Homeostasis in the human body

By the end of this section you should be able to:

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 The structure and function of the human kidney
➢ The kidneys are a pair of bean-shaped organs just above the waist.
➢ They are important organs with many functions in your body, including producing hormones,
absorbing minerals, and filtering blood and producing urine.
➢ Internally, the kidney has three regions:
❖ An outer cortex
❖ A medulla in the middle and
❖ The renal pelvis, which is the expanded end of the ureter.
➢ The renal cortex contains the nephrons, which is the functional unit of the kidney.
➢ The renal pelvis collects the urine and leads to the ureter on the outside of the kidney.
➢ The ureters are urine-bearing tubes that exit the kidney and empty into the urinary bladder.
➢ Blood flows into the kidney along the renal artery.
➢ The blood is filtered, so fluid containing water, salt, urea, glucose and many other substances
is forced out into the kidney tubules.
➢ Then everything the body needs is taken back (reabsorbed),including all of the sugar and the
mineral ions needed by the body.
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➢ The roles of the different areas of a single
kidney tubule in the production of urine are:
❑ Bowman’s capsule is the site of the ultrafiltration
of the blood.
➢ The blood vessel feeding into the capsule is wider
than the vessel leaving the capsule, which means
the blood in the capillaries is under a lot of
pressure.
➢ The amount of water reabsorbed depends on the
needs of the body.
➢ The waste product urea and excess ions and
unwanted water of the body are released as urine.
➢ Each kidney has a very rich blood supply and is
made up of millions of tiny microscopic tubules
(nephrons)where all the filtering and reabsorption
takes place.

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➢ Several layers of cells, the wall of the blood capillaries and the wall of the capsule act
as a filter and the blood cells and the large blood proteins cannot leave the blood vessels as
they are too big to fit through the gaps.
➢ However water, salt, glucose, urea and many other substances are forced out into the start of
the tubule.
➢ In fact, the concentration of substances in the liquid in the capsule is the same as that in the
blood itself. This process is known as ultrafiltration-filtration on a very small scale.
❑ Glomerulus: This is the knot of blood vessels in the Bowman's capsule where the pressure
builds up so that ultrafiltration occurs.
➢ The volume of the blood leaving the glomerulus is about 15%less than the blood coming
in which is a measure of the liquid which has moved into the capsule as a result of
ultrafiltration.
❑ First coiled (convoluted)tubule: This is the liquid which enters this first tubule is known as
the glomerular filtrate.
➢ The first tubule is where much of the reabsorption takes place.
➢ All of the glucose is actively taken back into the blood along with around 67%of the
sodium ions and around 80% of the water.
➢ Simon
It has many microvilli to increase the surface area for absorption.
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❑Loop of Henle: is part of a kidney where the urine is concentrated and more water is
conserved.
❑Second coiled(convoluted)tubule: is part of a kidney where the main water balancing is done.
➢ If the body is short of water, more is reabsorbed into the blood in this tubule under the
influence of the anti-diuretic hormone or ADH.(Diuresis means passing urine, so anti
diuresis means preventing or reducing urine flow.)
➢ Ammonium ions and some drugs (if they have been taken into the body)are secreted from
the blood into this tubule to get rid of them.
➢ By the end of this second coiled tubule all of the salt which is needed by your body has
been reabsorbed, leaving the excess in the filtrate along with most of the urea.
❑ Collecting duct: is also part of a kidney where the liquid(essentially urine)is collected.
➢ It contains about 1% of the original water, with no glucose at all.
➢ The level of salt in the urine will depend on the amount of salt in your diet and the water
content of the urine.
➢ There is also a much higher concentration of urea(about 60 times more)in the urine than
in the blood. But, if your body badly needs more water, more may be reabsorbed along the
collecting duct again under the influence of ADH-until the urine passes into the pyramid
of the
Simon kidney and on into your bladder.
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❑Urine: is formed constantly in your kidneys as it drips down to collect in your bladder.
➢ The bladder is a muscular sac which can hold between 600 and 800 cm3 urine, although we
usually empty it when it contains only 150- 300 cm3
➢ The amount of water lost from the kidney in the urine is controlled by a sensitive feedback
mechanism involving the hormone ADH.
➢ If the water content of the blood is too low(the salt concentration of blood increases)special
sense organs known as osmoreceptors in the brain detect this.
➢ They stimulate the pituitary gland in the brain to release ADH into the blood.
➢ This hormone affects the second coiled tubules of the kidneys, making them more permeable
so more water is reabsorbed back into the blood.
➢ This means less water is left in the kidney tubules and so a more concentrated urine is
formed. At the same time the amount of If the water content of the blood is too high, the
pituitary gland releases much less ADH into the blood.
➢ The kidney then water in the blood increases so that the concentration of salts in the blood
returns to normal.
➢ Reabsorbs less water back into the blood, and thereby producing a large volume of dilute
urine.
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➢ Water is effectively lost from the blood and concentration of salts returns to normal.
➢ This system of osmoregulation is an example of negative feedback.As the water concentration
of the blood falls, the level of ADH produced rises.
➢ Then as the water concentration of the blood rises again, the level of ADH released falls.
➢ On an average day the kidneys will produce around 1801 (that's about 50 gallons)of liquid
filtered out of the blood in the glomerulus(glomerular filtrate)-but only about 1.51 (just over
2.5 pints)of urine. So more than 99% of the liquid filtered out of the blood is eventually
returned to it.
1. Thermoregulation
➢ It is vitally important that wherever we go and whatever we do our body temperature is
maintained at the temperature (around 37 ℃) at which our enzymes work best.
➢ It is not the temperature at the surface of an organism which matters as the skin temperature
can vary enormously without causing harm.
➢ It is the temperature deep inside the body, known as the internal or core body temperature,
which must be kept stable. Human beings are good examples of homeotherms.
➢ The body temperature is controlled by a number of physiological mechanisms which work
together to allow gain or lose heat you need to.
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 2. Osmoregulation
➢ If the concentration of the body fluids changes, water will move into or out of the cells by
osmosis and they could be damaged or destroyed. Yet some days you may drink several liters
of water of liquid and other days much less.
How is the balance maintained?
➢ We gain water when we drink and eat. We lose water constantly from the lungs when we
breathe out, when water evaporates into the air in the lungs and is breathed out. This water
loss is constant. Whenever we exercise or get hot we sweat and lose more water.
➢ The water balance is maintained by the kidneys. They remove any excess water which leaves
the body as urine.
➢ If we are short of water we produce very little urine and most water is saved for use in the
body.
➢ If we have too much water then our kidneys produce lots of urine to get rid of the excess.
➢ The ion concentration of the body- particularly ordinary salt is also important. We take in
mineral ions with our food. Some are lost via our skin when we sweat. Again the kidney
is most important organ to keep an ion balance.
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➢ Excess mineral ions are removed by the kidneys and lost in the urine. The balance of water
and salts in the body is very important because of the osmotic impact of the cells.
➢ If the balance is wrong controlling this balance is known as osmoregulation.
➢ The kidneys are vitally important in two aspects of homeostasis, both in excretion and in
osmoregulation.
3. Chemical regulation
➢ Human liver plays a vital role in maintaining a constant internal environment.
➢ It is the largest individual organ in the body that makes up around 5%of the body mass.
➢ The liver cells are very active in carrying out a wide range of functions, many of which help
to maintain a constant internal environment.
➢ The liver has a very special blood supply in addition to the usual artery and vein (hepatic
artery and vein)there is another blood vessel which comes to the liver directly from the gut.
➢ This is the hepatic portal vein and it brings the products of digestion to the liver to be dealt
with.
➢ A large number of reactions take place in the liver. Many of them are involved in homeostasis
in one way or another.
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➢ It plays a part in all of the following
functions:
✓ It controls the sugar levels in the body
(through stored glycogen in the
liver itself).
✓ It controls and balances the fats that
you eat and the cholesterol levels in
the blood.
✓ It is an important organ where protein
metabolism takes place. The liver
breaks down excess amino acids and
forms urea.
❑ If you eat more carbohydrate or fat
than you need in the diet the body
simply stores the excess energy as fat.

