Essentials of the Living World Second Edition George B. Johnson Jonathan B. Losos Chapter 28 Maintaining the Internal Environment Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

28.1 How the Animal Body Maintains Homeostasis Homeostasis is defined as the dynamic constancy of the internal environment  conditions fluctuate continuously within narrow limits  most of the regulatory mechanisms of the vertebrate body that are not devoted to reproduction are concerned with maintaining homeostasis

28.1 How the Animal Body Maintains Homeostasis To maintain internal constancy, the vertebrate body needs  sensors that are able to measure each condition of the internal environment  an integrating center that contains the set point, or proper value for a particular internal condition  effectors are generally muscles or glands that can change the value of the condition back toward the set point

28.1 How the Animal Body Maintains Homeostasis The integrating center is often a particular region of the brain or spinal cord, but could also be cells of endocrine glands  it receives messages from several sensors and then determines if the condition is deviating from the set point  it sends a message to the certain effectors to either decrease or increase their activity the activity of the effectors is influenced by the effects they produce in a negative feedback loop

Figure 28.1 A generalized diagram of a negative feedback loop

28.1 How the Animal Body Maintains Homeostasis Examples of negative feedback loops in homeostasis  regulating body temperature humans, as well as mammals and birds, are endothermic –this means that they can maintain relatively constant body temperature other vertebrates are ectothermic, meaning their body temperatures depend more or less on the environmental temperature –but they can modify their behavior to affect body temperature  regulating blood glucose

Figure 28.2 Control of blood glucose levels

28.2 Regulating the Body’s Water Content Animals use various mechanisms for osmoregulation, the regulation of the body’s osmotic composition  this refers to how much water and salt the body contains  the proper operation of many vertebrate organ systems of the body requires that the osmotic concentration of the blood be kept within narrow bounds

28.2 Regulating the Body’s Water Content In many animals, the removal of water and salts from the body is coupled with the removal of metabolic wastes through the excretory system  for example, protists, like Paramecium, employ contractile vacuoles water and metabolic wastes are collected by endoplasmic reticula that connect to feeder canals, which lead to the vacuole the water and wastes are then expelled through a pore

Figure 28.3 Contractile vacuoles in Paramecium

28.2 Regulating the Body’s Water Content Flatworms employ a system of excretory tubules, called protonephridia, to expel fluids and wastes from the body  the protonephridia branch throughout the body into bulblike flame cells, which open to the outside of the body only the beating action of cilia within the flame cells draws in fluid from the body, which is passed into a collecting tube water and metabolites are reabsorbed, while the wastes are expelled through excretory pores

Figure 28.4 The protonephridia of flatworms

28.2 Regulating the Body’s Water Content Other invertebrates have a system of tubules that open both to the inside and to the outside of the body  in annelids, these tubules are called nephridia the nephridia obtain fluid from the body cavity through a process of filtration into funnel-shaped structures called nephrostomes the filtration process excludes, under pressure, particles large than a certain size as the fluid passes through the nephridia, NaCl is removed by active transport in the process of reabsorption the end result is that the urine excreted is more dilute (hypotonic) than the body fluids

Figure 28.5 The nephridia of annelids

28.2 Regulating the Body’s Water Content The excretory organs in insects are called Malpighian tubules, which extend are extensions of the digestive tract  urine is not formed by filtration but, instead waste molecules and K + are added to the tubules by secretion  the secretion of the K + creates an osmotic gradient that causes water to enter the tubules by osmosis  most of the water and K + is then reabsorbed into the circulatory system via the epithelium of the hindgut

Figure 28.6 The Malpighian tubules of insects Kidneys are the excretory organs in vertebrates  kidneys created a tubular fluid by filtration  the filtrate contains many valuable nutrients in addition to waste products selective reabsorption ensures that these nutrients and water are reabsorbed into the blood, while wastes remain in the filtrate

28.3 Evolution of the Vertebrate Kidney The kidney is a complex organ made up of many, many units called nephrons  blood pressure forces the fluid in the blood through a capillary bed at the top of each nephron, called a glomerulus the glomerulus excludes blood cells, proteins, and other large molecules from the filtrate  the remainder of the nephron tube reabsorbs anything else useful from the filtrate

Figure 28.7 Basic organization of the vertebrate nephron

28.3 Evolution of the Vertebrate Kidney Because the original glomerular filtrate is isotonic blood, all vertebrates can produce a urine that is  isotonic to blood by reabsorbing ions  hypotonic to blood by making the urine more dilute Only birds and mammals can reabsorb water from the glomerular filtrate to produce a urine that is hypertonic (more concentrated than) blood

28.3 Evolution of the Vertebrate Kidney Kidneys are thought to have evolved first among the freshwater fish  because the body fluids of a freshwater fish have a greater osmotic concentration than the surround water, these animals face two serious problems water tends to enter the body from the environment solutes tend to leave the body and enter the environment

28.3 Evolution of the Vertebrate Kidney Freshwater fish address these problems by  not drinking water  excreting a large volume of dilute urine  reabsorbing ions (mainly NaCl) across the nephron tubule from the glomerular filtrate  actively transporting NaCl across the gills from the surrounding water into the blood

28.3 Evolution of the Vertebrate Kidney Marine fish probably evolved from freshwater ancestors  because their bodies are hypotonic to the surrounding seawater, they faced problems in that water tends to leave their bodies through osmosis across the gills they lose water in their urine  to compensate, marine fish drink lots of seawater they excrete isotonic urine

