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Excretion Chapter 35 Excretion • Excretion – removal of cellular (metabolic) waste products from an organism • Metabolic wastes includes: – Carbon dioxide & water (respiration) – Water (Dehydration synthesis) – Mineral salts (Metabolic processes) – Nitrogenous wastes – (Protein metabolism) – Heat – (Metabolic processes) • Metabolic wastes may be toxic or nontoxic Nitrogenous Wastes • Formed by process of deamination. • Deamination – removal of the amine group from a protein • Amine group converted into: – Ammonia – highly toxic, very soluble. Requires large amounts of water for removal – Urea – less toxic, requires less water for removal – Uric acid – harmless, water soluble, crystalline solid. Requires very little or no water for removal Types of Excretion • Protozoa – simple diffusion out to environment • Plants – recycle gases (O2 & CO2), wall off toxics in vacuoles • Hydra – similar to Protozoa • Earthworms – pairs of nephridia excrete water, salts, & urea • Nephridia filter the blood & interstitial fluids Types of Excretion • Insects – Use Malpighian tubules to filter hemocoel • Excrete uric acid & mineral salts w/ digestive wastes • Uric acid is a water –conserving mechanism developed in terrestrial organisms Major Excretory Organs • Lungs – remove respiratory gas, some heat & some water vapor • Skin – remove heat, some water, very little salts & urea • Remove heat by evaporative cooling • Evaporative cooling – use heat energy to evaporate water off the skin surface Major Excretory Organs • Liver – converts ammonia into urea, converts hemoglobin into bile, removes toxins from the blood (Alcohol, some drugs) • Urea – ammonia + CO2 • Urea released back into the blood • Removed by urinary system Urinary System • The urinary system maintains the chemical composition of the blood and extracellular fluid – Important for maintaining cellular metabolism • Urinary system functions are performed through three basic processes 1. Blood (or equivalent fluid) is filtered, removing water and small dissolved substances 2. Nutrients are selectively reabsorbed from filtrate 3. Any remaining water and dissolved wastes are excreted from the body The Human Urinary System • Several structures make up the human urinary system – Kidneys – Ureters – Bladder – Urethra The Kidneys • Paired organs located on either side of the spinal column, just above the waist • Blood enters the kidneys by renal arteries • Blood leaves the kidneys by renal veins The Ureters • Urine flows from each kidney into a muscular tube, called a ureter • Rhythmic contractions of the ureter transports urine to the bladder The Bladder • The ureters deposit urine in a hollow, muscular chamber called the bladder • The bladder wall is lined with smooth muscle and is expandable – Accommodates about 500 milliliters of urine The Bladder • Urine is contained within the bladder by sphincter muscles at its base – Internal sphincter: closest to bladder; under involuntary control – External sphincter: beneath internal sphincter; under voluntary control – When open, allow urine to flow into the urethra The Bladder • When the bladder becomes distended, the urination reflex is initiated – The bladder contracts – The internal sphincter opens involuntarily – The external sphincter opens voluntarily – Urine flows into the urethra The Urethra • A single tube that extends from the base of the bladder to the external environment – In males, it is about 8 inches long – In females, it is about 1.5 inches long The Anatomy of the Kidney • Made of an outer layer of tissue (renal cortex) overlying deeper tissues (renal medulla) • The subdivided inner chamber is called the renal pelvis – Collects urine and directs it into the ureter • The tissues of each kidney are made up of more than 1 million microscopic tubules called nephrons Urine Is Formed in the Nephrons • Each nephron has three major parts – Bowman’s capsule: cuplike structure that contains a capillary glomerulus; blood is filtered here – Tubule: long twisted tube; composed of the proximal tubule, loop of Henle, and distal tubule – Collecting ducts: collect fluid from many nephrons and deposits it in the renal pelvis Filtration in the Glomerulus • The kidneys receive more than one liter of blood per minute • Blood flows from the renal artery, which branches • An arteriole directs blood to each nephron • The arteriole branches within the Bowman’s capsule into a capillary tuft, called the glomerulus Filtration in the Glomerulus • Glomerular capillary walls are permeable to water and small dissolved molecules – Large molecules and cells cannot pass • Blood pressure drives filtration of water and small dissolved substances from blood through glomerular walls – Filtrate is deposited in the Bowman’s capsule Filtration in the Glomerulus • Blood remaining in the glomerulus contains 20% less fluid • Blood leaves the glomerulus by an outgoing arteriole • The arteriole branches to form porous capillaries that intertwine the nephron tubule – These capillaries can reacquire nutrients from the nephron filtrate Urine Formation • Filtrate in the Bowman’s capsule contains water, wastes, and essential nutrients • As filtrate flows through the nephron tubule, urine is formed by 2 processes – Tubular reabsorption – Tubular secretion Tubular Reabsorption • Occurs primarily in the proximal tubule, although water and other nutrients are also reabsorbed in other tubule areas • Tubule cells actively transport many nutrients – Examples: salts, amino acids, glucose • Water follows nutrients by osmosis – 99% of water reabsorbed from filtrate Tubular Secretion • Occurs primarily in the distal tubule • Tubule cells actively transport wastes and excess substances from blood into filtrate – Examples: hydrogen and potassium ions, ammonia, and many drugs The Loop of Henle • The loop of Henle allows urine to become concentrated • Mammal and bird urine has a higher concentration of dissolved materials than the blood – Due to the actions of the nephron tubule and collecting duct • The loop of Henle (in the renal medulla) forms a salt and urea concentration gradient around the nephron tubule and collecting duct The Loop of Henle • The salt and urea gradient causes an osmotic gradient between the filtrate and surrounding fluids • The longer the loop, the stronger the gradient • The most concentrated fluid surrounds the bottom of the loop The Loop of Henle • As filtrate descends into the loop of Henle and collecting duct, – It is exposed to the osmotic gradient surrounding the nephron – Water leaves the filtrate (by osmosis) and enters the surrounding capillaries • Filtrate becomes urine when it enters the collecting duct – Urine can be more than four times as concentrated as the blood Role of Kidney in Homeostasis • One drop of blood passes through a kidney about 35 times per day • Each time, the kidneys fine-tune blood composition and help maintain homeostasis • Regulating blood water content • Regulating blood pressure and oxygen levels • Monitoring and regulating dissolved substances in blood Water Content of the Blood • If the kidneys did not reabsorb water, 50 gallons of urine would be produced daily • The ability of kidneys to reabsorb water is under the influence of antidiuretic hormone (ADH) – Produced by the posterior pituitary – Makes the distal tubule and collecting ducts more permeable to water Water Content of the Blood • ADH release is controlled by negative feedback – ADH is released when the hypothalamus detects the increased blood osmotic concentration (dehydration) – ADH is released into the blood from the posterior pituitary – ADH increases the permeability of the distal tubule and collecting duct to water – More water is reabsorbed from urine Blood Pressure • When blood pressure falls, the kidneys release the hormone renin into the bloodstream • Renin catalyzes the formation of the hormone angiotensin in the blood • Angiotensin causes arterioles constriction, elevating blood pressure • Kidney arteriole constriction reduces filtration – Results in less water removed from the blood and adding to increase in blood pressure Oxygen Levels • When blood oxygen levels are low, the kidneys release the hormone erythropoietin – Stimulates bone marrow to make more red blood cells – More red blood cells increase the oxygen carrying capacity of the blood Dissolved Substances • The kidneys monitor blood composition during filtration • Tubular reabsorption and secretion rates are adjusted to maintain blood homeostasis Dissolved Substances • Substances the kidneys monitor include – Glucose – Amino acids – Vitamins – Urea – Sodium ions – Potassium ions – Chloride ions – Sulfate ions Dissolved Substances • Hydrogen and bicarbonate ions are monitored and regulated to maintain blood pH • Harmful foreign substances are removed from the blood by tubular secretion – Examples: drugs, food additives, pesticides, and nicotine Vertebrate Kidney Adaptations • Freshwater and saltwater environments pose challenges to animals • Animals are immersed in a solution with a higher or lower osmolarity than their body fluids • Animals have evolved homeostatic mechanisms, including kidney adaptations, to maintain water and salt within their bodies (osmoregulation) Vertebrate Kidney Adaptations • Freshwater fish live in a hypotonic environment – Water continuously leaks into their bodies by osmosis – Salts diffuse out • To compensate, their kidneys can excrete large quantities of extremely dilute urine Vertebrate Kidney Adaptations • Saltwater fish live in a hypertonic environment – Water is constantly leaving their tissues by osmosis – Salt constantly diffuses in • Fish kidney tubules lack loops of Henle, and cannot produce concentrated urine • To conserve water, they excrete only small quantities of urine containing salts not eliminated by their gills Vertebrate Kidney Adaptations • Mammals have structurally different kidneys depending on the availability of water in their natural habitat • If water availability is low, water can be conserved by producing concentrated urine – The longer the loop of Henle, the more concentrated the urine • Example: beaver nephrons – Nephrons have only short loops of Henle – Urine can be twice as concentrated as blood plasma Vertebrate Kidney Adaptations • Example: human nephrons – Some nephrons have short loops, others have long loops – Urine can be four times as concentrated as blood plasma • Example: kangaroo rat nephrons – Live in deserts – Nephrons have only very long loops of Henle – Urine can be fourteen times as concentrated as blood plasma