NOTES: Urinary System - Processes (Ch 15, part 2) *Recall: the primary function of the urinary system is to filter the blood of ions and nitrogenous wastes; when combined with water, these wastes make up URINE. • Kidneys receive about 20-25% of total cardiac output – ~1200mL of blood goes through the kidneys per minute Blood Supply to Kidneys • Descending aorta Renal artery interlobar arcuate cortical afferent arterioles • The afferent arterioles deliver blood to nephrons NEPHRONS • NEPHRONS: the functional units of the kidneys -each kidney contains about a million nephrons! Parts of a NEPHRON: • GLOMERULUS: tangled cluster of blood capillaries • GLOMERULAR CAPSULE (a.k.a. Bowman’s capsule): thin-walled structure surrounding glomerulus Parts of a NEPHRON: • PROXIMAL CONVOLUTED TUBULE • LOOP OF HENLE -descending limb -ascending limb • DISTAL CONVOLUTED TUBULE Parts of a NEPHRON: • COLLECTING DUCT (where distal tubules from several nephrons converge and drain into; from here, urine empties into the RENAL PELVIS) Blood Supply of a Nephron: -blood is brought to a nephron from an afferent arteriole; -from here, it is passed to an efferent arteriole; -this gives rise to a system of peritubular capillaries that surround the renal tubules URINE FORMATION *nephrons remove wastes from blood and regulate water and electrolyte concentrations. URINE IS THE END PRODUCT! Three Organic Wastes Products of Urine 1) Urea: most abundant, from breakdown of amino acids 2) Creatinine: generated in skeletal muscle when creatine phosphate is broken down (creatine phosphate is important for muscle contractions) 3) Uric Acid: waste product of formed when recycling nitrogenous bases from RNA molecules (makes proteins) • Removal of the 3 organic wastes results in unavoidable water loss • Kidneys are able to produce urine with an osmotic concentration of 1200-1400 mOsm/L which is 4 x that of plasma – If kidneys unable to do this, this would lead to fatal dehydration Three Steps of Urine Formation: 1) FILTRATION – in the glomerulus 2) REABSORPTION – “good stuff” is reabsorbed into the bloodstream; occurs in the tubular portion of the nephron 3) SECRETION – substances the body needs to get rid of at a faster rate are secreted; occurs in the tubular portion of the nephron GLOMERULAR FILTRATION: • Blood pressure forces water and solutes across the glomerular capillaries into capsular space • Analogy Drip coffee machine – Gravity forces hot water through filter, water carries dissolved solutes into pot. Coffee grounds too big to penetrate filter GLOMERULAR FILTRATION: • water and dissolved materials filter out of glomerular capillaries • the composition of the filtrate is similar to that of tissue fluid (water, salts, proteins, etc.) Filtrate goes into the PCT GLOMERULAR FILTRATION: • occurs due to pressure pushing the fluid into the capillaries *FILTRATION RATE varies with FILTRATION PRESSURE TUBULAR REABSORPTION: • substances are selectively reabsorbed from the glomerular filtrate • the capillaries around the nephron have increased permeability • most reabsorption occurs in the PROXIMAL TUBULE TUBULAR REABSORPTION: • substances may be reabsorbed via: -ACTIVE TRANSPORT: glucose, amino acids, sodium ions (Na+) -OSMOSIS: water • substances that remain in the filtrate become more concentrated as water is reabsorbed TUBULAR REABSORPTION: • Removal of water and solutes… – Goes into the peritubular fluid – Most material is reabsorbed because body can use – Materials eventually reenter the bloodstream TUBULAR REABSORPTION: • Tubular maximum (Tm) is the concentration at saturation – If goes past this, material will stay in tubular fluid and appear in urine (renal threshold) – Example: if you eat a meal high in carbs, plasma glucose may exceed Tm briefly, liver will kick in quickly lowering the amount of glucose • Glucose in urine (glycosuria) can be an indicator for diabetes mellitis TUBULAR SECRETION: • transports substances from the plasma to the tubular fluid to be excreted with the urine • necessary because filtration doesn’t force all of the dissolved material out of the plasma • acts like a back up to filtration • also a way to get rid of drugs TUBULAR SECRETION: • substances actively secreted include: -penicillin -creatine -histamine • substances secreted actively and passively along the tubule: -hydrogen ions (H+): important in regulating pH -potassium ions (K+) Antidiuretic Hormone (ADH) • Hypothalamus constantly secretes low amounts of ADH • In the absence of ADH, water is not reabsorbed in the DCT – All fluids reaching the DCT is lost as urine – Urine looks dilute (less yellow) – Can be an indicator of diabetes insipidus Antidiuretic Hormone (ADH) • As ADH levels rise, the DCT and collecting ducts become more permeable and the amount of water absorbed increases – Urine becomes more concentrated (more yellow) URINE COMPOSITON: about 95% water usually contains urea and uric acid (byproducts of nitrogen metabolism) electrolytes, amino acids, glucose (only if in excess) URINE ELIMINATION (“MICTURITION”) • urine drains out of the kidneys into the URETERS • URETERS convey urine to the URINARY BLADDER via peristaltic waves • urine is stored in the URINARY BLADDER until the “micturition reflex” is triggered: MICTURITION (URINATION) REFLEX: 1) bladder is filled and stretched stretch receptors stimulated 2) micturition reflex center in spinal cord sends motor impulses detrusor muscle in bladder wall 3) as bladder fills, internal pressure increases, forcing the INTERNAL URETHRAL SPHINCTER open MICTURITION (URINATION) REFLEX: 4) a second reflex relaxes the EXTERNAL URETHRAL SPHINCTER (can stay contracted under voluntary control…if you need to “hold it”!) 5) nerve centers in the brain and brainstem aid control of urination.