1 Maintaining the Internal Environment Chapter 49

2 Outline Need to Maintain Homeostasis Antagonistic Effectors and Positive Feedback Osmolality and Osmotic Balance Osmoregulatory Organs Evolution of the Vertebrate Kidney The Mammalian Kidney Transport Processes in Mammalian Nephron Ammonia, Urea, and Uric Acid Hormones Control Homeostatic Functions

3 Need to Maintain Homeostasis Homeostasis may be defined as dynamic constancy of the internal environment. Negative feedback loops – The vertebrate body must have sensors to measure conditions of the internal environment.  information relayed to integrating center  When a deviation occurs, the integrating center sends a message to increase or decrease the activity of particular effectors.

4 Negative Feedback Loop

5 Need to Maintain Homeostasis Regulating body temperature – Hypothalamus responds to increased body temperature by promoting the dissipation of heat through sweating, dilation of blood vessels, and other mechanisms.  Coordinates a different set of responses such as shivering and blood vessel constriction for decreased body temperature.

6 Need to Maintain Homeostasis Regulating blood glucose – Glucose levels are constantly monitored by the islets of Langerhans in the pancreas.  When levels increase, the islets secrete insulin which stimulate blood glucose uptake.  In this case, the islets are the sensor and the integrating center.

7 Antagonistic Effectors and Positive Feedback Antagonistic effectors – Increasing activity of one effector is accompanied by decreasing activity of an antagonistic effector. Positive feedback loops – feedback loops that accentuate a disturbance  Deviations cause the effector to drive the value of the controlled variable even farther from the set point.

8 Positive Feedback During Childbirth

9 Osmolality and Osmotic Balance Osmolality and osmotic pressure – Because the total solute concentration of a solution determines its osmotic behavior, the total moles of solute per kilogram is expressed as osmolality of the solution. – Osmotic pressure of a solution is a measure of its tendency to take in water by osmosis.  hypertonic, hypotonic, isotonic

10 Osmolality and Osmotic Balance Osmoconformers and osmoregulators – osmoconformers - Osmolality of body fluid is same as that of surroundings.  no osmotic gradient  most marine invertebrates – osmoregulators - Maintain a relatively constant blood osmolality despite different concentrations in the surrounding environment.

11 Osmolality and Osmotic Balance Freshwater vertebrates are hypertonic to their environment. – Water tends to enter their bodies.  Must actively transport ions back into their bodies. Most marine vertebrates are hypotonic to their environment. – in danger of losing water by osmosis  drink seawater and eliminate excess ions through kidneys and gills

12 Osmoregulatory Organs In many animals, removal of water or salts is coupled with removal of metabolic wastes through the excretory system. – flatworms - flame cells – earthworms - nephridia

13 Osmoregulatory Organs Insects - Malpighian tubules – Create an excretory fluid by secreting K + into tubules.  Creates an osmotic gradient. Vertebrates - Kidneys create a fluid (urine) by filtration of the blood under pressure.

14 Malpighian Tubules of Insects

15 Evolution of the Vertebrate Kidney Kidney is made up of thousands of repeating units (nephrons), each with the structure of a bent tube. – Blood pressure forces the fluid in blood past a filter, glomerulus, at the top of each nephron.  Water and small molecules pass through filter and into the nephron tube.  Sugars and ions are removed by active transport.

16 Vertebrate Nephron Organization

17 Evolution of the Vertebrate Kidney Freshwater fish – Body fluids have greater osmotic concentration than surrounding water. – Water enters body from environment.  They do not drink water and excrete large amounts of dilute urine. – Solutes tend to leave the body.  reabsorb ions across nephron tubules

18 Evolution of the Vertebrate Kidney Marine bony fish – Body fluids are hypotonic to surrounding seawater.  Water tends to leave body via osmosis.  drink large amounts of seawater  Actively transport ions out of the blood across the gill surfaces.  Excrete urine isotonic to body fluids.

19 Evolution of the Vertebrate Kidney Cartilaginous fish – Reabsorb urea from nephron tubules and maintain a blood urea concentration 100 times higher than that of mammals.  Blood is approximately isotonic to surrounding sea.

