The Excretory System

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The Excretory System
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• Excretion is the removal of the waste products of
  metabolism from living organisms.
• The accumulation of these waste products would be
  toxic if they were not eliminated.
• In plants and simple animals, waste products are
  removed by diffusion. Plants, for example,
  excrete O2, a product of photosynthesis.

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METABOLIC WASTE A BY-PRODUCT OF
Water Dehydration synthesis and cellular respiration
Carbon dioxide Cellular respiration
Salts Neutralization reactions
Urea Deamination (removal of amino group from amino acids
Uric acid Nucleic acid breakdown
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Organs of Excretion

• There are four major organs involved in
  excretion
• 1. The lungs
• 2. The liver
• 3. The skin
• 4. The kidneys

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THE LUNGS

• Cellular respiration occurs in every living cell
  in your body.
• Carbon dioxide is produced as a waste product. 
  As the carbon dioxide accumulates in body cells,
  it eventually diffuses out of the cells and into
  the bloodstream, which eventually circulates to
  the lungs.
• In the alveoli of the lungs, carbon dioxide
  diffuses from the blood, into the lung tissue,
  and then leaves the body every time we exhale. 
  Some water vapor also exits the body during
  exhalation.

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THE SKIN

• As you already know, sweat comes out of pores in
  your skin. 
• Sweat is a mixture of three metabolic wastes
  water, salts, urea.
•   So as you sweat, your body accomplishes two
  things
• 1) sweating has a cooling effect on the body,
• 2) metabolic wastes are excreted.

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• The skin is made up of two layers
• 1. The thin epidermis at the top
• 2. The thicker dermis below.  This is where
  oil glands, hair follicles, fatty layers, nerves,
  and sweat glands are found.

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• The sweat gland is a tubular structure tangled
  with capillaries (the smallest of blood vessels).
• This close association of tubes allows wastes
  (water, salts urea) to diffuse from the blood 
  into the sweat gland.
• When body temperature rises, the fluid (sweat) is
  released from the gland, travels through the
  duct, and reaches the skin surface through
  openings called pores.

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THE LIVER

• The liver is a large, important organ.  In fact
  it is the largest internal organ in our bodies. 
  Its numerous functions make it "part" of the
  circulatory, digestive, and excretory systems.
• In excretion, the liver removes the NH2 from
  amino acids in proteins. This is called
  deamination

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• The by-product of deamination is ammonia, a
  water-soluble gas.
• Ammonia is extremely toxic so it is combined with
  CO2 to make urea.
• Urea is much less toxic and can dissolve in the
  blood for excretion through the sweat or through
  urine.
• Uric acid is formed by the breakdown of nucleic
  acids in the liver.

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THE KIDNEY

• The kidney is the major organ of excretion.
• The kidney is also a major regulatory organ.
• Through the formation of urine, it is
  responsible for the following
• Removal of organic wastes urea, uric acid and
  the breakdown products of hemoglobin and hormones
• Regulation of concentrations of important ions
  sodium, potassium, calcium, magnesium, sulfate
  and phosphate ions

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• Regulation of pH balance of body control levels
  of H, HCO3- and NH4
• Regulation of Red Blood Cell production. The
  kidneys release erythropoietin, which regulates
  the production of RBCs in the bone marrow.
• Regulation of blood pressure regulation of
  fluid volume of the body.
• Limited control of blood glucose and blood amino
  acid concentration eliminate excess amounts

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Structure of the Kidney
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• Renal Capsule A smooth semitransparent membrane
  that adheres tightly to the outer surface of the
  kidney.
• Renal Cortex The region of the kidney just below
  the capsule. This part of the kidney is rich is
  arterioles and venules.

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• Renal Medulla The region below the cortex that
  is segregated into triangular regions. The
  triangular regions are the renal pyramids, which
  are striated (or striped) in appearance due to
  the collecting ducts running through them.
• Renal Pelvis A cavity within the kidney that is
  continuous with the ureter. The pelvis has
  portions that extend towards the renal pyramids.
  These extensions are called calyces.

