I. Metabolic Wastes Metabolic Waste: a) Carbon Dioxide: by-product of cellular respiration. b) Water: by-product of cellular respiration & dehydration synthesis reactions. c) Inorganic Salts: by-product of acid-base reactions. d) Ketones: by-product of fat (fatty acid) metabolism. e) Nitrogenous Wastes: by-product of the breakdown of amino & nucleic acids. Nitrogenous Wastes Deamination: Types of Nitrogenous Waste products: a) Ammonia: soluble, highly toxic base that must be diluted with copious amounts of water from circulatory fluid. Urination involves a great deal of water loss. Thus ammonia is only produced by aquatic organisms that can offset water loss by the uptake of water from the surrounding environment. b) Urea: soluble compound produced in the liver (NH 4 + CO 2 ). Is 100,000x s less toxic than ammonia & does not require as much water for dilution. Thus urea is produced by marine fishes, adult amphibians, & terrestrial mammals as a means of water conservation for life on land. A disadvantage of urea is that energy must be expended to produce it from ammonia. c) Uric Acid: least toxic of all nitrogenous wastes & require no water for dilution. Is produced by reptiles, birds, & insects as a semisolid paste is even more energetically costly to produce than urea. Figure 1: Nitrogenous Wastes
II. Human Excretion Excretion: Excretory systems produce urine by 3 major processes: filtration, secretion, & selective reabsorption. a) Filtration: body fluids are exposed to a filtering device that retains proteins & other large molecules in the body fluid. Blood pressure forces other small molecules (salts, sugars, amino acids, etc) through the device into the excretory system (filtrate). b) Secretion: passage of substances into filtrate across the tubule epithelium. Serves to regulate blood ph, electrolyte balance, & osmotic movement of water into the filtrate. c) Selective Reabsorption: essential substances are actively transported back to the body fluid from the filtrate. Excretory Organs Lungs: primary excretory function is to liberate CO 2 (via cell respiration), H 2 O vapor (via cell respiration) & ketones (via fatty acid catabolism). Sweat Glands: primary excretory function is the liberation of excess heat from metabolism. The evaporation of perspiration (water, salts, & urea) from the surface of the skin has an overall cooling effect on the body. Liver: functions in the deamination of nitrogenous compounds (e.g. amino acids, nitrogenous bases) to form urea, the major nitrogenous waste of mammals. Kidneys: major excretory organ. Filters blood of metabolic wastes (water, nitrogenous wastes, salts, ions, etc) to form urine. The act of urination, Diuresis, is the primary means by which wastes are excreted from the body. Kidney Anatomy & Physiology Nephron: a) Each nephron is supplied with blood by an Afferent Arteriole, a branch of the renal artery that subdivides into the capillaries of the glomerulus. The capillaries converge as they leave the glomerulus, forming the Efferent Arteriole. The efferent arteriole subdivides into an extensive capillary network to reabsorb essential materials. b) The capillaries derived directly from the efferent arteriole are the Peritubular Capillaries, which intermingle with the proximal & distal convoluted tubules of the nephron. Additional capillaries extend downward to form the Vasa Recta, the capillary system that serves the loop of Henle. c) The nephron & its surrounding capillary network do not exchange materials directly. The tubules & capillaries are immersed in intercellular fluid, through which substances can pass back & forth between the plasma in the capillaries & filtrate in the nephron. Figure 4: Urinary System & Kidney Anatomy
Stages of Urine Production Filtration: Secretion: Selective Reabsorption: From Blood Filtrate to Urine 1) Filtration occurs as blood pressure forces water, urea, salts, & other small solutes from the blood in the glomerulus into Bowman s capsule. The porous capillaries, along with cells called Podocytes, act as a filter, being permeable to water & small solutes but not to blood cells or plasma proteins. 