20 Osmotic Problems

21 Evolution of the Vertebrate Kidney Amphibians and reptiles – Amphibian kidney is identical to that of freshwater fish. – Reptile kidneys are very diverse.  Marine species eliminate excess salt through salt glands.  Terrestrial reptiles reabsorb much of salt and water in nephron tubules.

22 Evolution of the Vertebrate Kidney Mammals and birds – Only vertebrates able to produce urine with a higher osmotic concentration than their body fluids.  Hypertonic urine accomplished by loop of Henle portion of the nephron. – Birds have relatively few or no loops, and thus cannot produce urine as concentrated as that in mammals.  Marine birds excrete excess salt from salt glands.

23 Ammonia, Urea, and Uric Acid When amino acids and nucleic acids are catabolized, they produce nitrogenous wastes that must be eliminated from the body. – First step is the removal of the amino (-NH 2 ) group and its combination with H + to form ammonia (NH 3 ) in the liver.  toxic to cells

24 Ammonia, Urea, and Uric Acid Elasmobranchs, adult amphibians, and mammals eliminate nitrogenous wastes in the form of urea. Reptiles, birds, and insects excrete nitrogenous wastes in the form of uric acid. – Most mammals have enzyme uricase which converts uric acid into a more soluble derivative, allantoin.

25 Nitrogenous Wastes

26 The Mammalian Kidney Each kidney receives blood from a renal artery, and produces urine. – Urine drains from each kidney through a ureter which carries urine to urinary bladder. – Within the kidney, mouth of ureter flares to form renal pelvis.  Divided into renal cortex and renal medulla.

27 Urinary System of a Human Female

28 The Mammalian Kidney Nephron structure and function – Blood is carried by an afferent arteriole to the glomerulus.  Blood is filtered as it is forced through porous capillary walls.  Glomerular filtrate enters Bowman’s capsule.  Moves to the proximal convoluted tubule.

29 The Mammalian Kidney Fluid then moves down the medulla and back into the cortex in a loop of Henle. – After leaving the loop, the fluid is delivered to a distal convoluted tubule in the cortex that drains to a collecting duct.  merges with other collecting ducts to empty its contents into the renal pelvis

30 Mammalian Kidney Nephron

31 The Mammalian Kidney Reabsorption and secretion – Most of the water and dissolved solutes that enter the glomerular filtrate must be returned to the blood. – Reabsorption of glucose and amino acids, is driven by active transport carriers. – Secretion of waste products involves transport across capillary membranes and kidney tubules. Excretion

32 Transport Processes in the Mammalian Nephron Some mechanism is needed to create an osmotic gradient between the glomerular filtrate and the blood, allowing reabsorption. Proximal convoluted tubule – Approximately two-thirds of NaCl and water filtered in Bowman’s capsule is immediately reabsorbed across the walls of the proximal convoluted tube.

33 Transport Processes in the Mammalian Nephron Loop of Henle – Descending limb is permeable to water, thus water leaves via osmosis. – Water loss in the descending limb multiples concentration achieved at each loop. – Ascending limb actively extrudes N + and Cl follows. – NaCl pumped out of ascending limb is trapped within surrounding interstitial fluid. – countercurrent multiplier system

34 Transport Processes in the Mammalian Nephron Distal tubule and collecting duct – Permeability of the collecting duct to water is adjusted by antidiuretic hormone (ADH - vasopressin).  Kidneys also regulate the balance of electrolytes in the blood by reabsorption and secretion.

35 Reabsorption of Salt and Water

36 Hormones Control Homeostatic Functions Antidiuretic hormone – Stimulates reabsorption of water by the kidneys.

37 Hormones Control Homeostatic Functions Aldosterone – Promotes reabsorption of NaCl and water across the distal convoluted tubule and the secretion of K + into the tubule. Atrial natriuretic hormone – decreases NaCl reabsorption

38 Summary Need to Maintain Homeostasis Antagonistic Effectors and Positive Feedback Osmolality and Osmotic Balance Osmoregulatory Organs Evolution of the Vertebrate Kidney The Mammalian Kidney Transport Processes in Mammalian Nephron Ammonia, Urea, and Uric Acid Hormones Control Homeostatic Functions

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