The Urinary (Excretory) System

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The Urinary (Excretory) System
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Functions of the Urinary System/Kidneys

• Removal of metabolic wastes, such as urea,
  ammonia, creatinine, and uric acid.
• Maintenance of water-salt balance in body
  (blood). Ions such as Na, Cl-, K, NH4, Ca2,
  among others are managed.
• Maintenance of acid-base balance in body (blood).
  Ions such as H and HCO3- are managed.
• Hormonal function Erythropoietin described in
  Circulatory System as well as Renin (an enzyme
  involved in the urinary hormone, aldosterones
  pathway).
• - Refer to p. 305 for a summary of each.

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Excretion

• Excretion is the process by which metabolic
  wastes are removed from the body (in this unit,
  our main focus will be on nitrogen-containing
  (nitrogenous) wastes).
• The metabolism of amino acids, nucleotides, and
  creatine phosphate results in nitrogenous wastes
  that need to be disposed of by the body.
• Common nitrogenous wastes that humans excrete
  include, in order of amount urea, creatinine,
  uric acid, and ammonia (which tends to buffer H
  present in urine forming NH4 (ammonium)).
  Ammonia is present in trace amounts.
• Feces is not considered to be metabolic wasteit
  consists of materials that never actually
  entered the body the ridding of feces is known
  as elimination.

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• Amino Acid Metabolism
• the deamination of amino acids (ie. the removal
  of the amino (-NH2) group) occurs in the liver in
  order to convert three amino acids into one
  glucose molecule.
• Deamination results in the formation of ammonia
  (NH3), which is a basic and toxic chemical
  requiring a large amount of water for safe
  dilution and subsequent removal from any animal.
• Thus, to conserve water for the body, the liver
  converts the majority of the ammonia into urea,
  which is far less toxic, and requires much less
  water for disposal than ammonia.
• Urea is filtered out of the blood into the urine
  by the kidneys.

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Ammonia reacts with CO2 in the liver to produce
UREA

• 2 NH3 CO2

H2O
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• Nucleotide Metabolism
• if too many DNA/RNA/ADP nucleotides exist within
  the body or if they become degraded to a
  non-functional state, they are metabolized in
  order to conserve space within cells.
• The metabolism of nucleotides results in the
  formation of uric acid, which is a relatively
  insoluble nitrogenous waste (bird pee the
  white stuff).
• Uric acid is filtered out of the blood by the
  kidneys for excretion.
• If too much uric acid accumulates in the blood,
  it may precipitate out of solution, most commonly
  in joints (known as gout), less commonly in the
  urinary tract as a kidney stone (most kidney
  stones are Ca2-based).

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• Creatine Phosphate Metabolism
• creatine phosphate is a donator molecule that
  donates a phosphate (PO43-) group to ADP
  (adenosine diphosphate) in order to re-create
  ATP.
• This occurs solely within skeletal muscle cells
  and serves to speed up the production of ATP
  during times of stress/exercise.
• Once creatine phosphate has donated its phosphate
  group, its name changes to creatinine, which is a
  nitrogenous waste that is filtered out of the
  blood by the kidneys.

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• Urea, creatinine, uric acid, and ammonia/ammonium
  are all nitrogenous wastes that exist in urine.
• Urine is produced within the kidneys (the kidneys
  essentially filter the blood) for expulsion from
  the body.
• The general composition of urine is as follows
• 95 water 5 solids/dissolved substances
• Solids/Dissolved substances include (per 500 mL
  urine)
• - 10 g urea
• - 0.33-0.67 g creatinine
• - 0.33 g uric acid
• - 8-9 g of the following Na, K, Ca2, Mg2,
  NH4 (a combination of H and NH3), SO42-, PO43-,
  HCO3-
• - varying levels of toxins, drugs, antibiotics,
  histamines.

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Anatomy of the Urinary System (fig. 16.1 p. 304)

• All organs and blood vessels of the urinary
  system work to maintain homeostasis (ie. they
  work to maintain a constant internal environment)
  with respect to water-salt balance, pH balance,
  and nitrogenous waste management.
• The kidneys are paired, kidney bean-shaped organs
  that exist on either side of the spinal cord,
  just below the diaphragm a kidney is about the
  size of a fist.
• Protection of kidneys they lie within concave
  depressions in the back muscles and are covered
  by the lower ribs.

