animal excretory systems

• O2 unloaded to tissues at rest and during exercise
• Hemoglobin retains less O2 at lower pH (higher CO2 concentration)
• CO2 produced during cellular respiration lowers blood pH and decreases the affinity of hemoglobin for O2; this is called the Bohr shift
• Hemoglobin plays a minor role in transport of CO2 and assists in buffering the blood
• Osmoregulation balances the uptake and loss of water and solutes
• The driving force for movement of solutes and water is a concentration gradient of one or more solutes across the plasma membrane
• Osmolarity


  The solute concentration of a solution, determines the movement of water across a selectively permeable membrane
• Osmoconformers, consisting only of some marine animals, are isoosmotic with their surroundings and do not regulate their osmolarity
• Osmoregulators expend energy to control water uptake and loss in a hyperosmotic or hypoosmotic environment
• Adaptations to reduce water loss are key to survival on land for terrestrial animals
• Osmoregulators must expend energy to maintain osmotic gradients
• The type and quantity of an animal's waste products may greatly affect its water balance
• Forms of nitrogenous waste

  • Ammonia
  • Urea
  • Uric acid
• Excretory systems regulate solute movement between internal fluids and the external environment
• Excretory processes


  1. Filtration

  2. Reabsorption

  3. Secretion

  4. Excretion
• Kidneys, the excretory organs of vertebrates, function in both excretion and osmoregulation
• Nephrons


  • Highly organized tubules in the kidney
  • Involved in filtration, reabsorption, secretion, and excretion
• The filtrate produced in Bowman's capsule contains salts, glucose, amino acids, vitamins, nitrogenous wastes, and other small molecules
• From blood filtrate to urine

  1. Proximal tubule: Reabsorption of ions, water, and nutrients

  2. Descending limb of loop of Henle: Reabsorption of water

  3. Ascending limb of loop of Henle: Salt but not water able to diffuse from tubule

  4. Distal tubule: Regulates K+ and NaCl concentrations, contributes to pH regulation

  5. Collecting duct: Reabsorption of solutes and water
• Descending Limb of the Loop of Henle

  1. Reabsorption of water continues through channels formed by aquaporin proteins

  2. Movement is driven by the high osmolarity of the interstitial fluid, which is hyperosmotic to the filtrate

  3. The filtrate becomes increasingly concentrated
• Ascending Limb of the Loop of Henle

  1. In the ascending limb of the loop of Henle, salt but not water is able to diffuse from the tubule into the interstitial fluid

  2. The filtrate becomes increasingly dilute
• Distal Tubule


  1. The distal tubule regulates the K+ and NaCl concentrations of body fluids

  2. The controlled movement of ions (H+ and HCO3−) contributes to pH regulation
• Collecting Duct


  1. The collecting duct carries filtrate through the medulla to the renal pelvis

  2. One of the most important tasks is reabsorption of solutes and water

  3. Urine is hyperosmotic to body fluids
• The mammalian kidney's ability to conserve water is a key terrestrial adaptation
• Hyperosmotic urine can be produced only because considerable energy is expended to transport solutes against concentration gradients
• The two primary solutes affecting osmolarity are NaCl and urea
• Concentrating Urine in the Mammalian Kidney

  1. In the proximal tubule, filtrate volume decreases as water and salt are reabsorbed, but osmolarity remains the same

  2. As the filtrate flows to the descending limb of the loop of Henle, solutes become more concentrated due to water leaving the tubule by osmosis

  3. NaCl diffusing from the ascending limb maintains a high osmolarity in the interstitial fluid of the renal medulla
• Concentrating Urine in the Mammalian Kidney (continued)

  1. Energy is expended to actively transport NaCl from the filtrate in the upper part of the ascending limb

  2. The countercurrent multiplier system involving the loop of Henle maintains a high salt concentration in the kidney

  3. This system allows the vasa recta to supply the kidney with nutrients, without interfering with the osmolarity gradient
• Concentrating Urine in the Mammalian Kidney (continued)

  1. In the collecting duct, osmosis extracts water from the filtrate as it passes from cortex to medulla and encounters interstitial fluid of increasing osmolarity

  2. Urine produced is isoosmotic to the interstitial fluid of the inner medulla, but hyperosmotic to blood and interstitial fluids elsewhere in the body
• Juxtamedullary nephron


  • Key to water conservation in terrestrial animals
• Mammals that inhabit dry environments have long loops of Henle, while those in fresh water have relatively short loops
• Mammals can control the volume and osmolarity of urine in response to changes in salt intake and water availability
• Antidiuretic hormone (ADH)


  Also called vasopressin
• Homeostatic Regulation of the Kidney

  1. A combination of nervous and hormonal controls manages the osmoregulatory functions of the mammalian kidney

  2. These controls contribute to homeostasis for blood pressure and blood volume
• Antidiuretic Hormone (ADH)


  1. Osmoreceptor cells in the hypothalamus monitor blood osmolarity and regulate release of ADH from the posterior pituitary

  2. When osmolarity rises above its set point, ADH release into the blood stream increases

  3. Binding of ADH to receptor molecules leads to a temporary increase in the number of aquaporin proteins in the membrane of collecting duct cells

  4. This reduces urine volume and lowers blood osmolarity
• The Renin-Angiotensin-Aldosterone System (RAAS)


  1. A drop in blood pressure near the glomerulus causes the juxtaglomerular apparatus (JGA) to release the enzyme renin

  2. Renin triggers the formation of the peptide angiotensin II

  3. Angiotensin II raises blood pressure and decreases blood flow to the kidneys

  4. Angiotensin II stimulates the release of the hormone aldosterone, which increases blood volume and pressure
• Coordination of ADH and RAAS Activity

  1. ADH and RAAS both increase water reabsorption, but only RAAS will respond to a decrease in blood volume

  2. Another hormone, atrial natriuretic peptide (ANP), opposes the RAAS

  3. ANP is released in response to an increase in blood volume and pressure and inhibits the release of renin
• Vasa recta

  Specialized blood vessels that run parallel to the loop of Henle in the kidney. They maintain the osmolarity gradient established by the loop of Henle by allowing for the passive diffusion of water and solutes.