Excretion

1
Excretion

• Anatomy and Physiology of the Urinary System

2
Excretion Defined

• Elimination of metabolic wastes

3
Systems Involved in Excretion

• Urinary system
• _____
• Respiratory system
• ____________
• Digestive system
• _____
• Integumentary system
• _____

Urine
Carbon dioxide
Feces
Sweat
4
Functions of the Urinary System

• Excretion of metabolic wastes
• Regulates fluid and electrolyte balance
• Regulates blood pH
• Produces hormones
• Calcitriol (vitamin D) helps regulate calcium
• Erythropoietin regulates RBC formation
• Regulates blood pressure by enzyme renin

5
Anatomy of the urinary system

• Kidneys filter blood and produce urine
• Ureters transport urine to urinary bladder
• Urinary bladder stores urine until eliminated
• Urethra is tube for elimination of urine

Kidney-----
Ureter----------
Urinary bladder----------
Urethra----------------------
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Kidney Anatomy

• Renal capsule sheaths kidney
• Renal cortex is outer region
• Renal medulla is inner region with 8-18 renal
  pyramids
• Renal papilla is apex or tip of renal
  pyramid
• Renal columns are cortical tissue between
  pyramids

Renal capsule-----
Renal papilla------
Renal cortex-----
Renal pyramid--------
Renal column---------------
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Anatomy of Kidney Continued

• Minor calyx receives urine from renal papilla
• Major calyx collects urine from two or more minor
  calyces
• Renal pelvis collects urine from all calyces
• Ureter carries urine to bladder

---------------Minor calyx
Major calyx---------
Renal pelvis--------------
---Ureter
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Nephron

• Microscopic filtering units of kidneys composed
  of renal tubules and a blood supply
• Called functional units make most of urine
• About 1.25 million per kidney
• Mostly in renal cortex with some tubules
  extending into renal pyramids of medulla

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Nephron Continued

• Structure
• Renal corpuscle
• Loops of capillaries called glomerulus
• Double layered Bowmans capsule surrounds
  glomerulus
• Renal tubules
• Proximal convoluted tubule (PCT) connects to
  Bowmans capsule
• Loop of Henle with descending and ascending limbs
• Distal convoluted tubule (DCT) is last part of
  nephron
• DCT of several nephrons connect to a collecting
  duct (CD)

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Nephron Anatomy
Bowmans capsule----------------
-------------Glomerulus
PCT------------------------------
----------DCT
Descending limb of Henle----------------
-----------Ascending limb of Henle
---------CD
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Renal Corpuscle Anatomy
---Bowmans capsule
Afferent arteriole----------
-------Glomerulus
DCT-----------------
-------PCT
Efferent arteriole-----
--------Capsular space
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Nephron Blood Supply
Peritubular capillaries----------------
-------Efferent arteriole
-------Afferent arteriole
----------------Vasa recta
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Physiology of the Urinary System

• Three processes required for urine formation

1. Filtration from the glomerulus to Bowman's
capsule
2. Tubular reabsorption from renal tubules to
blood
3. Tubular secretion from blood into the renal
tubules
Urine
1
2
3
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Filtration

• Water and many different solutes pass from
  glomerulus to the Bowmans capsule
• Resulting fluid in capsular space called filtrate
• Filtrate contains
• Wastes such as urea and uric acid that must be
  eliminated in the urine
• Useful (vital) substances such as water, organic
  nutrients and electrolytes that must be kept

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Filtrate Formation

• High glomerular blood hydrostatic pressure of
  about 55 mmHg forces water and solutes through
  filtration membrane
• Formed by glomerular capillaries and visceral
  layer of Bowmans capsule

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Filtration Continued

• Amount of blood flow through kidneys in one
  minute renal blood flow (RBF)
• About 1200 mL/minute
• Amount of filtrate formed in one minute
  glomerular filtration rate (GFR)
• Between 105 and 125 mL/minute
• So around 10 of RBF becomes filtrate

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Regulation of GFR How to keep GFR in
homeostasis as BP changes

• Regulation by controlling glomerular blood flow
• Dilation of afferent arteriole or constriction of
  efferent arteriole increases GFR
• Increased blood flow in glomerulus causes
  increased filtration
• Constriction of afferent arteriole or dilation of
  efferent arteriole decreases GFR
• Decreased blood flow in glomerulus causes
  decreased filtration

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Three Methods of Regulation

• Autoregulation
• Regulation of GFR by kidneys themselves
• Neural regulation
• Autonomic nervous system affects GFR
• Hormonal regulation
• Hormones indirectly affect GFR
• All make adjustments in glomerular blood pressure