2) From Bowman s capsule, filtrate (salts, glucose, vitamins, & urea) passes through the following regions: a) Proximal Tubule: most of the NaCl & water of the filtrate is reabsorbed. When blood H + concentrations become too high, it is secreted into the filtrate to prevent it from becoming too acidic, the tubular epithelium produces & secretes NH 3 into the filtrate. This region also functions in the active transport of nutrients such as glucose & amino acids into the intercellular fluid, & then into the blood within the peritubular capillaries. b) Descending Loop of Henle: permeable to water but not to salt. The ICF bathing the tubules is hyperosmotic to the filtrate, increasing in osmolarity from the renal cortex to the renal medulla. Consequently, water moves out of the tubule into the ICF where it is reabsorbed by the peritubular capillaries. Thus the filtrate within the tubule becomes more concentrated. c) Ascending Loop of Henle: is permeable to salt but not to water & consists of a thin & thick segment. As filtrate ascends the thin segment, NaCl, which became concentrated in the descending limb, diffuses out of the tubule into the ICF (contributes to its high osmolarity in the renal medulla). The loss of salt continues in the thick segment, resulting in the filtrate becoming more dilute as it ascends up the tubule toward the cortex. d) Distal Tubule: plays a key role in regulating the amount of K + & NaCl concentration of body fluids by varying the amount of K + that passes into the filtrate & the amount of NaCl that is reabsorbed. Like the proximal tubule, this region helps regulate blood ph by the controlled secretion of H + & reabsorption of bicarbonate. e) Collecting Duct: carries the filtrate back in the direction of the medulla & renal pelvis. Is permeable to water but not salt. Thus as the collecting duct traverses the gradient of osmolarity in the ICF, the filtrate loses more & more water by osmosis to the hyper osmotic fluid outside the duct this concentrates the urea in the filtrate. At the bottom of the collecting duct, the tubule is permeable to urea due to the high concentration of urea, some of it diffuses into the ICF (contributes to the osmolarity gradient from cortex to medulla). The end product is concentrated urine consisting of urea, salts, & water. Figure 5: Urine Production (Filtration, Secretion, Reabsorption)
IV. Renal Regulation Antidiuretic Hormone (ADH): a) Excessive water loss from sweating or diarrhea could cause an increase in blood osmolarity (solute concentration). This will lead to a corresponding decrease in blood volume as more water is retained in the filtrate within the nephrons. b) A decrease in blood volume may result in reduced blood pressure & cardiac output. In order to maintain these conditions, the hypothalamus (via the posterior pituitary) will release ADH (vasopression). c) This hormone will mainly target the collecting ducts of the nephrons, stimulating the accumulation of aquaporins to increase the recovery of water into the bloodstream. Consequently, less water is excreted with urine (leading to less frequent urination of more concentrated urine). d) In an example of negative feedback, the return of blood osmolarity within its normal range, indicating an increase in blood volume & pressure, reduces the activity of osmorecptor cells in the hypothalamus, thus reducing the amount of ADH secreted. e) When drinking large volumes of water, blood osmolarity decreases (leading to an increase in blood volume & pressure). This results in a reduction of the amount of ADH released by pituitary gland. Consequently, less water is reabsorbed by the capillaries from filtrate, resulting in the production of large volumes of dilute urine. As a result, urination is more frequent, a condition called diruresis. Figure 6: ADH (Vasopressin) Regulation
Renin-Angiostensin-Aldoesterone Pathway (RAAS) a) Renin intiates the conversion of the inactive plasma protein angiotensiongen to Angiotensin II, a hormone that increases blood pressure by constricting arterioles to decrease blood flow through capillaries, including those of the kidney (reduces amount of fluid lost from circulatory system as filtrate). Angiotensin II also stimulates proximal tubules to reabsorb more Na & water (increases blood volume). b) Angiotensin II also stimulates the adrenal glands to release the hormone Aldoesterone, stimulating the distal tubules to reabsorb more Na & water to increase blood volume/pressure. c) The hormone Atrial Natriuretic Factor (ANF), secreted by the heart, opposes RAAS when blood pressure becomes too high. ANF is stored in granules in atrial muscle cells when blood volume increases, the atria stretch & respond by releasing ANF into circulation. ANF inhibits rennin secretion from the JGA & aldosterone from the adrenal glands, thereby reducing Na & water reabsorption (reduces blood volume & pressure). Figure 6.1: RAAS