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Diverse Excretory Systems and Their Functions

This chapter discusses the diverse excretory systems found in different animals, including invertebrates and vertebrates, and their key functions in regulating solute movement and excretion. It also explores the structure and function of nephrons in the mammalian kidney.

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Diverse Excretory Systems and Their Functions

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  1. Chapter 44 Osmoregulation and Excretion

  2. Concept 44.3: Diverse excretory systems are variations on a tubular theme • Excretory systems regulate solute movement between internal fluids and the external environment Excretory Processes • Most excretory systems produce urine by refining a filtrate derived from body fluids • Key functions of most excretory systems: • Filtration: pressure-filtering of body fluids • Reabsorption: reclaiming valuable solutes • Secretion: adding toxins and other solutes from the body fluids to the filtrate • Excretion: removing the filtrate from the system

  3. Capillary Filtration - Filter out the bad Excretory tubule Filtrate Reabsorption - Reclaim the goods • Secretion • Get rid of the bad that was missed • the first time Urine Excretion - Release the bad

  4. Survey of Excretory Systems • Systems that perform basic excretory functions vary widely among animal groups • They usually involve a complex network of tubules • Invertebrates: • Planarians have flame cells • Earthworm/Annelids have nephridia • Insects have Malpighian tubules

  5. Protonephridia: Flame-Bulb Systems • A protonephridium is a network of dead-end tubules lacking internal openings • The smallest branches of the network are capped by a cellular unit called a flame bulb/cell • These tubules excrete a dilute fluid and function in osmoregulation

  6. Cilia Interstitial fluid filters through membrane where cap cell and tubule cell interdigitate (interlock) Tubule cell Flame bulb Protonephridia (tubules) Tubule Nephridiopore in body wall

  7. Metanephridia • Each segment of an earthworm has a pair of open-ended metanephridia • Metanephridia consist of tubules that collect coelomic fluid and produce dilute urine for excretion

  8. Coelom Capillary network Bladder Collecting tubule Nephridiopore Nephrostome Metanephridium

  9. Malpighian Tubules • In insects and other terrestrial arthropods, Malpighian tubules remove nitrogenous wastes from hemolymph and function in osmoregulation • Insects produce a relatively dry waste matter, an important adaptation to terrestrial life

  10. Digestive tract Rectum Hindgut Intestine Midgut (stomach) Malpighian tubules Anus Feces and urine Salt, water, and nitrogenous wastes Malpighian tubule Rectum Reabsorption of H2O, ions, and valuable organic molecules HEMOLYMPH

  11. Vertebrate Kidneys • Kidneys, the excretory organs of vertebrates, function in both excretion and osmoregulation

  12. Concept 44.4: Nephrons and associated blood vessels are the functional unit of the mammalian kidney • The mammalian excretory system centers on paired kidneys, which are also the principal 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 called the ureter • Both ureters drain into a common urinary bladder where it is stored • Urine leaves the bladder through the urethra

  13. Posterior vena cava Renal artery and vein Kidney Renal medulla Aorta Renal cortex Ureter Renal pelvis Urinary bladder Urethra Ureter Excretory organs and major associated blood vessels Section of kidney from a rat Kidney structure Juxta- medullary nephron Cortical nephron Afferent arteriole from renal artery Glomerulus Bowman’s capsule Proximal tubule Peritubular capillaries Renal cortex Collecting duct SEM 20 µm Efferent arteriole from glomerulus Renal medulla Distal tubule To renal pelvis Collecting duct Branch of renal vein Descending limb Loop of Henle Nephron Ascending limb Vasa recta Filtrate and blood flow

  14. Structure and Function of the Nephron and Associated Structures • The mammalian kidney has two distinct regions: an outer renal cortex and an inner renal medulla

  15. The nephron, the functional unit of the vertebrate kidney, consists of a single long tubule and a ball of capillaries called the glomerulus which is cupped by the Bowman’s capsule

  16. Long tubule after the Bowman’s capsule consists of 4 major regions • Proximal convoluted tubule • Loop of Henle • Distal convoluted tubule • Collecting duct