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The Nephron

• The functional unit of the kidney is the nephron.
  Each kidney contains over one million of these
  microscopic filters.
• About 20 of the total blood pumped by the heart
  each minute will enter the kidneys to undergo
  filtration. This is called the filtration fraction

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The nephron
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Urine Formation

• Occurs in the nephron through three processes
• Glomerular Filtration
• Tubular Reabsorption
• Tubular Secretion

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Glomerular filtration

• The transfer of fluid and solutes from the
  glomerular capillaries into Bowmans capsule.
• Blood enters the glomerulus under pressure. This
  causes water, small molecules (urea, glucose,
  amino acids) and ions to filter through the
  capillary walls into Bowmans capsule.
• Large blood components (RBCs, WBCs, platelets
  and proteins) cannot filter through.

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• The fluid in the Bowman's capsule appears very
  much like interstitial fluid without the
  proteins. It is called the nephric filtrate
• The glomerular capillaries are substantially more
  (100 to 1000X) leaky than regular capillaries and
  have 2-3 times more pressure than regular
  capillaries.
• Glomerular filtration rate is fairly constant.
  (130 ml/min or 7.8 l/hr). This means that about
  190 L of filtrate is formed every 24 hours by
  both kidneys

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Tubular Reabsorption

• Occurs in the Proximal convoluted tubule, the
  Loop of Henle and the Distal convoluted Tubule.
• Materials which are required by the organism are
  returned to the bloodstream water, ions,
  glucose, amino acids etc
• The kidney does not work by a process of
  identifying what is bad rather it works by
  identifying those things that are good for the
  body

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• Much of the urea is lost simply because the
  kidney chooses not to recover it after it has
  been filtered.
• Any small foreign molecule that has entered our
  blood, even if it has not existed in human
  evolutionary history (drugs, new pollutants, etc)
  can be removed by the kidney.

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Proximal convoluted tubule

• The cells of the tubule are lined with
  microvilli. Why?
• Reabsorption of glucose, amino acids, and most
  inorganic salts occurs here.

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• Na ions are transported out of the tubule by
  active transport, through carrier molecules.
• Cl- ions and HCO3- ions follow by charge
  attraction.
• As these solutes move out of the tubule, they
  create an osmotic gradient and water moves out of
  the tubule and back into the blood, through
  osmosis.
• About 80-85 of the water in the filtrate is
  reabsorbed in the proximal tubule.

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• Glucose and amino acids attach to carrier
  molecules and are transported out by active
  transport.
• This requires a lot of energy so there are many
  mitochondria in the cells of the proximal tubule.
• There is a limit to the amount of sodium, glucose
  and amino acids that can be reabsorbed by the
  carrier molecules the threshold limit. When
  this limit is reached, these substances are
  excreted in the urine.

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• H ions are secreted into the proximal tubule.
  This helps regulate pH.
• About 50 of the urea that was in the nephric
  filtrate is reabsorbed in the tubule. This is a
  passive mechanism. The rest is excreted in the
  urine.

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The loop of Henle

• The descending loop of Henle is permeable to
  water. Water is reabsorbed into the peritubular
  capillaries by osmosis.
• The filtrate decreases in volume,but increases in
  osmotic concentration.
• Salt (NaCl) becomes concentrated in the filtrate
  as the loop penetrates the inner medulla of the
  kidney.

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• The ascending loop of Henle is permeable to salt
  but not to water. Sodium is actively transported
  out of the filtrate and chlorine follows by
  charge attraction.
• The volume of the filtrate does not change, but
  the concentration decreases.
• The peritubular capillaries ensure a rich blood
  supply for reabsorption

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The Distal Convoluted Tubule

• More sodium is reclaimed by active transport, and
  still more water follows by osmosis.
• Although 97 of the sodium has already been
  removed, it is the last 3 that determines the
  final balance of sodium.
• This determines the water content and blood
  pressure in the body.
• The reabsorption of sodium in the distal tubule
  and the collecting tubules is closely regulated,
  chiefly by the action of the hormone aldosterone.

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The Collecting Duct.

• The filtrate now flows into the collecting duct,
  which gathers fluid from many nephrons.
• Urine flows from the collecting ducts into the
  renal pelvis to the ureters and into the bladder.

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• Water regulation occurs here.
• The plasma membranes of the cells of the distal
  tubule and the collecting duct have transmembrane
  channels made of a protein called aquaporin.
• When these channels are open, water can pass
  through very quickly. These water channels are
  responsive to levels of antidiuretic hormone
  (ADH).

