Controlling the Internal Environment Chapter 40

The Big Picture The excretory system is a regulatory system that helps to maintain homeostasis within the body The excretory system is a regulatory system that helps to maintain homeostasis within the body Homeostasis = the dynamic constancy of the internal environment Homeostasis = the dynamic constancy of the internal environment The excretory system eliminates wastes and functions in osmoregulation. The excretory system eliminates wastes and functions in osmoregulation. Osmoregulation= the control of salt and water balance Osmoregulation= the control of salt and water balance

Osmoconformers vs. Osmoregulators Osmoconformers Osmoconformers –Do not actively adjust their internal osmolarity Osmoregulators Osmoregulators –Animals whose body fluids are not isotonic with the external environment must manipulate solute concentrations in their body fluids Which one are we?? Which one are we??

Passive vs. Active Transport Passive transport = does not require an input of energy by the cell Passive transport = does not require an input of energy by the cell Active transport = requires an input of energy by the cell Active transport = requires an input of energy by the cell

The Vertebrate Excretory System Kidneys, ureters, urinary bladder, urethra, renal artery, renal vein Kidneys, ureters, urinary bladder, urethra, renal artery, renal vein Also… Also… –Lungs: excrete carbon dioxide –Skin: excretes water, salts, and a small amount of urea (in sweat)

Anatomy of the Vertebrate Excretory System Kidneys = bean-shaped organs, about the size of your fist in humans Kidneys = bean-shaped organs, about the size of your fist in humans Blood enters the kidney through the renal artery, and exits the kidney through the renal vein Blood enters the kidney through the renal artery, and exits the kidney through the renal vein Urine exits the kidney through the ureter Urine exits the kidney through the ureter The ureter from each kidney goes to the urinary bladder The ureter from each kidney goes to the urinary bladder Urine leaves the body through the urethra Urine leaves the body through the urethra

Human Excretory System

Blood Supply to the Kidney Into the kidney: Into the kidney: –Renal artery to afferent arteriole to capillaries of the glomerulus Capillaries converge into efferent arteriole, which subdivides into peritubular capillaries Capillaries converge into efferent arteriole, which subdivides into peritubular capillaries Vasa recta = looped capillary system that serves the loop of Henle Vasa recta = looped capillary system that serves the loop of Henle Out of the Kidney: renal vein Out of the Kidney: renal vein Kidneys receive ~20% of the blood pumped with each heartbeat Kidneys receive ~20% of the blood pumped with each heartbeat

Kidney Structure Divided into renal cortex (outer portion) and renal medulla (inner portion) Divided into renal cortex (outer portion) and renal medulla (inner portion) Renal pelvis: where the ureter meets the kidney Renal pelvis: where the ureter meets the kidney

The Nephron The functional unit of the kidney (ie. The smallest part of the kidney that can still perform all the tasks associated with it) The functional unit of the kidney (ie. The smallest part of the kidney that can still perform all the tasks associated with it) There are approximately 1 million nephrons in each kidney There are approximately 1 million nephrons in each kidney Nephron consists of a single long tubule and a ball of capillaries (glomerulus) Nephron consists of a single long tubule and a ball of capillaries (glomerulus) Made up of Bowman’s Capsule, Proximal Tubule, Loop of Henle, Distal Tubule, (Collecting Duct) Made up of Bowman’s Capsule, Proximal Tubule, Loop of Henle, Distal Tubule, (Collecting Duct)

The Nephron

80% of nephrons are cortical nephrons (located in the renal cortex) 80% of nephrons are cortical nephrons (located in the renal cortex) –Have reduced loops of Henle 20% of nephrons are juxtamedullary nephrons (extend into renal medulla) 20% of nephrons are juxtamedullary nephrons (extend into renal medulla) –Have well-developed loops of Henle –Allow for the conservation of water

The Nephron: An Overview

Regulation of Blood Composition Filtration: Filtration: –Blood pressure forces fluid from the capillaries of the glomerulus into Bowman’s Capsule Capillaries function as a filter: permeable to water and small solutes but not to blood cells or large molecules like plasma proteins Capillaries function as a filter: permeable to water and small solutes but not to blood cells or large molecules like plasma proteins Nonselective with regard to small molecules Nonselective with regard to small molecules

Regulation of Blood Composition Secretion: Secretion: –Substances are transported into the tubule from the surrounding interstitial fluid (ISF) –Occurs mostly in the proximal and distal tubules –Net effect is the addition of plasma solutes to the filtrate within the tubule –A very selective process involving both passive and active transport –Responsible for maintenance of a constant pH level (secretion of H+ ions)

Regulation of Blood Composition Reabsorption: Reabsorption: –The selective transport of substances across the epithelium of the excretory tubule from the filtrate to the ISF –Because filtration is nonselective, it’s important that small molecules essential to the body be returned to the ISF and blood plasma –Occurs in the proximal tubule, loop of Henle, distal tubule, and collecting duct –Involves both passive and active transport

Bowman’s Capsule Tube of nephron that surrounds the glomerulus, which is a ball of capillaries Tube of nephron that surrounds the glomerulus, which is a ball of capillaries Blood pressure forces water, urea, salts and other small solutes from blood in the glomerulus into Bowman’s Capsule; this fluid is now called filtrate Blood pressure forces water, urea, salts and other small solutes from blood in the glomerulus into Bowman’s Capsule; this fluid is now called filtrate

Proximal Tubule Reabsorption and secretion Reabsorption and secretion Reabsorption: Reabsorption: –Nutrients (glucose, amino acids) –Water* –NaCl* Secretion: Secretion: –Drugs and other poisons from the liver –Ammonia Maintains a constant pH: H+ and HCO 3 Maintains a constant pH: H+ and HCO 3

Descending Loop of Henle Permeable to water but not to salt Permeable to water but not to salt ISF is hypertonic to filtrate, and water moves out of the tubule by osmosis ISF is hypertonic to filtrate, and water moves out of the tubule by osmosis NaCl becomes more and more concentrated in the tubule NaCl becomes more and more concentrated in the tubule

Ascending Loop of Henle Permeable to salt, but impermeable to water Permeable to salt, but impermeable to water Two regions: Two regions: –Thin: NaCl diffuses out into the ISF –Thick: NaCl is actively transported into the ISF Result: filtrate becomes more and more dilute because it loses salt without losing water (ie. salt concentration decreases) Result: filtrate becomes more and more dilute because it loses salt without losing water (ie. salt concentration decreases)

Distal Tubule Site of selective secretion and absorption Site of selective secretion and absorption –Secretion: K+ K+ –Reabsorption: Water Water NaCl NaCl Regulates pH levels Regulates pH levels –Secretion of H+ –Reabsorption of HCO 3 -

Collecting Duct Shared by many nephrons Shared by many nephrons Carries the filtrate back towards the medulla and renal pelvis Carries the filtrate back towards the medulla and renal pelvis Permeable to water but impermeable to salt Permeable to water but impermeable to salt Filtrate becomes more and more concentrated again as water is lost by osmosis to the hypertonic ISF Filtrate becomes more and more concentrated again as water is lost by osmosis to the hypertonic ISF

The Nephron

An Overview of Kidney Function

Why do animals have to get rid of nitrogen? The metabolism of proteins and nucleic acids produces toxic by-products ammonia The metabolism of proteins and nucleic acids produces toxic by-products ammonia Ammonia: Ammonia: –a small and very toxic molecule –can be transported and excreted only in a very dilute solution Many animals convert ammonia to urea or uric acid, which are much less toxic, but require ATP to produce Many animals convert ammonia to urea or uric acid, which are much less toxic, but require ATP to produce