Urinary System Introduction A. The urinary system consists of two kidneys that filter the blood, two ureters, a urinary bladder, and a urethra to convey waste substances to the outside. Kidneys A. The kidney is a reddish brown, bean-shaped organ 12 centimeters long; it is enclosed in a tough, fibrous capsule. B. Kidney Structure 1. A medial depression in the kidney leads to a hollow renal sinus into which blood vessels, nerves, lymphatic vessels, and the ureter enter. 2. Inside the renal sinus lies a renal pelvis that is subdivided into major and minor calyces; small renal papillae project into each minor calyx. 3. Two distinct regions are found within the kidney: a renal medulla and a renal cortex. a. The renal medulla houses tubes leading to the papillae. b. The renal cortex contains the nephrons, the functional units of the kidney. C. Location of the Kidneys 1. The kidneys are positioned retroperitoneally on either side of the vertebral column between the twelfth thoracic and third lumbar vertebrae. D. Kidney Functions 1. The kidneys function to regulate the volume, composition, and pH of body fluids and remove metabolic wastes from the blood in the process. 2. The kidneys also help control the rate of red blood cell formation by secreting erythropoietin, and regulate blood pressure by secreting renin. E. Renal Blood Vessels 1. The abdominal aorta gives rise to renal arteries leading to the kidneys. 2. As renal arteries pass into the kidneys, they branch into successively smaller arteries: interlobar arteries, arcuate arteries, interlobular arteries, and afferent arterioles leading to the nephrons. 3. Venous blood is returned through a series of vessels that generally correspond to the arterial pathways. F. Nephrons 1. Nephron Structure a. A kidney contains one million nephrons, each of which consists of a renal corpuscle and a renal tubule. b. The renal corpuscle is the filtering portion of the nephron; it is made up of a ball of capillaries called the glomerulus and a glomerular capsule that receives the filtrate. c. The renal tubule leads away from the glomerular capsule and first becomes a highly coiled proximal convoluted tubule, then leads to the nephron loop, and finally to the distal convoluted tubule. d. Several distal convoluted tubules join to become a collecting duct. 2. Blood Supply of a Nephron a. The glomerulus receives blood from a fairly large afferent arteriole and passes it to a smaller efferent arteriole. b. The efferent arteriole gives rise to the peritubular capillary system, which surrounds the renal tubule. 3. Juxtaglomerular Apparatus a. At the point of contact between the afferent and efferent arterioles and the distal convoluted tubule, the epithelial cells of the distal tubule form the macula densa. b. Near the macula densa on the afferent arteriole are smooth muscle cells called juxtaglomerular cells. c. The macula densa together with the juxtaglomerular cells make up the juxtaglomerular apparatus. Urine Formation A. Urine formation involves glomerular filtration, tubular reabsorption, and tubular secretion. B. Glomerular Filtration 1. Urine formation begins when the fluid portion of the blood is filter by the glomerulus and enters the glomerular capsule as glomerular filtrate. C. Filtration Pressure 1. The main force responsible for moving substances by filtration through the glomerular capillary wall is the hydrostatic pressure of the blood inside. 2. Due to plasma proteins, osmotic pressure of the blood resists filtration, as does hydrostatic pressure inside the glomerular capsule. D. Filtration Rate 1. The factors that affect the filtration rate are filtration pressure, glomerular plasma osmotic pressure, and hydrostatic pressure in the glomerular capsule. 2. When the afferent arteriole constricts in response to sympathetic stimulation, filtration pressure, and thus filtration rate, declines. 3. When the efferent arteriole constricts, filtration pressure increases, increasing the rate of filtration. 4. When osmotic pressure of the glomerular plasma is high, filtration rate decreases. 5. When hydrostatic pressure inside the glomerular capsule is high, filtration rate declines. 6. On the average, filtration rate is 125 milliliters per minute or 180 liters in 24 hours, most of which is reabsorbed further down the nephron. E. Regulation of Filtration Rate 1. Glomerular filtration rate is relatively constant, although sympathetic impulses may decrease the rate of filtration. 2. Another control over filtration rate is the renin-angiotensin system, which regulates sodium excretion. a. When the sodium chloride concentration in the tubular fluid decreases, the macula densa senses these changes and causes the juxtaglomerular cells to secrete renin. b. Secretion of renin triggers a series of reactions leading to the production of angiotensin II, which acts as a vasoconstrictor; this may, in turn, affect filtration rate. c. Presence of angiotensin II also increases the secretion of aldosterone, which stimulates reabsorption of sodium. d. The heart can also increase filtration rate when blood volume is high. F. Tubular Reabsorption 1. Changes in the fluid composition from the time glomerular filtrate is formed to when urine arrives at the collecting duct are largely the result of tubular reabsorption of selected substances. 