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• The kidneys are supported within their back
  muscle depressions by connective tissue known as
  renal fascia.
• Kidneys serve to produce urine, whereas other
  urinary structures serve to transport, store, and
  excrete urine.
• Blood carrying dissolved wastes enters each
  kidney on its concave side (called the hilium)
  via the renal artery note the kidneys comprise
  only 1 of the mass of a human body but they
  receive about 20 of the blood pumped with each
  heartbeat.
• Blood is essentially filtered by the kidneys as
  it flows through a series of arterioles and
  capillaries.
• Filtered blood exits the kidneys via the renal
  vein.

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• Once filtering is complete, urine (the waste
  fluid) exits each kidney through a duct known as
  a ureter.
• Ureters are narrow, muscular tubes that utilize
  peristalsis to carry urine to its storage area,
  the urinary bladder.
• The urinary bladder is a muscular organ that
  expands as urine enters it it can store up to
  600 mL of urine due to its distensible walls.
• When the bladder holds around 250 mL, a person
  gets the urge to urinate at around 500 mL, it
  becomes uncomfortable.
• The urinary bladder houses special stretch
  receptors that send sensory impulses to the
  spinal cord, which sends motor impulses back to
  the bladder signaling it to contract. Thus, the
  bladder is partly controlled via a reflex arc.

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• So why dont we pee our pants when this occurs?
• The brain plays a role as well there is a
  voluntary holding of your urine (ie. The
  clench) that occurs if facilities are not
  nearby or if you are too busy.
• Urine that leaves the bladder enters the urethra,
  which is the final tube it passes through before
  leaving the body the passage of urine from the
  bladder to the urethra is mitigated by two
  muscle-controlled sphincters.
• The first sphincter is under parasympathetic,
  involuntary control (spinal cord/reflex control)
  and is relaxed when the bladder contracts (ie. we
  have no choice in the matter).
• The second sphincter is under the conscious,
  voluntary, somatic control of the brain this is
  how a person can hold their pee.
• See fig. 16.2 p. 305

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• The urethra extends from the bladder to the
  exterior opening known as the external urethral
  orifice.
• The urethra is about 4 cm long in females, and
  about 20 cm long in males (this is why females
  are more susceptible to urinary tract infections
  (bacterial)). Read Health Focus on p. 306.
• The prostate gland encircles the urethra in men
  and can cause a restriction of urination if it
  becomes enlarged.

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FEMALE URETHRA
MALE URETHRA
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Kidneys

• When a kidney is sliced lengthwise, it is evident
  that the renal artery, the renal vein, and the
  ureter all enter/exit the kidney at the hilium
  (fig. 16.3a p.307).
• The longitudinal section also provides one with a
  glimpse into the three major regions (layers) of
  a kidney the renal cortex, renal medulla, and
  renal pelvis (fig. 16.3b p. 307).
• The renal cortex is the outer, overlying,
  protective layer that houses an ECF with a
  relatively low osmotic pressure.
• The renal medulla is the inner, fan-shaped
  layer that houses an ECF with a relatively high
  osmotic pressure this region consists of
  multiple cone-shaped tissue masses called renal
  pyramids, which serve to collect and drain urine
  into the third region, the renal pelvis the
  innermost cavity that collects urine and allows
  it to drain into a ureter

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• On a micro scale, a kidney is composed of about
  one million nephrons, which are tiny
  urine-producing factories.
• Each nephron serves to produce a tiny bit of
  urine, which eventually drains into the renal
  pelvis.
• Each nephron possesses structures that exist in
  the renal cortex or the renal medulla. In fact,
  the collecting ducts of nephrons (the final
  tubules of nephrons) are responsible for the
  fan-shaped appearance of the renal pyramids in
  the medullar region of the kidneys.

