12/7/10. Excretory System. The basic function of the excretory system is to regulate the volume and composition of body fluids by:

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1 Excretory System The basic function of the excretory system is to regulate the volume and composition of body fluids by: o o removing wastes returning needed substances to the body for reuse Body systems that perform Excretion (p. 306) Respiratory system: Lungs excrete CO2 and water vapour Skin: Water, salts and some urea are excreted in perspiration Digestive system: Excretes water, salts, lipids, pigments (elimination of food residue (feces) is not considered a process of excretion) Excretory (urinary) system: Excretes dissolved metabolic wastes 1

2 Organs of the Excretory System (p ) Kidneys Humans have 2 fist-sized kidneys Located in lower back A cushioning fat layer surrounds the kidneys The Kidney Three main functions: 1. Daily elimination of toxic nitrogenous wastes such as: Uric acid from the break down of nucleic acids Ammonia which is converted in the liver to less toxic Urea is from the breakdown of amino acids Creatinine from the breakdown of creatine phosphate in muscles Each day the kidney filters liters of fluid from the blood to process out the nitrogen wastes as urine 2

3 Proteins Nucleic acids Amino acids Nitrogenous bases NH 2 Amino groups Most aquatic animals, including many fishes Mammals, amphibians, sharks, some bony fishes Birds, insects, many reptiles, land snails Ammonia Urea Uric acid Figure Regulation of water and salt balance Figure

4 hyponatraemia Boston Marathon 2002: Cynthia Lucero died from hyponatraemia - a severe lack of salt in the blood - because she ingested excessive volumes of a sports drink before and during the race. Too much water during exercise can kill 3. Regulation of ph Excess H + are secreted from the blood into the kidney Anatomy of the Kidneys: The Body s Blood-Cleansers (p. 307) (Inferior vena cava) Renal artery and vein Kidney (Aorta) Ureter Bladder Urethra A. THE EXCRETORY SYSTEM 4

5 Kidney tissue is divided into outer renal cortex and inner renal medulla The two human kidneys each contain about a million functional units called nephrons Renal pelvis Ureter Renal medulla Renal cortex Bowman s capsule Renal artery Renal vein Nephron tubule Collecting duct Renal cortex Renal medulla To renal pelvis B. THE KIDNEY C. ORIENTATION OF A NEPHRON WITHIN THE KIDNEY Figure 25.9B, C kidney aortic anneurism 5

6 Each nephron consists of a folded tubule and associated blood vessels The nephrons extract filtrate from the blood They refine the filtrate into a much smaller amount of urine Bowman s capsule Arteriole from renal artery Arteriole from glomerulus Branch of renal vein 2 Loop of Henle with capillary network Glomerulus 1 Proximal tubule Capillaries 3 Distal tubule From another nephron D. DETAILED STRUCTURE OF A NEPHRON (p. 308) Collecting duct Figure 25.9D Blood supply to the nephron A renal artery branches into a smaller arteriole that brings blood into each nephron the arteriole then splits into a fine network of capillaries called a glomerulus which is surrounded by Bowman s capsule an arteriole exits the glomerulus and branches into peritubular capillaries that surround the nephron tubule Peritubular capillaries merge into the renal vein carrying blood away from the nephron Urine Formation in the Nephron (p. 308, ) 1. Glomerular filtration Blood pressure forces water and many solutes (filtrate) from the blood vessels in the glomerulus into Bowman s capsule Passive process: no energy required Filtrate contains: water, glucose, amino acids, urea, uric acid, vitamins, ions such as Na +, Cl, HCO 3 Filtrate does not contain: RBC, platelets or blood plasma proteins 6