2. Most of the reabsorption occurs in the proximal convoluted tubule, where cells possess microvilli with carrier proteins. 3. Carrier proteins have a limited transport capacity, so excessive amounts of a substance will be excreted into the urine. 4. Glucose and amino acids are reabsorbed by active transport, water by osmosis, and proteins by pinocytosis. G. Sodium and Water Reabsorption 1. Sodium ions are reabsorbed by active transport, and negatively charged ions follow passively. 2. As sodium is reabsorbed, water follows by osmosis. H. Regulation of Urine Concentration and Volume 1. Most of the sodium ions are reabsorbed before the urine is excreted, and sodium is concentrated in the renal medulla by the countercurrent mechanism. 2. Normally the distal convoluted tubule and collecting duct are impermeable to water unless the hormone ADH is present. I. Urea and Uric Acid Excretion 1. Urea is a by-product of amino acid metabolism; uric acid is a by-product of nucleic acid metabolism. 2. Urea is passively reabsorbed by diffusion but about 50% of urea is excreted in the urine. 3. Most uric acid is reabsorbed by active transport and a small amount is secreted into the renal tubule. J. Tubular Secretion 1. Tubular secretion transports certain substances from the plasma into the renal tubule. 2. Active transport mechanisms move excess hydrogen ions into the renal tubule along with various organic compounds. 3. Potassium ions are secreted both actively and passively into the distal convoluted tubule and the collecting duct. K. Urine Composition 1. Urine composition varies from time to time and reflects the amounts of water and solutes that the kidneys eliminate to maintain homeostasis. 2. Urine is 95% water, and also contains urea, uric acid, a trace of amino acids, and electrolytes. Urine Elimination A. After forming in the nephrons, urine passes from the collecting ducts to the renal papillae, then to the minor and major calyces, and out the renal pelvis to the ureters, urinary bladder, and finally to the urethra, which conveys urine to the outside. B. Ureters 1. The ureters are muscular tubes extending from the kidneys to the base of the urinary bladder. 2. The wall of the ureter is composed of three layers: mucous coat, muscular coat, and outer fibrous coat. D. Micturition 1. Urine leaves the bladder by the micturation reflex. 2. The detrusor muscle contracts and the external urethral sphincter (in the urogenital diaphragm) must also relax. •Stretching of the urinary bladder triggers the micturation reflex center located in the sacral portion of the spinal cord. 4. Return parasympathetic impulses cause the detrusor muscle to contract in waves, and an urge to urinate is sensed. 5. When these contractions become strong enough, the internal urethral sphincter is forced open. 6. The external urethral sphincter is composed of skeletal muscle and is under conscious control. E. Urethra 1. The urethra is a tube that conveys urine from the urinary bladder to the outside. 2. It is a muscular tube with urethral glands that secrete mucus into the urethral canal. Water, Electrolyte, and Acid-Base Balance Introduction A. To be in balance, the quantities of fluids and electrolytes leaving the body should be equal to the amounts taken in. B. Anything that alters the concentrations of electrolytes will also alter the concentration of water, and vice versa. Distribution of Body Fluids A. Fluids occur in compartments in the body, and movement of water and electrolytes between compartments is regulated. B. Fluid Compartments 1. The average adult female is 52% water by weight, while a male is 63% water, the difference due to the female's additional adipose tissue. 2. The intracellular fluid compartment includes all the water and electrolytes within cells. 3. The extracellular fluid compartment includes all water and electrolytes outside of cells (interstitial fluid, plasma, and lymph). 4. Transcellular fluid includes the cerebrospinal fluid of the central nervous system, fluids within the eyeball, synovial fluid of the joints, serous fluid within body cavities, and exocrine gland secretions. C. Body Fluid Composition 1. Extracellular fluids have high concentrations of sodium, chloride, and bicarbonate ions, and lesser amounts of potassium, calcium, magnesium, phosphate, and sulfate ions. 2. Intracellular fluid has high concentrations of potassium, phosphate, and magnesium ions, and lesser amounts of sodium, chloride, and bicarbonate ions. D. Movement of Fluid between Compartments 1. Hydrostatic pressure and osmotic pressure regulate the movement of water and electrolytes from one compartment to another. 2. Although the composition of body fluids varies from one compartment to another, the total solute concentrations and water amounts are normally equal. 3. A net gain or loss of water will cause shifts affecting both the intracellular and extracellular fluids due to osmosis. Water Balance A. Water balance exists when water intake equals water output. B. Water Intake 1. The volume of water gained each day varies from one individual to the next. 2. About 60% of daily water is gained from drinking, another 30% comes from moist foods, and 10% from the water of metabolism. C. Regulation of Water Intake 1. The thirst mechanism is the primary regulator of water intake. 