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Urine Formation and Excretion Urine Formation & Excretion Basic Overview · 3 processes glomerular filtration - kidneys "dump" cell- and protein-free blood into renal tubule tubular reabsorption - everything the body needs is kept (glucose, amino acids, water, salt) tubular secretion - other things are selectively added to the tubule · final product = urine · kidneys process 180 L of filtrate a day · only 1% is eliminated as urine, the rest is returned to circulation Glomerular Filtration · a passive process driven pressure gradients · filtration membrane has large surface area and is thousands of times more permeable to water and solutes smaller molecules (3 nm or less) pass freely through membrane molecules larger than 5 nm (proteins and cells) stay in blood Filtration Rate · net filtration pressure (NFP)- forcing acting on glomerular capillaries that push water and solute through the filtration membrane · glomerular filtration rate (GFR) - volume of filtrate formed each minute by all 2 million glomeruli of the kidneys total surface area available for filtration filtration membrane permeability NFP · GFR is directly proportional to NFP Regulation of Glomerular Filtration · kidneys needs a relatively constant GFR to maintain homeostasis · reasons for increase excess body fluid dilation of afferent arterioles more urine (dilute or clear) · reasons for decrease conservation of body fluids constriction of afferent arterioles less urine (concentrated or darker) Tubular Reabsorption · filtered substances returned to bloodstream (from nephron to blood) · aims to recover useful materials · most reabsorption takes place in PCT all glucose and amino acids, 65% Na+ and water · loop of Henle descending limb - water ascending limb - Na+ and ions · DCT water, Na+ · collecting duct water, Na+ · other nutrients and ions are also reabsorbed throughout the nephron

Basic Overview Urine Formation & Excretion · Urine Formation and Excretion Urine Formation & Excretion Basic Overview ·3 processes glomerular filtration - kidneys "dump" cell- and

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Urine Formation and Excretion

Urine Formation & Excretion Basic Overview· 3 processes

glomerular filtration - kidneys "dump" cell- and protein-free blood into renal tubule

tubular reabsorption - everything the body needs is kept (glucose, amino acids, water, salt)

tubular secretion - other things are selectively added to the tubule

· final product = urine· kidneys process 180 L of filtrate a day· only 1% is eliminated as urine, the rest is returned to circulation

Glomerular Filtration· a passive process driven pressure gradients· filtration membrane has large surface area and is thousands of times more permeable to water and solutes

smaller molecules (3 nm or less) pass freely through membrane

molecules larger than 5 nm (proteins and cells) stay in blood

Filtration Rate· net filtration pressure (NFP)- forcing acting on glomerular capillaries that push water and solute through the filtration membrane· glomerular filtration rate (GFR) - volume of filtrate formed each minute by all 2 million glomeruli of the kidneys

total surface area available for filtrationfiltration membrane permeabilityNFP

· GFR is directly proportional to NFP

Regulation of Glomerular Filtration· kidneys needs a relatively constant GFR to maintain homeostasis· reasons for increase

excess body fluid dilation of afferent arterioles more urine (dilute or clear)

· reasons for decreaseconservation of body fluids constriction of afferent arterioles less urine (concentrated or darker)

Tubular Reabsorption· filtered substances returned to bloodstream (from nephron to blood)· aims to recover useful materials· most reabsorption takes place in PCT

all glucose and amino acids, 65% Na+ and water· loop of Henle

descending limb - waterascending limb - Na+ and ions

· DCTwater, Na+

· collecting ductwater, Na+

· other nutrients and ions are also reabsorbed throughout the nephron

Urine Formation and Excretion

Tubular Secretion· reabsorption in reverse· secretions enter nephron from bloodstream· major secretions - H+ (PCT and DCT) and K+ (DCT)

Hormones that Influence Urine Formation

· renin-angiotensin mechanismrenin angiotensin I angiotensin IIhelps maintain Na+ and water balance, and blood

pressure (glomerular filtration)· aldosterone

stimulates reabsorption of Na and secretion of K+ (tubular reabsorption and secretion)

· antidiuretic hormone (ADH)stimulates H20 reabsorption (tubular reabsorption)increase ADH, increase H20 reabsorption, decrease

urine volume (more concentrated)decrease ADH, decrease H20 reabsorption, increase

urine volume

hydrated

dehydrated

Urine Composition· physical characteristics

color and transparency - clear and pale to deep yellowabnormal color or cloudiness

may be a result of diet or infection

odor - slightly aromaticpH - slightly acidic (pH = 6)

· chemical characteristics95% water5% solutes - urea, uric acid, creatine, electrolytes

Urinary Bladder· smooth, collapsible, muscular sac that temporarily stores urine· when empty the inner lining is folded· inner lining becomes smooth as bladder fills with urine· trigone - triangular structure on internal floor with openings from 2 ureters and 1 urethra· 3 layers

mucosa - transitional epitheliumthick muscular layer - detrusor muscle forms the internal

urethral sphincterserosa

Urine Formation and Excretion

Micturition· aka urination· process that expels urine from the urinary bladder· distention of bladder wall stimulates stretch receptors which trigger micturition reflex· reflex center (spinal cord) send parasympathetic impulses to the detrusor muscle which begins to contract rhythmically (provides a sense of urgency)· max capacity = 600 mL· the urge to urinate begins around 150 mL· urgency kicks into around 300 mL· when a person decides to urinate, the external urethral sphincter relaxes, detrusor muscle contracts and urine is excreted via urethra