidneys as Thermoregulation
it drips down to collect in your bladder. o It is vitally important that wherever we
The bladder go and whatever we do our body
 is a muscular sac which can hold between temperature is maintained at the
600 and 800 cm3 urine, although we usually temperature (around 37 °C) at which our
empty it when it contains only 150–300 cm3. enzymes work best.
 The amount of water lost from the kidney in o It is not the temperature at the surface of
the urine is controlled by a sensitive an organism which matters as the skin
feedback mechanism involving the hormone temperature can vary enormously
ADH. without causing harm. It is the
 If the water content of the blood is too low temperature deep inside the body,
(so the salt concentration of blood increases) known as the internal or core body
special sense organs known as temperature, which must be kept stable.
osmoreceptors in the brain detect this. They o Human beings are good examples of
stimulate the pituitary gland in the brain to homeotherms. The body temperature is
release ADH into the blood. controlled by a number of physiological
 This hormone affects the second coiled mechanisms which work together to
tubules of the kidneys, making them more allow gain or lose heat you need to.
permeable so more water is reabsorbed back
into the blood This means less water is left in

2017 E.C 4TH QUARTER BIOLOGY NOTE FOR GRADE 12 BASED ON THE NEW CURRICULUM PREPARED BY GEZAHEGN SEBSIBE, FEB. 2017 E.C., ADAMA Page 2
1.Osmoregulation  The liver cells are very active in carrying
 If the concentration of the body fluids out a wide range of functions, many of
changes, water will move into or out of the which help to maintain a constant internal
cells by osmosis and they could be environment.
damaged or destroyed.  The liver has a very special blood supply in
 Yet some days you may drink several liters addition to the usual artery and vein
of water of liquid and other days much less. (hepatic artery and vein) there is another
How is the balance maintained? blood vessel which comes to the liver
 We gain water when we drink and eat. We directly from the gut. This is the hepatic
lose water constantly from the lungs when portal vein and it brings the products of
we breathe out, when water evaporates digestion to the liver to be dealt with.
into the air in the lungs and is breathed Figure 5.53 The liver is one of the most
out. This water loss is constant. active organs in the body
 Whenever we exercise or get hot we sweat  A large number of reactions take place in
and lose more water. The water balance is the liver. Many of them are involved in
maintained by the kidneys. They remove homeostasis in one way or another. It plays
any excess water which leaves the body as a part in all of the following functions:
urine.  It controls the sugar levels in the body
 If we are short of water we produce very (through stored glycogen in the liver itself).
little urine and most water is saved for use  It controls and balances the fats that you
in the body. If we have too much water eat and the cholesterol levels in the blood.
then our kidneys produce lots of urine to  It is an important organ where protein
get rid of the excess. metabolism takes place. The liver breaks
 The ion concentration of the body – down excess amino acids and forms urea.
particularly ordinary salt – is also  If you eat more carbohydrate or fat than
important. We take in mineral ions with you need in the diet the body simply stores
our food. Some are lost via our skin when the excess energy as fat.
we sweat.  If you eat too much protein, it isn’t so easy.
 Again the kidney is most important organ The body cannot store the excess amino
to keep an ion balance. Excess mineral ions acids or simply convert protein to fat.
are removed by the kidneys and lost in the Instead the amino acids which make up the
urine. The balance of water and salts in the protein are broken down in the liver.
body is very important because of the  The amino (nitrogen containing) part of
osmotic impact of the cells. If the balance is the amino acid molecule is removed and
wrong controlling this balance is known as converted into ammonia and then urea in
osmoregulation. the liver. The rest of the amino acid can be
 The kidneys as discussed above are vitally used in cellular respiration or converted to
important in two aspects of homeostasis, fat for storage.
both in excretion and in osmoregulation.  The process of removing the amino group
2. Chemical regulation from excess amino acids is known as
 Human liver plays a vital role in deamination. This it is a very important
maintaining a constant internal function of the liver. It carries out the
environment. It is the largest individual breakdown of worn-out red blood cells – in
organ in the body that makes up around particular the red pigment hemoglobin.
5% of the body mass.

2017 E.C 4TH QUARTER BIOLOGY NOTE FOR GRADE 12 BASED ON THE NEW CURRICULUM PREPARED BY GEZAHEGN SEBSIBE, FEB. 2017 E.C., ADAMA Page 3
  It is vital organ for the formation of bile 2. Peripheral chemoreceptors:
which is made in the liver and stored in the  These include the aortic body, which
gall bladder before it is released into the detects changes in blood oxygen and
gut to emulsify fats and help in their carbon dioxide, but not in the pH, and the
digestion. carotid body which detects all three. They
 It controls toxins. The liver breaks down do not desensitize, but they have less
most of the poisons you take into the body, impact on the respiratory rate compared to
including alcohol. This is why the liver is so the central chemoreceptors.
often damaged when people drink heavily.  The need for different levels of respiration
 It is used to control temperature. Around varies with the physiologic state of the
500 different reactions take place in the organism (e.g., sleep, excitement, exercise).
liver at any time. The respiratory system must try to
 For many years it has been believed that as maintain constant levels of O2, CO2 and H+
a result of all these reactions the liver in the arterial blood which then ensures
generates a lot of heat which is then spread relatively constant levels of these
around the body by the bloodstream. important substances in the interstitial
 The regulation of tissue oxygenation is fluid.
another typical example for chemical  For O2, one needs an adequate supply to
regulation in the body. meet cellular metabolic requirements. For
 The respiratory chemoreceptors work by CO2 and H+, one needs to maintain the
sensing the pH levels of their environment acid–base status of the body's cells.
through the concentration of hydrogen  The respiratory system provides rapid, but
ions. usually incomplete, compensation for acid–
 Because most carbon dioxide is converted base disturbances through altered partial
to carbonic acid (and bicarbonate) in the pressure of CO2. (PCO2).
bloodstream, chemoreceptors are able to  Changes in the levels of O2, CO2 and H+ in
use blood pH as a way to measure the the blood cause compensatory changes in
carbon dioxide levels of the bloodstream. the level of ventilation.
 The main chemoreceptors involved in
respiratory feedback are:
1. Central chemoreceptors:
 These are located on the ventrolateral
surface of medulla oblongata and detect
changes in the pH of spinal fluid. They can
be desensitized over time from chronic
hypoxia (oxygen deficiency) and increased
carbon dioxide.

2017 E.C 4TH QUARTER BIOLOGY NOTE FOR GRADE 12 BASED ON THE NEW CURRICULUM PREPARED BY GEZAHEGN SEBSIBE, FEB. 2017 E.C., ADAMA Page 4