2. The thirst mechanism derives from the osmotic pressure of extracellular fluids and a thirst center in the hypothalamus. 3. Once water is taken in, the resulting distention of the stomach will inhibit the thirst mechanism. D. Water Output 1. Water is lost in urine, feces, perspiration, evaporation from skin (insensible perspiration), and from the lungs during breathing. 2. The route of water loss depends on temperature, relative humidity, and physical exercise. E. Regulation of Water Output 1. The distal convoluted tubules and collecting ducts of the nephrons regulate water output. 2. Antidiuretic hormone from the posterior pituitary causes a reduction in the amount of water lost in the urine. 3. When drinking adequate water, the ADH mechanism is inhibited, and more water is expelled in urine. Electrolyte Balance A. An electrolyte balance exists when the quantities of electrolytes gained equals the amount lost. B. Electrolyte Intake 1. The electrolytes of greatest importance to cellular metabolism are sodium, potassium, calcium, magnesium, chloride, sulfate, phosphate, bicarbonate, and hydrogen ions. 2. Electrolytes may be obtained from food or drink or produced as a by- product of metabolism. C. Regulation of Electrolyte Intake 1. A person ordinarily obtains sufficient electrolytes from foods eaten. 2. A salt craving may indicate an electrolyte deficiency D. Electrolyte Output 1. Losses of electrolytes occur through sweating, in the feces, and in urine. E. Regulation of Electrolyte Output 1. The concentrations of the cations, especially sodium, potassium, and calcium, are very important. 2. Sodium ions account for 90% of the positively charged ions in extracellular fluids; the action of aldosterone on the kidneys regulates sodium reabsorption. 3. Aldosterone also regulates potassium ions; potassium ions are excreted when sodium ions are conserved. 4. Calcium concentration is regulated by parathyroid hormone, which increases the concentrations of calcium and phosphate ions in extracellular fluids and by calcitonin which does basically the reverse. 5. Generally, the regulatory mechanisms that control positively charged ions secondarily control the concentrations of anions. Acid-Base Balance A. Electrolytes that ionize in water and release hydrogen ions are acids; those that combine with hydrogen ions are bases. B. Maintenance of homeostasis depends on the control of acids and bases in body fluids. C. Sources of Hydrogen Ions 1. Most hydrogen ions originate as by-products of metabolic processes, including: the aerobic and anaerobic respiration of glucose, incomplete oxidation of fatty acids, oxidation of amino acids containing sulfur, and the breakdown of phosphoproteins and nucleic acids. D. Strengths of Acids and Bases 1. Acids that ionize more completely are strong acids; those that ionize less completely are weak acids. 2. Bases release hydroxyl and other ions, which can combine with hydrogen ions, thereby lowering their concentration. E. Regulation of Hydrogen Ion Concentration 1. Acid-base buffer systems, the respiratory center in the brain stem, and the kidneys regulate pH of body fluids. 2. Acid-Base Buffer Systems a. The chemical components of a buffer system can combine with a strong acid and convert it to a weaker one. b. The chemical buffer systems in body fluids include the bicarbonate buffer system, the phosphate buffer system, and the protein buffer system. 3. The Respiratory Center a. The respiratory center in the brain stem helps to regulate hydrogen ion concentration by controlling the rate and depth of breathing. b. During exercise, the carbon dioxide, and thus the carbonic acid, levels in the blood increase. c. In response, the respiratory center increases the rate and depth of breathing, so the lungs excrete more carbon dioxide. 4. The Kidneys a. Nephrons secrete excess hydrogen ions in the urine. 5. Rates of Regulation a. Chemical buffers are considered the body's first line of defense against shifts in pH; physiological buffer systems (respiratory and renal mechanisms) function more slowly and constitute secondary defenses. Acid-Base Imbalances A. Chemical and physiological buffer systems usually keep body fluids within very narrow pH ranges but abnormal conditions may prevent this. 1. A pH below 7.35 produces acidosis while a pH above 7.45 is called alkalosis. B. Acidosis 1. Two major types of acidosis are respiratory and metabolic acidosis. a. Respiratory acidosis results from an increase of carbonic acid caused by respiratory center injury, air passage obstructions of problems with gas exchange. b. Metabolic acidosis is due to either an accumulation of acids of a loss of bases and has many causes including kidney disease, vomiting, diarrhea and diabetes mellitus. c. Increasing respiratory rate or the amount of hydrogen ions released by the kidney can help compensate for acidosis. C. Alkalosis 1. Alkalosis also has respiratory and metabolic causes. a. Respiratory alkalosis results from hyperventilation causing an excessive loss of carbon dioxide. b. Metabolic alkalosis is caused by a great loss of hydrogen ions or a gain in base perhaps from vomiting or use of drugs.