1. Maintaining the Internal Environment
Chapter 32
Maintaining the Internal Environment
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2. 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

3. 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

4. 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

5. A generalized diagram of a negative feedback loop

6. 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

7. Control of blood glucose levels

8. 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

9. 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

10. 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

11. Basic organization of the vertebrate nephron

12. Vertebrate Kidney Function
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

13. Evolution of the Vertebrate Kidney
Kidneys are thought to have evolved first among the freshwater fish
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

14. The Vertebrate Kidney - Fish
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

15. Fish Kidneys 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

16. Freshwater and marine teleosts (bony fish) face different osmotic problems

17. Sharks’ (and their realtives’)Kidneys
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

18. Figure 32.9 Osmoregulation in elasmobranchs

19. 32.3 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

20. Other Vertebrate Kidneys
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

21. Osmoregulation by some vertebrates

22. Marine birds drink seawater and then excrete the salt through the salt glands

23. Functions of the Mammalian Urinary System
Removal of metabolic wastes (especially nitrogenous wastes e.g. urea & uric acid).
Water balance (and therefore blood pressure).
Control of electrolyte balance.
Control of pH.
Removal of toxins.

24. 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

25. The mammalian urinary system contains two kidneys, each of which contain about a million nephrons within the renal cortex and renal medulla

26. 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

27. About 25% of your cardiac output flows through your kidneys each minute!

28. The Nephron: functional unit of the kidney
Interlobular artery
Afferent Arteriole
Glomeruli

29. Urine formation
The mammalian nephron is composed of three regions. Each region has specific roles.
Filtration
the filtration device at the top of each nephron is called the Bowman’s capsule which receives filtrate from the glomerular capillaries
Tubular reabsorption and secretion
the Bowman’s capsule is connected to a long renal tubule, which includes the Loop of Henle, that acts as a reabsorption device
Water and pH balance
the renal tubule empties into a collecting duct that operates as a water conservation device

30. Formation of urine in the kidney
Filtration (glomerulus)
Tubular reabsorption (kidney tubules)
Tubular secretion (kidney tubules)
Water conservation (collecting system)

31. 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

32. Filtration pressures: NFP must be positive for U2P

33. The filtration membrane

34. Tubular reabsorption
Reabsorption of filtered solutes occurs in the Proximal Convoluted Tubules.
Reabsorbed substances:
glucose
amino acids
water
vitamins
fatty acids
urea
electrolytes (Na, K, HCO3- etc.)

35. Reabsorption of water
The “thin” segment of 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

36. 32.4 The Mammalian Kidney NaCl (salt) reabsorption
as the filtrate passes up the ascending arm of the Loop of Henle, the walls become thicker and permeable to salts (but not to water) 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.

37. 32.4 The Mammalian Kidney Tubular secretion
Mostly occurs in the “distal convoluted tubule” and the collecting duct. This involves active transport of other nitrogenous wastes, such as uric acid and ammonia, as well as excess hydrogen ions

38. 32.4 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

40. 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

41. The first step in the metabolism of amino acids and nucleic acids is the removal of the amino (--NH2 group)
this group is then combined with H+ to form ammonia (NH3)
this takes place in the liver

42. Ammonia is toxic
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

43. Figure 32.14 Nitrogenous wastes

44. How mammals do it!
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

45. Clinical Diseases that are harmful to the urinary system: Hypertension
Diabetes mellitus
Liver dysfunction
Congestive heart failure
Hormone imbalances
These can all lead to renal failure! Some can also result in …

46. Kidney stones
Renal Calculi

47. “Well Mr. Osborne, I don’t think that it’s kidney stone after all”