basic overview urine formation & excretion · urine formation and excretion urine formation...
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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