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❑ If you eat too much protein, it isn’t so easy. The body cannot store the excess amino acids or
simply convert protein to fat. Instead the amino acids which make up the protein are broken
down in the liver.
❑ The amino(nitrogen containing)part of the amino acid molecule is removed and converted
into ammonia and then urea in the liver.
❑ The rest of the amino acid can be used in cellular respiration or converted to fat for storage.
❑ The process of removing the amino group from excess amino acids is known as deamination.
This it is a very important function of the liver.
✓ It carries out the breakdown of worn-out red blood cells –in particular the red pigment
hemoglobin.
✓ It is vital organ for the formation of bile which is made in the liver and stored in the gall
bladder before it is released into the gut to emulsify fats and help in their digestion.
✓ It controls toxins. The liver breaks down most of the poisons you take into the body,
including alcohol.
✓ This is why the liver is so often damaged when people drink heavily.
✓ It is used to control temperature. Around 500 different reactions take place in the liver at
any time. For many years it has been believed that as a result of all these reactions the
liver
Simon generates a lot of heat which is then spread around the body by the bloodstream.
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❑ The regulation of tissue oxygenation is another typical example for chemical regulation in
the body.
❑ The respiratory chemoreceptors work by sensing the pH levels of their environment through
the concentration of hydrogen ions. Because most carbon dioxide is converted to carbonic
acid(and bicarbonate)in the bloodstream, chemoreceptors are able to use blood pH as a way
to measure the carbon dioxide levels of the bloodstream.
❑ 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.
2. Peripheral chemoreceptors: These include the aortic body, which detects changes in
blood oxygen and carbon dioxide, but not in the pH, and the carotid body which detects
all three.
➢ They do not desensitize, but they have less impact on the respiratory rate compared to
the central chemoreceptors.

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➢ The need for different levels of respiration varies with the physiologic state of the organism
(e.g. sleep, excitement, exercise).
➢ The respiratory system must try to maintain constant levels of CO2
➢ CO2 and H+ in the arterial blood which then ensures relatively constant levels of these
important substances in the interstitial fluid.
➢ For O2, one needs an adequate supply to meet cellular metabolic requirements.
➢ For CO2 and H+, one needs to maintain the acid-base status of the body’s cells.
➢ The respiratory system provides a rapid, but usually incomplete, compensation for acid-base
disturbances through altered partial pressure of CO2.(P CO2).
➢ Changes in the levels of O2 ,CO2 and H+ in the blood cause compensatory changes in the
level of ventilation.

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