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Nephron Structure (fig. 16.4 p. 308)

• TWO MAJOR PARTS Blood Vessels and Urinary Tract
  (also keep in mind the existence of ECF between
  these two major parts).
• Blood Vessels
• The renal arteries are the vessels carrying blood
  to the kidneys (to be filtered and to provide O2
  and nutrients to the kidney cells).
• A renal artery splits into about one million
  afferent arterioles, each of which carry blood to
  a nephron.
• An afferent arteriole carries blood to the first
  nephronal capillary bed known as the glomerulus,
  a knot-like ball of capillary vessels located
  in the Bowmans (Glomerular) Capsule of the
  nephron.

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• Some filtering occurs here, then blood leaves the
  glomerulus and enters an efferent arteriole,
  which carries blood to the second nephronal
  capillary bed called the peritubular capillary
  network (the efferent arteriole is another
  example of a portal system as it connects two
  capillary beds).
• This network surrounds the majority of the
  nephron like a spiders web more filtering
  occurs here.
• Blood then flows into a renal venule, and
  eventually the renal vein, where its composition
  is quite different from what it was in the renal
  artery. In fact, blood in the renal vein is
  hypotonic compared to blood in the renal artery
  (due to the filtering that occurs between the
  glomerulus/peritubular network and the
  ECF/Urinary tract).

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• The Urinary Tract
• The nephron is really a tube (with a lumen) that
  acts to receive and process material from the
  blood.
• It consists of several parts Bowmans
  (Glomerular) Capsule, Proximal Convoluted Tubule,
  Loop of Henle (Loop of the Nephron), Distal
  Convoluted Tubule, and Collecting Duct.
• Bowmans Capsule
• a goblet-like structure found at the blind end
  of the nephron.
• Outer layer comprised of protective epithelial
  cells inner layer comprised of special cells
  called podocytes, which integrate with the
  glomerulus to form pores (slits) that allow for
  simple size-based filtration to occur between the
  glomerulus and Bowmans Capsule.

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• Proximal Convoluted Tubule (PCT)
• Comprised of special epithelial cells that
  possess microvilli, which form a brush border
  that serves to increase the SA for the tubular
  reabsorption that occurs here (microvilli collect
  materials from lumen of tubule and allow entrance
  to epithelial cells, which perform tubular
  reabsorption). Fig. 16.5 p. 309
• This reabsorption process (described later) is an
  active one requiring ATP and carrier proteins.
  Thus, a plethora of mitochondria exist within
  each epithelial cell of the proximal convoluted
  tubule.
• both the Bowmans Capsule and the PCT exist in
  the renal cortex of the kidney.

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• Loop of Henle
• Begins in the renal cortex, dips deep into the
  renal medulla, and returns (due to a hairpin turn
  at its base) to the cortex.
• Thus, the loop is discussed in two parts, its
  descending arm and its ascending arm as they both
  have distinctly different structures/functions.
• Both arms possess a thin-walled and a
  thick-walled portion whose effects will be
  discussed during urine formation (fig. 16.7 p.
  312).

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• Distal Convoluted Tubule (DCT)
• Lined by epithelial cells (this time, no
  microvilli) that possess many mitochondria for
  active transport during a process known as
  tubular secretion.
• Here, substances tend to actively move from the
  blood into the DCT for excretion.
• More tubular reabsorption occurs here as well.
• Exists solely within the renal cortex.
• Unlike most diagrams of the nephron, the DCT
  actually neighbours the afferent arteriole (ie.
  they touch) at a region known as the
  juxtaglomerular apparatus (fig. 16.8 p. 313).

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Such a twisting back of the nephron allows each
nephron to be skinnier so that a million can
exist within a kidney.
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• Collecting Duct
• The nephrons last urinary structure is the
  collecting duct, which dives back into the renal
  medulla region of the kidney.
• Comprised of epithelial cells that are permeable
  to water but not to salts.
• Several DCTs drain into one collecting duct
  (about 200-300 collecting ducts per kidney why
  you see branches coming off of the coll. duct
  in pictures).
• Each collecting duct drains collected urine
  into the renal pelvis for eventual expulsion from
  the body.
• The collecting duct does not have the capillary
  network as close to it as the other nephron
  structures do. The material that it releases
  enter the ECF and contribute to the higher than
  normal osmotic pressure that exists in the renal
  medulla.