Osmoregulation & Excretion

• The processes involved in the formation of urine are summarized in this image.
• Nephrogenic diseases are diseases of the human urinary system.
• A mammalian nephron is intimately associated with a capillary network and forms urine by changing fluid composition inside a tubule.
• Mammalian nephrons function in both osmoregulation and excretion.
• Mammalian nephrons receive filtrate from blood instead of coelomic fluid.
• Mammalian nephrons have a transport epithelium.
• Unlike an earthworm's metanephridia, a mammalian nephron is not intimately associated with a coelomic fluid network.
• Increased blood osmolarity in humans can lead to increased permeability of the collecting duct to water, production of more dilute urine, release of ADH by the pituitary gland, increased thirst, and reduced urine production.
• The high osmolarity of the renal medulla is maintained by diffusion of salt from the thin segment of the ascending limb of the loop of Henle, active transport of salt from the upper region of the ascending limb, the spatial arrangement of juxtamedullary nephrons, diffusion of urea from the collecting duct, and diffusion of salt from the descending limb of the loop of Henle.
• Natural selection should favor the highest proportion of juxtamedullary nephrons in species with the highest water consumption.
• The process in the nephron that is least selective is reabsorption.
• African lungfish, which are often found in small stagnant pools of fresh water, produce urea as a nitrogenous waste.
• The osmolarity of body fluids is maintained at 300 mosm.
• Human beings accumulate urea before excreting nitrogenous waste.
• Flame cells are primitive excretory organs found in annelids.
• Mammals use uric acid crystals as an antioxidant in their cells.
• Excretory systems are central to homeostasis because they dispose of metabolic wastes and control body fluid composition by adjusting the rates of loss of particular solutes.
• Excretory systems regulate solute movement between internal fluids and the external environment.
• Key functions of most excretory systems are filtration, reabsorption, secretion, and excretion.
• Animals across a wide range of species produce a fluid waste called urine through the basic steps shown in Fig 10.
• Most excretory systems produce a filtrate by pressure filtering body fluids and then modify the filtrate's contents.
• The basic process of excretion involves body fluid collection, filtration, selective reabsorption, and excretion.
• While excretory systems are diverse, nearly all produce urine in a process that involves several steps.
• Body fluid (blood, coelomic fluid, or hemolymph) is collected in the initial fluid collection, usually involving filtration through selectively permeable membranes consisting of a single layer of transport epithelium.
• Hydrostatic pressure (blood pressure in many animals) forces water and small solutes into the excretory system, driving the process of filtration, this fluid is called the filtrate.
• The process of selective reabsorption recovers useful molecules and water from the filtrate and returns them to the body fluids.
• Valuable solutes including glucose, certain salts, vitamins, hormones, and amino acids are reabsorbed by active transport.
• Nonessential solutes and wastes are left in the filtrate or are added to it by selective secretion, which also occurs by active transport.
• The pumping of various solutes adjusts the osmotic movement of water into or out of the filtrate.
• Physiological systems of animals operate in a fluid environment.
• In contrast to the descending limb, the transport epithelium of the ascending limb of the loop of Henle is permeable to salt, not water.
• In the thin part of the ascending loop of Henle, Na + and Cl - ions diffuse into the interstitial fluid.
• In the thick part, these same ions are actively transported into the interstitial fluid.
• Because salt but not water is lost, the filtrate becomes more dilute as it travels up the limb of the loop of Henle.
• Among the vertebrates, only mammals and some birds have loops of Henle.
• In the distal convoluted tubule, K + and H + ions are selectively secreted into the filtrate, while Na +, Cl -, and HCO 3 - ions are reabsorbed to maintain pH and electrolyte balance in the blood.
• The distal tubule plays a key role in regulating the K + and NaCl concentration of body fluids.
• Like the proximal tubule, the distal tubule contributes to pH regulation by the controlled secretion of H + and reabsorption of HCO 3 -.
• The collecting duct carries the filtrate through the medulla to the renal pelvis and reabsorbs NaCl (actively transported out of filtrate).
• The filtrate flows from all of the collecting ducts of the kidney into the renal pelvis, which is drained by the ureter and stored into the urinary bladder.