Maintaining the Internal Environment

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 (-NH2) group and its combination with H+ to form ammonia (NH3) 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 convolutedtubule 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