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Home , Detrusor muscle, Glomerulus (kidney), Tubular fluid, Urine

NOTES: - Processes (Ch 15, part 2) *Recall: the primary function of the urinary system is to filter the of ions and nitrogenous ; when combined with , these wastes make up . • Kidneys receive about 20-25% of total cardiac output – ~1200mL of blood goes through the kidneys per minute Blood Supply to Kidneys

• Descending aorta   interlobar  arcuate  cortical afferent • The deliver blood to

NEPHRONS

• NEPHRONS: the functional units of the kidneys -each contains about a million nephrons! Parts of a :

• : tangled cluster of blood

• GLOMERULAR CAPSULE (a.k.a. Bowman’s capsule): thin-walled structure surrounding glomerulus Parts of a NEPHRON: • PROXIMAL CONVOLUTED TUBULE

• -descending limb -ascending limb

•

Parts of a NEPHRON:

• COLLECTING DUCT (where distal tubules from several nephrons converge and drain into; from here, urine empties into the ) Blood Supply of a Nephron: -blood is brought to a nephron from an afferent ; -from here, it is passed to an ; -this gives rise to a system of 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) : most abundant, from breakdown of amino acids 2) : generated in skeletal muscle when creatine phosphate is broken down (creatine phosphate is important for muscle contractions) 3) : product of formed when recycling nitrogenous bases from RNA molecules (makes ) • 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 to fatal Three Steps of Urine Formation:

1) – in the glomerulus 2) – “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:

• forces water and solutes across the glomerular capillaries into capsular space • Analogy  Drip 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 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 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 () can be an indicator for 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 - 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 ) – 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 )  electrolytes, amino acids, glucose (only if in excess) URINE ELIMINATION (“MICTURITION”) • urine drains out of the kidneys into the

• URETERS convey urine to the via peristaltic waves

• urine is stored in the URINARY BLADDER until the “micturition reflex” is triggered: MICTURITION () REFLEX:

1) bladder is filled and stretched  stretch receptors stimulated 2) micturition reflex center in spinal cord sends motor impulses  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) centers in the brain and brainstem aid control of urination