THE URINARY SYSTEM II URINE FORMATION FILTRATION REGULATION OF FILTRATION, CONCENTRATION, AND VOLUME COMPOSITION OF URINE

URINE FORMATION involves 3 processes in the nephron involves 3 processes in the nephron 1.glomerular filtration – capillaries of the glomerulus filter blood coming in The filtered fluid goes from the glomerulas to the glomerulary capsule and then into the tubule of the nephron. The filtered fluid goes from the glomerulas to the glomerulary capsule and then into the tubule of the nephron. 2.tubular reabsorption happens next – in the tubule needed substances (water, electrolytes and glucose) are reabsorbed from the filtrate fluid.

URINE FORMATION 3 processes 3. tubular secretion happens simultaneously with the blood secreting specific unwanted materials directly into the renal tubule - -some substances the body must eliminate (H+ and certain toxins) are removed faster than through filtration alone

the final product of these 3 processes  urine the final product of these 3 processes  urine amount filtered at glomerulas amount filtered at glomerulas - amount reabsorbed by tubule - amount reabsorbed by tubule + amount secreted by tubule + amount secreted by tubule = amount excreted in the urine = amount excreted in the urine

Glomerular Filtration Happens in the renal corpuscle (glomerulas and glomerular capsule) part of the nephron Happens in the renal corpuscle (glomerulas and glomerular capsule) part of the nephron urine formation begins when filtrate is made by water and certain dissolved substances being filtered out of glomerular capillaries urine formation begins when filtrate is made by water and certain dissolved substances being filtered out of glomerular capillaries many tiny openings in glomerular capillary walls make glomerular capillaries more permeable than capillaries in other tissues many tiny openings in glomerular capillary walls make glomerular capillaries more permeable than capillaries in other tissues Blood cells and plasma proteins are too big to go through the tiny openings and so go back out of the glomerulas through the efferent arteriole. Blood cells and plasma proteins are too big to go through the tiny openings and so go back out of the glomerulas through the efferent arteriole. Almost all other materials go through the openings to make up the filtrate fluid. Almost all other materials go through the openings to make up the filtrate fluid.

Glomerular Filtration The filtrate fluid goes from the glomerulus to the surrounding glomerular capsule (and then into the tubule of the nephron). The filtrate fluid goes from the glomerulus to the surrounding glomerular capsule (and then into the tubule of the nephron). glomerular filtration produces 180 liters of fluid every 24 hours (more than 4 times the total body water) glomerular filtration produces 180 liters of fluid every 24 hours (more than 4 times the total body water) most of this filtered fluid returns to the body through tubular reabsorbtion most of this filtered fluid returns to the body through tubular reabsorbtion

Filtration Rate glomerular filtration rate is directly proportional to net filtration pressure glomerular filtration rate is directly proportional to net filtration pressure factors that affect filtration pressure affect filtration rate factors that affect filtration pressure affect filtration rate ex. decrease in diameter of the afferent arteriole (vasoconstriction) or an obstruction (kidney stone or enlarged prostate) would decrease glomerular filtration rate. ex. decrease in diameter of the afferent arteriole (vasoconstriction) or an obstruction (kidney stone or enlarged prostate) would decrease glomerular filtration rate.

Regulation of Filtration Rate glomerular filtration rate is relatively constant glomerular filtration rate is relatively constant to help maintain homeostasis, glomerular filtration rate may increase when body fluids are in excess and decrease when body fluids must be conserved to help maintain homeostasis, glomerular filtration rate may increase when body fluids are in excess and decrease when body fluids must be conserved if blood pressure or volume decrease, afferent arterioles vasoconstrict  decreasing filtration rate if blood pressure or volume decrease, afferent arterioles vasoconstrict  decreasing filtration rate

less urine forms when the body needs to conserve water (urine more concentrated) less urine forms when the body needs to conserve water (urine more concentrated) vasodilation of afferent arterioles increases filtration rate vasodilation of afferent arterioles increases filtration rate more urine forms when fluids are in excess (urine diluted) more urine forms when fluids are in excess (urine diluted)

Some kidney cells secrete renin in response to decreased blood pressure Some kidney cells secrete renin in response to decreased blood pressure renin through a series of reactions creates angiotensin II which helps maintain sodium balance, water balance, and blood pressure renin through a series of reactions creates angiotensin II which helps maintain sodium balance, water balance, and blood pressure renin  angiotensin II renin  angiotensin II angiotensin II also stimulates the secretion of aldosterone which stimulates tubular reabsorption of sodium angiotensin II also stimulates the secretion of aldosterone which stimulates tubular reabsorption of sodium

Tubular Reabsorption process by which filtered substances are returned to the bloodstream (internal environment) process by which filtered substances are returned to the bloodstream (internal environment) recovers useful materials that have entered filtrate recovers useful materials that have entered filtrate the reabsorption reclaims the right amount of water, electrolytes, and glucose that the body needs the reabsorption reclaims the right amount of water, electrolytes, and glucose that the body needs

o most reabsorption occurs in the PCT -- PCT reabsorbs 60 – 70% of the filtrate PCT absorbs – PCT absorbs – 1. more than 99% of glucose, amino acids, and other organic nutrients 1. more than 99% of glucose, amino acids, and other organic nutrients 2. several ions: sodium, potassium, magnesium, phosphate, sulfate, and bicarbonate 2. several ions: sodium, potassium, magnesium, phosphate, sulfate, and bicarbonate liters of water daily liters of water daily 4. urea, chloride ions, lipid-soluble materials 4. urea, chloride ions, lipid-soluble materials

loop of Henle absorbs – loop of Henle absorbs – 1. roughly ½ of the remaining water and 2/3 of the remaining sodium and chloride ions 1. roughly ½ of the remaining water and 2/3 of the remaining sodium and chloride ions DCT absorbs – DCT absorbs – 1. sodium, chloride, and calcium 1. sodium, chloride, and calcium And water only 15 – 20% of initial filtrate reaches DCT only 15 – 20% of initial filtrate reaches DCT

collecting duct absorbs – collecting duct absorbs – 1. sodium 1. sodium 2. bicarbonate 2. bicarbonate 3. urea 3. urea And water

Review Questions 1. Where does the majority of reabsorption occur? 1. Where does the majority of reabsorption occur? proximal convoluted tubule (PCT) proximal convoluted tubule (PCT) 2. What is the function of tubular secretion? 2. What is the function of tubular secretion? to get rid of substances not filtered in the glomerulus to get rid of substances not filtered in the glomerulus