  17. Blood Vessels Associated with the Nephrons • Each nephron is supplied with blood by an afferent arteriole, a branch of the renal artery that divides into the capillaries • The capillaries converge as they leave the glomerulus, forming an efferent arteriole • The vessels divide again, forming the peritubular capillaries, which surround the proximal and distal tubules

  18. Filtration of the Blood • Filtration occurs as blood pressure forces fluid from the blood in the glomerulus into the lumen of Bowman’s capsule • glomerulus & Bowman’s capsule make up the renal corpuscle • Fluid that enters the Bowman’s capsule in now called the filtrate • Rest of the blood enters the efferent arteriole and into the peritubular capillaries

  19. Pathway of the Filtrate • From Bowman’s capsule, the filtrate passes through three regions of the nephron: the proximal tubule, the loop of Henle, and the distal tubule • Fluid from several nephrons flows into a collecting duct

  20. From Blood Filtrate to Urine: A Closer Look • Filtrate becomes urine as it flows through the mammalian nephron and collecting duct • Secretion and reabsorption in the proximal tubule greatly alter the filtrate’s volume and composition

  21. From Blood Filtrate to Urine: A Closer Look • Filtrate becomes urine as it flows through the mammalian nephron and collecting duct • Secretion and reabsorption in the proximal tubule greatly alter the filtrate’s volume and composition • Reabsorption of water continues as filtrate moves into the descending limb of the loop of Henle • In the ascending limb of the loop of Henle, salt diffuses from the permeable tubule into the interstitial fluid

  22. The distal tubule regulates the K+ and NaCl concentrations of body fluids • The collecting duct carries filtrate through the medulla to the renal pelvis and reabsorbs NaCl

  23. Proximal tubule Distal tubule Nutrients NaCl H2O HCO3– K+ HCO3– H2O NaCl H+ H+ NH3 K+ CORTEX Descending limb of loop of Henle Thick segment of ascending limb Filtrate H2O Salts (NaCl and others) HCO3– H+ Urea Glucose; amino acids Some drugs NaCl H2O OUTER MEDULLA NaCl Thin segment of ascending limb Collecting duct Key Urea NaCl Active transport Passive transport H2O INNER MEDULLA

  24. Regulation of Kidney Function • The concentration of the urine is regulated by nervous and hormonal control of water and salt reabsorption in the kidneys • Antidiuretic hormone (ADH) increases water reabsorption in the distal tubules and collecting ducts of the kidney • An example of an ADH hormone is vasopressin

  25. Osmoreceptors in hypothalamus Thirst Hypothalamus Drinking reduces blood osmolarity to set point ADH Increased permeability Pituitary gland Distal tubule H2O reab- sorption helps prevent further osmolarity increase STIMULUS osmoreceptorcells in the hypothalamus detect anincrease in the osmolarity of the blood Collecting duct Homeostasis: Blood osmolarity

  26. Regulation of Kidney Function • The osmolarity of the urine is regulated by nervous and hormonal control of water and salt reabsorption in the kidneys • Antidiuretic hormone (ADH) increases water reabsorption in the distal tubules and collecting ducts of the kidney • An example of an ADH hormone is vasopressin • Lack of vasopressin is diabetes insipidus • Lots of dilute urine is produced • Drinking alcohol inhibits the release of vasopressin and mimics diabetes insipidus  frequent urination

  27. The renin-angiotensin-aldosterone system (RAAS) is part of a complex feedback circuit that functions in homeostasis • Purpose: Maintain blood volume and pressure

  28. Homeostasis: Blood pressure, volume Increased Na+ and H2O reab- sorption in distal tubules STIMULUS: The juxtaglomerular apparatus (JGA) responds to low blood volume or blood pressure (due to dehydration or loss of blood) Aldosterone Arteriole constriction Adrenal gland Angiotensin Distal tubule Angiotensinogen JGA Renin production Renin

  29. Another hormone, atrial natriuretic factor (ANF), opposes the RAAS • Inhibits renin secretion and thus the production of angiotensin, and stimulates aldosterone release. • Its effect is increased excretion of water and sodium and a lowering of blood pressure, which reduces the workload of the heart.

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