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Tubular Reabsorption

• Most of filtrate moves from the nephron tubules
  back into the blood
• Water, organic nutrients and electrolytes are
  kept
• Wastes leave with urine
• Most reabsorption in proximal convoluted tubule

Blood of peritubular capillaries
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Tubular Reabsorption Continued

• Water reabsorbed by osmosis
• Solutes reabsorbed by diffusion, active
  transport, cotransport, countertransport and
  pinocytosis

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Reabsorption Continued

• Most actively transported substances have a
  transport maximum (Tm)
• The Tm refers to how many milligrams of a
  substance the nephron tubules can reabsorb in a
  minute
• When Tm exceeded, substance shows up in urine
• Renal threshold is the maximum blood
  concentration at which a substance begins to
  appears in the urine
• Renal threshold for glucose 180 milligrams per
  100 mL (deciliter)

22
Tubular Secretion

• Some solutes move from the blood of the nephron
  capillaries into the filtrate
• Potassium, ammonium, hydrogen ions and
  bicarbonate ions are secreted into filtrate
• Helps regulate acid-base balance and electrolyte
  concentrations

Blood of peritubular capillaries
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Water Reabsorption

• Each day we lose about as much water as we gain
• If loss exceeds gain, become dehydrated
• If gain exceeds loss, become over-hydrated
• Controlled by reabsorption of varying amounts of
  water

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Water Reabsorption Continued

• Each day, between 150-180 L of filtrate
• Each day, 1-2 L of urine
• What does this tell you?
• Amounts of H2O reabsorbed
• 65 in proximal convoluted tubules
• 15 in descending limbs of Henle
• None in ascending limb of Henle
• 10-15 in distal convoluted tubules
• Remainder in collecting ducts

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Types of Water Reabsorption

• Obligatory water reabsorption
• In PCT and descending LOH
• More constant
• Facultative water reabsorption
• In DCT and CD
• Variable, depending on degree of hydration
• This reabsorption controlled by hormones

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Setting Stage for Water Reabsorption

• Remember, 65 of water that is reabsorbed is
  reabsorbed in the PCT
• 15 is reabsorbed in descending LOH
• Water not reabsorbed by ascending LOH
• Salt (sodium chloride) is actively reabsorbed by
  ascending LOH
• Filtrate becomes about 4X as concentrated at
  bottom of LOH lot more salt and urea at bottom

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Water Reabsorption Continued
H2O-65
Less salt and urea
H2O-15
ISF and Blood
Urea
More salt and urea
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Steps in Concentration of Urine

• 1. As water loss exceeds gain, hypothalamus
  stimulates secretion of antidiuretic hormone
  (ADH) from the posterior pituitary
• 2. ADH makes the cells of the DCT and CD more
  permeable to water adds water pores
• 3. More water leaves the DCT and CD and enters
  concentrated interstitial fluid and blood of the
  pyramid

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Steps in Concentration of Urine

• 4. Concentrated interstitial fluid and blood take
  more water out of the filtrate and concentrate
  the urine
• 5. Result is less of a dark, and concentrated
  urine
• Aldosterone from adrenal cortex increases salt
  reabsorption from tubules and thus helps to
  concentrate urine
• How does this work?

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Concentration of Urine-ADH Present
ADH
DCT and CD more permeable to water and more water
reabsorbed
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Steps in Dilution of Urine

• 1. As water gain exceeds loss, hypothalamus stops
  secreting ADH
• DCT and CD become less permeable to water water
  pores decrease
• Less water is reabsorbed
• More water stays in filtrate and urine

32
Steps in Dilution of Urine

• 5. Results in more of a dilute urine
• 6. Adrenal cortex decreases aldosterone
  secretion, thus less salt is reabsorbed from
  tubules and this helps to dilute urine

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Dilution of Urine
No ADH, so DCT and CD less permeable to water and
less water reabsorbed
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Urine the final product

• Urine composition
• 95 H2O
• 5 solutes (solids)
• Electrolytes
• Nitrogenous wastes such as urea, creatinine and
  uric acid
• Urea from __________________
• Creatinine from ________________
• Uric acid from ____________________

protein metabolism
muscle metabolism
nucleic acid metabolism
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Urine Continued

• Characteristics of Urine
• Color
• Turbidity
• Specific gravity
• pH

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Clinical Terms

• Glomerulonephritis
• Acute and Chronic Renal failure
•  
• Hemodialysis (kidney dialysis)
• Renal Calculi
•  

37
Polycystic Disease

• Incurable, kidney disease in which cysts form in
  kidney, and destroy it. Cysts can be clearly seen
  in kidney below it weighed 17 pounds.

Cyst-------------