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Tubular secretion

• This is the third mechanism of excretion.
• Secretion occurs when substances are transported
  from the blood directly into the distal tubule.
• Hydrogen, potassium, and ammonium ions are
  actively secreted into the tubule. This helps to
  regulate pH.

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• If the pH of the blood becomes too acid, more H
  ions are secreted into the tubule.
• If the pH of the blood becomes too alkaline, then
  secretion of H is reduced.
• The pH of urine can vary from 4.5 to 8.5
• Certain drugs, such as penicillin, are also
  secreted into the tubule for excretion.
• Secretion occurs in the proximal tubule, the
  distal tubule and the collecting duct.

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• While we think of the kidney as an organ of
  excretion, it is more than that.
• It does remove wastes, but it also removes normal
  components of the blood that are present in
  greater-than-normal concentrations and reclaim
  these components when they are present in the
  blood in less-than-normal amounts.
• Thus the kidney continuously regulates the
  chemical composition of the blood within narrow
  limits. The kidney is one of the major
  homeostatic devices of the body.

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Urine Testing

• Color Normal urine will vary from light straw to
  amber color.
• The color of normal urine is due to a pigment
  called urochrome, which is the end product of
  hemoglobin breakdown 
• Hemoglobin ? hematin ? bilirubin
  ?urochromogen ? urochrome

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• The following changes in colour have pathological
  implications
• a. Milky presence of pus, bacteria, fat.
• b. Reddish amber presence of urobilinogen or
  porphyrin. Urobilinogen is produced in the
  intestine by the action of bacteria on bile
  pigments. Porphyrin may be evidence of liver
  cirrhosis, jaundice, Addison's disease, and other
  conditions.
• c. Brownish yellow or green presence of bile
  pigments.  
• d. Red to smoky brown presence of blood and
  blood pigments.
•  

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• Carrots may cause increased yellow color due to
  carotene, while beets cause reddening and rhubarb
  may cause the urine to become brown. These food
  items and certain drugs may color the urine, yet
  have no pathological significance.

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Glucose

• glucose is not normally present in the urine
  because all of it is usually reabsorbed from the
  renal tubules into the blood.
• When the glucose concentration of the filtrate is
  within the normal limits (70-110 mg per 100 ml),
  there is a sufficient number of carrier molecules
  in the renal tubules to transport all the glucose
  back into the blood.

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• However, if the blood glucose level exceeds its
  threshold (for glucose, about 180 mg per 100 ml),
  there will not be enough carrier molecules to
  reabsorb all of the glucose.
• The untransported glucose will end up in the
  urine and result in a condition known as
  glycosuria.
• The main cause of glycosuria is diabetes mellitus
  

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Ketones (Ketonuria)

• Normally, no ketones are present in urine.
• Detectable levels of ketone may occur in urine
  during physiological stress conditions such as
  fasting, pregnancy, and frequent strenuous
  exercise.
• When there is carbohydrate deprivation, such as
  in starvation or high-protein diets, the body
  relies increasingly on the metabolism of fats for
  energy.

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• This pattern is also seen in people with diabetes
  mellitus, where the lack of insulin prevents the
  body cells from utilizing the large amounts of
  glucose available in the blood.
• When the production of the intermediate products
  of fatty acid metabolism (ketone bodies) exceeds
  the ability of the body to metabolize these
  compounds, they accumulate in the blood and spill
  over into the urine (ketonuria).

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pH

• Freshly voided urine is usually acidic (around pH
  6) but the normal range is between 4.8 and 7.5.
• The pH will vary with the time of day and diet.
  High acidity is present in acidosis, fevers, and
  high protein diets. Excess alkalinity may be due
  to urine retention in the bladder, chronic
  cystitis, anemia, and obstructing gastric ulcers.

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Protein

• Since proteins are very large molecules, they are
  not normally present in measurable amounts in the
  glomerular filtrate or the urine.
• The detection of proteins in the urine,
  therefore, may indicate that the permeability of
  the glomerulus is abnormally increased.
• This may be caused by renal infections
  (nephritis), or it may be caused by other
  diseases that affect the kidney,such as diabetes
  mellitus, jaundice, or hyperthyroidism.