7 2. Tubular reabsorption (from filtrate into capillaries) In the proximal tubule, nutrients (glucose, amino acids, Na + and K+) move by active transport (requires ATP so proximal tubule cells contain many mitochondria) These nutrients move from inside the tubule OUT into the ECF, where they are absorbed by the peritubular capillaries (blood stream) negatively charged ions such as Cl -, follow passively, (attracted by the positively charged transported ions) Nephron tubule FILTRATION H 2 O, other small molecules TUBULAR REABSORPTION TUBULAR SECRETION Urine EXCRETION Capillary Na + is actively transported out of ascending loop of Henle Water is reabsorbed by osmosis (but not in ascending loop of Henle because it is impermeable to water). A high sugar meal may result in trace amounts of glucose being excreted in the urine Diabetes Mellitus is an insulin disorder characterized by large amounts of glucose in the urine Blood Bowman s capsule Proximal tubule NaCl H 2 O HCO 3 Glucose and amino acids Distal tubule H 2 O NaCl HCO 3 Filtrate H 2 O Salts (NaCl, etc.) HCO 3 Some NH 3 H + drugs and poisons CORTEX MEDULLA K + H + Collecting duct H + Urea Glucose Loop of Henle NaCl Amino acids Some drugs H 2 O NaCl Reabsorption Active transport Passive transport NaCl Urea H 2 O Secretion (active transport) Urine (to renal pelvis) 7

8 3. Tubular Secretion adds additional wastes from the blood (capillaries) into the filtrate occurs mainly in the distal tubule Hydrogen ions (H + ) are actively secreted as necessary, to maintain the ph of the blood penicillin and other drugs are secreted in the distal tubule FILTRATION Nephron tubule REABSORPTION SECRETION EXCRETION H 2 O, other small molecules Urine Capillary Figure Water reabsorption (water is returned to the blood) since most of the water is reabsorbed in the collecting duct, it functions as a waterconservation device this water reabsorption causes the filtrate to become very concentrated it is now called urine. Pathway of urine: Collecting tubule ---> renal pelvis ---> ureter ---> bladder (urine stored) ---> urethra REABSORPTION SECRETION EXCRETION Urine Figure

9 Urine: urine is generally clear and pale to deep yellow certain diseases change urine compositon so most physical exams include urinalysis urine is like a window into the body NOT normally found in the urine: RBCs, WBCs, blood proteins, hemoglobin and bile Homeostasis and the excretory system (text page 316) 1. Regulation of Water Reabsorption The amount of water reabsorbed from the filtrate by the kidneys affects: Blood volume (and blood pressure) Concentration of plasma solutes Solute concentration in the blood is constant regardless of how much water a person consumes Regulation of water reabsorption takes place in the hypothalamus in the brain Osmoreceptors in the hypothalamus detect when blood plasma becomes too concentrated (osmotic pressure increases) 9

10 When this happens, the hypothalamus signals the pituitary gland to release antidiuretic hormone (ADH) ADH makes the distal tubule and collecting duct in the nephron more permeable so more water is reabsorbed into the blood A smaller volume of more concentrated urine is produced If blood plasma is too dilute, osmoreceptors in the hypothalamus prevent the release of ADH Distal tubule and collecting duct become less permeable to water More, dilute urine is produced Alcohol and caffeine are diuretics they decrease release of ADH so more urine is produced 10

11 Diabetes insipidus Not enough ADH is produced Results in excessive urination (4-8 L per day) Dehydration Ion imbalances Synthetic ADH is often the treatment 2. Reabsorption of salts The kidneys regulate blood salt balance under the influence of the hormone aldosterone A drop of Na + concentration in the blood results in the release of aldosterone This hormone stimulates the distal tubule and collecting ducts to reabsorb Na + Water and chloride ions follow passively Both ADH and aldosterone regulate how much water is reabsorbed into the blood They have a role in regulating blood pressure (by changing blood volume) 11

12 3. Maintaining Blood ph Main buffer in the blood is carbonic acid (H 2 CO 3 ) and bicarbonate ions (HCO 3- ) The kidneys regulate blood ph by excreting H + or reabsorbing HCO 3 - as needed. This buffer maintains blood ph at 7.4 If blood is too acidic: H + is excreted HCO 3 - is reabsorbed If blood is too basic H + is not excreted HCO 3 - is not reabsorbed Disorders of the Excretory System Urinary tract infections: bacterial or viral infections of the urethra and bladder 12

13 Kidney stones: urine has crystallized into a solid stone (usually calcium compounds) Kidney Dialysis Machine compensates for kidney failure (renal insufficiency) It performs the function of the nephrons by removing wastes from the blood and maintaining its solute concentration 13

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