Figure 28.8 Freshwater and marine teleosts (bony fish) face different osmotic problems

28.3 Evolution of the Vertebrate Kidney Elasmobranchs are the most common subclass of cartilaginous fish  they solve their osmotic problem posed by their seawater environment by reabsorbing urea from the nephron tubules  this elevates the osmotic concentration in the blood so that they do not have to continually drink seawater the blood is approximately isotonic to the surrounding sea

Figure 28.9 Osmoregulation in elasmobranchs

28.3 Evolution of the Vertebrate Kidney The amphibian kidney is identical to that of freshwater fish  amphibians produce a very dilute urine and actively transport Na + across their skin The kidneys of terrestrial reptiles absorb much salt and water in the nephron tubules  their urine is still hypotonic but they can absorb additional water in the cloaca

28.3 Evolution of the Vertebrate Kidney Because mammals and birds can produce hypertonic urine, they can excrete their waste products in a small volume of water  the production of the hypertonic urine is possible due to a looped portion of the nephron, called the Loop of Henle  marine birds additionally drink sea water and excrete excess salt through salt glands

Figure Osmoregulation by some vertebrates

Figure Marine birds drink seawater and then excrete the salt through the salt glands

28.4 The Mammalian Kidney Each kidney receives blood from a renal artery, and it is from this blood that urine is produced  urine drains from each kidney through a ureter  the ureters carry urine to a urinary bladder  urine passes out of the body through the urethra

Figure (a) The mammalian urinary system contains two kidneys, each of which contain about a million nephrons that lie in the renal cortex and renal medulla

28.4 The Mammalian Kidney Within the kidney, the mouth of the ureter flares open to form a funnel-like renal pelvis  the renal pelvis has cup-like extensions that receive urine from the renal tissue  the renal tissue is divided into an outer renal cortex an inner renal medulla

Figure (b) The mammalian urinary system contains two kidneys, each of which contain about a million nephrons that lie in the renal cortex and renal medulla

28.4 The Mammalian Kidney The mammalian kidney is comprised of roughly1 million nephrons, each of which is composed of three regions  filter the filtration device at the top of each nephron is called the Bowman’s capsule which receives filtrate from the glomerular capillaries  tube the Bowman’s capsule is connected to a long renal tubule, which includes the Loop of Henle, that acts as a reabsorption device  duct the renal tubule empties into a collecting duct that operates as a water conservation device

28.4 The Mammalian Kidney There are five steps involved in the formation of urine in the kidney 1.pressure filtration 2.reabsorption of water 3.selective reabsorption 4.tubular secretion 5.further reabsorption of water

28.4 The Mammalian Kidney Pressure filtration  occurs when, driven by blood pressure, small molecules are pushed across the thin walls of the glomerulus into the Bowman’s capsule  large particles, such as blood cells and proteins, are excluded from the filtrate  the filtrate contains water, nitrogenous wastes (mostly urea), nutrients (principally glucose and amino acids), and a variety of ions

28.4 The Mammalian Kidney Reabsorption of water  the descending arm of the Loop of Henle is permeable to the passage of water, but impermeable to salts and urea  this leaves a more concentrated filtrate to go to the ascending arm of the Loop of Henle

28.4 The Mammalian Kidney Selective reabsorption  as the filtrate passes up the ascending arm of the Loop of Henle, the walls become permeable to salts which pass out into the surrounding tissues and enter the blood  in the upper regions of the ascending arm of the Loop of Henle, active transport channels pump out more salt  the filtrate is at this point mostly concentrated urea

28.4 The Mammalian Kidney Tubular secretion  this also occurs in the ascending arm of the Loop of Henle and involves active transport of other nitrogenous wastes, such as uric acid and ammonia, as well as excess hydrogen ions

28.4 The Mammalian Kidney Further reabsorption of water  in the collecting duct, the lower reaches are permeable to urea, which exits to the surrounding tissues  this makes water even more likely to leave

Figure (c) The mammalian urinary system contains two kidneys, each of which contain about a million nephrons that lie in the renal cortex and renal medulla

28.5 Eliminating Nitrogenous Wastes Amino acids and nucleic acids are nitrogen-containing molecules When animals metabolize these substances, they produce nitrogen- containing by-products, called nitrogenous wastes, that must be eliminated by the body

28.5 Eliminating Nitrogenous Wastes The first step in the metabolism of amino acids and nucleic acids is the removal of the amino (--NH 2 group)  this group is then combined with H + to form ammonia (NH 3 )  this takes place in the liver

28.5 Eliminating Nitrogenous Wastes Ammonia is quite toxic and is safe only in very dilute concentrations  for fish and tadpoles, ammonia can be directly eliminated across the gills or excreted in dilute urine  in sharks, adult amphibians, and mammals, the nitrogenous waste is eliminated as urea, which is less toxic  reptiles, birds, and insects excrete nitrogenous wastes in the form of uric acid, which can be excreted with very little water

Figure Nitrogenous wastes

28.5 Eliminating Nitrogenous Wastes Mammals also make some uric acid but it is a waste product of the breakdown of nucleotides  most mammals have an enzyme, uricase, that converts the uric acid into a more soluble form called allantoin  humans, apes, and dalmation dogs lack this enzyme and must excrete the uric acid in humans, an excessive accumulation of uric acid is called gout

Inquiry & Analysis How does the oxygen consumption of awake hummingbirds change as air temperature falls? How does the oxygen consumption of sleeping torpid hummingbirds change as air temperature falls? Graph of Effect of Temperature on O 2 Consumption