CHAPTER 44 (Osmoregulation& Excretion)

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What is osmoregulation? Why is it important

  • Osmoregulation = regulation of solute concentrations, balancing the gain and loss of water
    • Osmoregulation involves controlled movement of solutes between internal fluids and the external environment

What is excretion?

  • Excretion = process that gets rid of nitrogenous metabolites and other waste products

What is osmolarity?

  • Osmolarity = solute concentration of a solution

Determines the movement of water across a selectively permeable membrane


What does isoosmotic, hyperosmotic, and hypoosmotic mean?

  • If the movement of water is equal in both directions between two solutions = isoosmotic
  • If two solutions differ in osmolarity = the net flow of water is from the hypoosmotic to the hyperosmotic solution

When two solutions differ in osmolarity, which way will water move?



How are freshwater animals different from marine animals in regards to osmoregulation and excretion?

Freshwater Animals

  • Constantly take in water by osmosis from their hypoosmotic environment
  • They lose salts by diffusion
  • Replace salts through food and uptake across the gills
  • They maintain water balance by excreting large amounts of dilute urine

Marine Animals

  • Most marine invertebrates are osmoconformers
  • Most marine vertebrates are osmoregulators
  • Marine bony fishes are hypoosmotic to sea water
  • They lose water by osmosis
  • They gain salt by diffusion and from food
  • They balance water loss by drinking seawater and excreting salts

How do land animals balance osmoregulation and excretion? How do they gain water and lose water?

  • Manage water balance by drinking and eating moist foods and using metabolic water
  • Desert animals get major water savings from anatomical features and behaviors (Example: nocturnal lifestyle)
    • Animals regulate the composition of body fluid that bathes their cells

What are transport epithelia?

  • Transport epithelia = specialized epithelial cells that regulate solute movement
  • Essential components of osmotic regulation and waste disposal
  • Arranged in complex tubular networks

What are the forms of nitrogenous waste? How toxic are they? How are they excreted? Are they energetically expensive?

Different animals excrete nitrogenous wastes in different forms:

  • Ammonia- Most toxic form: Animals that excrete this need a lot of water:Requires the least amount of energy
  • Urea- Moderately toxic: Requires more energy: Excretion requires less water than ammonia
  • Uric acid- Least Toxic:Requires the most energy: Birds/land snails/many reptiles excrete this

What is an excretory system?

  • A tubular system
  • Regulate solute movement between internal fluids and the external environment
  • Most produce urine by refining a filtrate derived from body fluids

What is filtrate? What does it usually contain?

  • Filtration – pressure-filtering of body fluids

What are the four major functions of an excretory system? Define them.

  1. Filtration – pressure-filtering of body fluids
  2. Reabsorption – reclaiming valuable solutes
  3. Secretion – adding toxins and other solutes from the body fluids to the filtrate
  4. Excretion – removing the filtrate from the system

What is the main excretory organ in vertebrates?

  • Kidneys = excretory organs of vertebrates
  • Function in both excretion and osmoregulation

How is the mammalian excretory system structured? How does it work?

  • Paired kidneys – the central site of water balance and salt regulation
  • Each kidney is supplied with blood by a renal artery and drained by a renal vein
  • Urine exits each kidney through a duct (ureter)
  • Both ureters drain into a common urinary bladder
  • Urine is expelled through a urethra

Describe kidney structure.

Kidney structure

  • Each kidney is supplied with blood by a renal artery and drained by a renal vein
  • Urine exits each kidney through a duct (ureter)
  • Both ureters drain into a common urinary bladder
  • Urine is expelled through a urethra

What is the renal cortex? What is the renal medulla?

Renal cortex – outer region
Renal medulla – inner region


What is a nephron?

  • Nephron = functional unit of the kidney

What are the major regions of the nephron? (Glomerulus, Bowman’s capsule, etc.)

  • Consists of a single long tubule and a ball of capillaries called the glomerulus
  • Bowman’s capsule surrounds and receives filtrate from the glomerulus

How is blood filtered?

  • Filtration occurs as blood pressure forces fluid from the blood in the glomerulus into the lumen of Bowman’s capsule
  • Filtration of small molecules is nonselective
  • The filtrate contains salts, glucose, amino acids, vitamins, nitrogenous wastes, and other small molecules

What are the major regions of the nephron tubule?

  1. Proximal tubule
  2. Loop of Henle
  3. Distal tubule
  • Fluid from several nephrons flows into a collecting duct and is drained by the ureter

What is reabsorbed at the proximal tubule? What gets secreted into the filtrate? How does filtrate volume change?

  • Proximal Tubule
  • Reabsorption of ions, water, and nutrients
  • Majority of reabsorption happens here**
  • Molecules transported actively and passively from the filtrate into the interstitial fluid and then capillaries
  • Some toxic materials are secreted into the filtrate
  • Filtrate volume decreases

What is reabsorbed at the descending loop of Henle? How is movement driven? What happens to the filtrate?

  • Descending Loop of Henle
  • Reabsorption of water continues
  • Movement is driven by the high osmolarity of the interstitial fluid (which is hyperosmotic to the filtrate)
  • Filtrate becomes increasingly concentrated

What is reabsorbed at the ascending loop of Henle? What happens to the filtrate?

  • A scending Loop of Henle
  • Salt reabsorption
  • Drives reabsorption of water at the descending loop
  • No water reabsorption from the tubule into the interstitial fluid
  • Filtrate becomes increasingly dilute

What does the distal tubule regulate?

Distal Tubule

  • Regulates the K+ and NaCl concentrations of body fluids
  • Controlled movement of ions contributes to pH regulation

What happens at the collecting duct? What happens to the filtrate?

Collecting duct

  • Filtrate can become more concentrated
  • Last chance to reabsorb NaCl , water, and urea
  • Urine is hyperosmotic to body fluids

How is NaCl involved in water reabsorption?

  • NaCl is the main player for water reabsorption – contributing the most to interstitial fluid osmolarity that allows kidneys to reabsorb water

How are hormones involved in osmoregulation and excretion?

Hormonal circuits link kidney function, water balance, and blood pressure

    • Mammals control the volume and osmolarity of urine
    • Use hormones to regulate this

What does antidiuretic hormone do? What triggers the release of ADH?

  • ADH increases water reabsorption in the distal tubules and collecting ducts of the kidney
  • An increase in osmolarity triggers the release of ADH – which helps to conserve water

What is RAAS? How is it activated, and what happens in response?

  • RAAS is part of a complex feedback circuit that functions in homeostasis
  • A drop in blood pressure near the glomerulus causes the enzyme renin to be released
  • Renin triggers the formation of the peptide angiotensin II
    • Angiotensin II
    • Raises blood pressure and decreases blood flow to the kidneys
    • Stimulates the release of the hormone aldosterone – which increases blood volume and pressure

What is ANP? What does it do?

  • ADH and RAAS both increase water reabsorption
  • Only RAAS will respond to a decrease in blood volume
  • Atrial natriuretic peptide (ANP) – is a hormone that opposes the RAAS
  • Released in response to an increase in blood volume and pressure
  • Inhibits the release of renin