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CHAPTER 44 Osmoregulation and Excretion 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 Capillary Filtrate Excretory tubule Filtration - Filter out the bad Reabsorption - Reclaim the goods Secretion -Get rid of the bad that was missed the first time Urine Excretion - Release the bad 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 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 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 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 Coelom Capillary network Bladder Collecting tubule Nephridiopore Nephrostome Metanephridium 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 Digestive tract Rectum Intestine Hindgut Midgut (stomach) Malpighian tubules Feces and urine Anus Salt, water, and nitrogenous wastes Malpighian tubule Rectum Reabsorption of H2O, ions, and valuable organic molecules HEMOLYMPH VERTEBRATE KIDNEYS Kidneys, the excretory organs of vertebrates, function in both excretion and osmoregulation 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 Posterior vena cava Renal artery and vein Kidney Renal medulla Renal cortex Renal pelvis Aorta Ureter Urinary bladder Urethra Ureter Excretory organs and major associated blood vessels JuxtaCortical medullary nephron nephron Afferent arteriole Glomerulus from renal Bowman’s capsule artery Proximal tubule Peritubular capillaries Renal cortex Collecting duct 20 µm Renal medulla To renal pelvis Nephron Section of kidney from a rat Kidney structure SEM Efferent arteriole from glomerulus Distal tubule Collecting duct Branch of renal vein Descending Loop limb of Henle Ascending limb Vasa recta Filtrate and blood flow 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 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 LONG TUBULE AFTER THE BOWMAN’S CAPSULE CONSISTS OF 4 MAJOR REGIONS Proximal convoluted tubule Loop of Henle Distal convoluted tubule Collecting duct 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 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 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 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 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 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 Proximal tubule Distal tubule NaCl Nutrients HCO3– H2O K+ H2 O H+ NH3 NaCl K+ HCO3– H+ CORTEX Descending limb of loop of Henle Filtrate H2O Salts (NaCl and others) HCO3– H+ Urea Glucose; amino acids Some drugs OUTER MEDULLA H2 O Thick segment of ascending limb NaCl NaCl Thin segment of ascending limb Key Active transport Passive transport Collecting duct Urea NaCl INNER MEDULLA H2 O 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 Osmoreceptors in hypothalamus Hypothalamus Thirst Drinking reduces blood osmolarity to set point ADH Pituitary gland Increased permeability Distal tubule STIMULUS osmoreceptor cells in the hypothalamus detect an increase in the osmolarity of the blood H2O reabsorption helps prevent further osmolarity increase Collecting duct Homeostasis: Blood osmolarity 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 The renin-angiotensin-aldosterone system (RAAS) is part of a complex feedback circuit that functions in homeostasis Purpose: Maintain blood volume and pressure Homeostasis: Blood pressure, volume Increased Na+ and H2O reabsorption 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 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.