Topic 11.3: The Kidney and The Osmoregulation Flashcards
Excretion
Excretion is the removal of waste products from the body
• Wastes are produced as a consequence of metabolism
Excretory systems perform two functions:
1) Removes nitrogenous wastes (toxic) from the body
2) Removes excess water (maintains osmolarity)
Nitrogenous Wastes and Animals
The type of nitrogenous waste produced differs according to an animal’s evolutionary history and predominant habitat
1) Aquatic animals excrete ammonia (toxic but water soluble)
2) Birds and reptiles excrete uric acid (requires minimal water)
3) Mammals excrete urea (can store at high concentrations)
Osmotic Conditions and Animals
1) Osmoconformers match their osmolarity to the environment
2) Osmoregulators maintain a constant internal osmolarity
Osmoregulation Advantages and Disadvantages
Osmoregulation is a more energy intensive process, but it also provides independence from environmental conditions
Structures for Osmoregulation
1) Insects use a Malpighian tubule system for water balance
2) Mammals (e.g. humans) possess kidneys for water balance
Human Kidney Components
1) Cortex
2) Medulla
3) Pelvis
4) Renal Vein
5) Renal Artery
6) Ureter
Blood Composition in the Renal Artery and the Renal Vein
1) Less urea (large amounts are excreted)
2) Less water (variable amounts are excreted)
3) Similar amounts of nutrients (mostly reabsorbed)
4) The same amount of proteins (not filtered)
Urinary Analysis
Kidneys filter waste products from the bloodstream
• Hence, the presence of non-waste substances in the
urine is a potential indicator of a disease condition
Urinary analysis can be used to test for:
1) Glucose: Presence in urine may indicate diabetes
2) Protein: Indicate certain diseases / hormonal conditions
3) Blood cells: Suggestive of infectious diseases or cancers
4) Drugs: Indicates illicit use (e.g. performance enhancers)
Kidney Disease
Kidney diseases incapacitate the ability of the kidney to filter waste products from the bloodstream (leading to toxic build up)
Treatments for Kidney Disease
1) Kidney failure can be treated by hemodialysis (a patient’s blood is pumped through an external machine to remove wastes)
• Hemodialysis treatments typically last several hours (~4 hrs) and must be performed multiple times in a week (~3×)
2) Kidney failure can alternatively be treated via kidney transplant with a compatible donor (donor can survive with one kidney)
Nephrons
Nephrons are the functional units of the kidneys
• Are situated in the cortex but descend into the medulla
Stages of Excretion
1) Ultrafiltration – filters out all cells and proteins
2) Selective reabsorption – retains nutrients / solutes
3) Osmoregulation – controls water retention
Nephron Structure
1) Glomerulus
2) Bowman’s Capsule
3) Proximal Convoluted Tubules
4) Loop of Henle
5) Vasa Recta
6) Distal Convoluted Tubule
7) Collecting Duct
Ultrafiltration
Ultrafiltration occurs at the Bowman’s capsule / glomerulus
• Separates cells and proteins from blood to form filtrate
Structure of the Bowman’s Capsule
1) Glomerular capillaries are fenestrated (have pores), which allows blood to freely exit the glomerulus
2) The capsule is lined with podocytes that have extensions (called pedicels) that the blood can freely pass between
3) The only filtration barrier is the basement membrane
that lies between the glomerulus and the capsule
Hydrostatic Pressure (ULTRAfiltration)
1) Blood is forced into a Bowman’s capsule at high pressure
2) Wide afferent arterioles (entry) lead into narrow efferent
arterioles (exit), increasing the pressure in the capsule
3) The extensive narrow branching of the arterioles increases the glomerular surface area available for filtration
Selective Reabsorption
Selective reabsorption occurs in the convoluted tubules
• Involves the reuptake of usable substances from filtrate
Transportation of Materials in the Selective Reabsorption
1) Materials are actively transported across the tubule’s apical membrane before diffusing across the basolateral membrane
2) Tubules are lined with microvilli to increase surface area
Materials of the Selective Reabsorption
1) Glucose and amino acids (via symport with sodium ions)
2) Mineral ions and vitamins (via protein pumps)
3) Water (follows ions and solutes via osmosis)
Osmoregulation
Osmoregulation is the control of water balance in the body
• Involves the loop of Henle and collecting ducts
Establishing a Salt Gradient in Osmoregulation
1) The loop of Henle creates a salt gradient in the medulla
2) The descending limb is permeable to water but not salt
3) The ascending limb is permeable to salts but not water
4) This means that as the loop descends into the medulla, the interstitial fluid becomes increasingly hypertonic
Antidiuretic Hormone (ADH)
1) As the collecting duct passes through the medulla, the
salt gradient draws water out of the duct (into blood)
2) The amount of water drawn from the ducts is controlled by ADH (released from the posterior pituitary gland)
3) ADH produces water channels (aquaporins) to faciliate water reabsorption by the collecting duct
4) Levels are high when dehydrated and low when hydrated
Water Conservation
Maintaining water balance is critical to survival (homeostasis)
• Dehydration causes blood pressure to drop (⇧ heart rate)
• Overhydration causes cells to swell (leads to organ damage)
Water Conservation and Loops of Henle
Desert animals will have longer loops of Henle to maximise water conservation (⇧ salt gradient = ⇧ water reabsorption)
