EXCRETORY SYSTEM

• Components of the excretory system


  • Pair of kidneys
  • Pair of ureters
  • Urinary bladder
  • Urethra

  
• Ureters


  Tubes that carry urine from the pelvis of the kidneys to the urinary bladder

  
• Urinary bladder


  Temporarily stores urine until it is released from the body
• Urethra


  Tube that carries urine from the urinary bladder to the outside of the body

  
• Sphincter


  Circular muscle that controls the outer end of the urethra
• Sections of the kidney


  • Cortex
  • Medulla
  • Pelvis

  
• Cortex


  Where the blood is filtered
• Medulla


  Contains the collecting ducts which carry filtrate to the pelvis
• Pelvis


  Hollow cavity where urine accumulates and drains into the ureter
• Adipose tissue


  Surrounds the kidneys (capsula adiposa)

  
• Kidney


  • Essential part of the urinary system
  • Nephrons are structural and functional (filtration) units of the kidney
  • Normal kidney contains 800,000 to one million nephrons

  
• Components of the nephron


  • Renal corpuscle
  • Renal tubule

  
• Renal corpuscle


  Filtering component consisting of glomerulus and Bowman's capsule

  
• Renal tubule


  Specialized for reabsorption and secretion, consisting of proximal tubule, loop of Henle, distal tubule and collecting tubules

  
• Renal corpuscle


  • Composed of a glomerulus and Bowman's capsule
  • Afferent arteriole enters the glomerulus and an efferent arteriole leaves it
  • Glomerulus is composed of a capillary tuft that receives blood from an afferent arteriole
  • Glomerulus is surrounded by Bowman's Capsule with visceral (inner) and parietal (outer) layers
  • Space between the two layers is where primary urine is produced
  • Parietal layer is simple squamous epithelium
  • Visceral layer is composed of podocytes
  • Ultrafiltration of blood plasma occurs
  • Filtration barrier is endothelium, podocytes and their basement membranes
  • Glomerular blood pressure provides the driving force for blood plasma to be filtered

  
• Juxtaglomerular apparatus


  • Consists of macula densa and attaches to the afferent arteriole
  • Affects blood pressure by renin production

  
• Renal blood circulation


  1. Renal artery from aorta divides into branches

  2. Branches divide into interlobular arteries

  3. Interlobular arteries give rise to afferent arterioles

  4. Blood flows to glomerulus through afferent arterioles

  5. Blood is collected by interlobular veins, then passes to arcuate veins, interlobar veins, and finally the renal vein

  
• Segments of the renal tubule


  • Proximal tubule
  • Loop of Henle
  • Distal tubule
  • Collecting duct

  
• Proximal tubule


  • Leads from Bowman's capsule to the Loop of Henle
  • Lined by simple cuboidal epithelium
  • Absorption of amino acids, proteins, glucose, lactate, urea
  • Ion transport of bicarbonate, sodium, phosphate, and water
• Loop of Henle


  • Descending limb is permeable to water
  • Ascending limb transports ions
• Distal tubule


  • Connects the loop of Henle to the collecting duct
  • Lined by simple cuboidal cells
  • Transport of ions
  • Absorption and secretion is under hormonal control
• Collecting duct system


  • Contains intercalated cells involved in acid-base balance and principal cells that interact with antidiuretic hormone
  • Participates in electrolyte and fluid balance through reabsorption and excretion
  • Regulates electrolytes including chloride, potassium, hydrogen ions, and bicarbonate
• Normal diuresis is 1.5-2 l/day. Polyuria is diuresis higher than 2 l/day, oliguria lower than 0.5 l/day and anuria lower than 0.1 l/day.
• Functions of the renal system


  • Excretion of wastes (urea, uric acid)
  • Hormone secretion (calcitriol, renin, erythropoietin)
  • Osmolality regulation
  • Acido-base homeostasis
  • Blood pressure regulation

  
• Aldosterone


  Hormone that increases the reabsorption of sodium and water and the release of potassium in the kidneys

  
• Antidiuretic hormone (ADH)


  • Controls the reabsorption of water in the tubules by affecting the tissue's permeability
  • Plays a key role in homeostasis, and the regulation of water, glucose, and salts in the blood

  
• Renin-angiotensin-aldosterone system (RAAS)


  • When blood volume is low, the juxtaglomerular cells secrete renin, which converts angiotensinogen to angiotensin I
  • Angiotensin-converting enzyme converts angiotensin I to angiotensin II, which causes blood vessels to constrict and stimulates the secretion of aldosterone
  • Aldosterone increases the reabsorption of sodium and water, increasing blood volume and pressure

  
• Substances animals accumulate and need to remove


  • Ammonia
  • Urea
  • Uric acid
  • Carbon dioxide
  • Water
  • Sodium ions
  • Potassium ions
  • Chloride ions
  • Phosphate
  • Sulphate
• Ammonia is the most toxic form and requires large amount of water for its elimination, whereas uric acid, being the least toxic, can be removed with a minimum loss of water
• Ammonotelism


  The process of excreting ammonia
• Animals that are ammonotelic


  • Many bony fishes
  • Aquatic amphibians
  • Aquatic insects
• Ureotelic animals


  • Mammals
  • Many terrestrial amphibians
  • Marine fishes
• Uricotelic animals


  • Reptiles
  • Birds
  • Land snails
  • Insects
• Excretory structures in invertebrates


  • Protonephridia or flame cells
  • Nephridia
  • Malpighian tubules
  • Antennal glands or green glands
• Human excretory system


  • Pair of kidneys
  • Pair of ureters
  • Urinary bladder
  • Urethra
• Each kidney of an adult human measures 10-12 cm in length, 5-7 cm in width, 2-3 cm in thickness with an average weight of 120-170 g
• Structure of kidney


  • Outer tough capsule
  • Outer cortex
  • Inner medulla divided into medullary pyramids
  • Renal columns (Columns of Bertini)
• Nephron


  • Glomerulus
  • Renal tubule
• Parts of the renal tubule


  • Bowman's capsule
  • Proximal convoluted tubule
  • Henle's loop
  • Distal convoluted tubule
  • Collecting duct
• Cortical nephrons have a short loop of Henle, while juxta medullary nephrons have a long loop of Henle