urine formation

nephron location

• glomerulus, proximal tubule and distal tubule are located in the cortex of the kidney

• collecting ducts and loop of Henle are located in the medulla (under cortex)

• most substances return to bloodstream through blood vessels

• remainder leaves in the form of urine

1st step: glomerular filtration

• moves water and solutes from blood plasma to glomerulus into the Bowman’s capsule

• glomeruli have pores in their tissue walls that allow water and salts to pass

• 90mL/min rate of filtration

• protein and blood cells can’t move through glomeruli

• substances in Bowman’s capsule = filtrate (water, Na, Cl, K, HCO3), amino acids, creatine, urea)

proximal tube reabsorption

• requires active transport

• positively charged ions, Na, K, glucose, amino acids are actively reabsorbed

• negatively charged ions like Cl are passively reabsorbed by electrical attraction

• water reabsorbed by osmosis

• secretion: active secretion of H+ ions, some drugs, and waste products like urea

descending loop of Henle reabsorption

• reabsorb water and ions and concentrate the filtrate

• descending loops goes to medulla (salty environment)

• water moves from descending loop to capillaries by osmosis

• concentration of Na+ in filtrate is highest at the bottom of the loop

ascending loop of henle reabsorption

• impermeable to water, permeable to solutes

• Na+ ions are passively reabsorbed from the filtrate

• higher portion of ascending loop: Na+ actively absorbed from filtrate to

  • replenish salty environment of medulla

  • makes filtrate less concentrated than surrounding cortex tissue

distal tubule reabsorption & secretion

• reabsorption of NaCl depending on the needs of the body

• reabsorption of ions decreases the concentration of filtrate, causing water to be reabsorbed by osmosis

• secretion: K+ & H+ actively secreted into distal tubule from the bloodstream

• hormone control: aldosterone secreted by adrenal glands that increase Na+ reabsorption, triggered by low blood sodium/volume

collecting duct reabsorption, hormone control, excretion

• final reabsorption of water and NaCl happens in the collecting duct

• passive reabsorption of water from filtrate by osmosis

• hormone control: antidiuretic hormone (ADH) release by hypothalamus, acts on collecting duct to increase water reabsorption

• excretion: collecting ducts empty into the renal pelvis, urine moves to bladder and removed from body

water balance

• is blood plasm has too much salt

  • osmoreceptors (sensors) in hypothalamus signal the release of ADH

  • ADH goes to kidneys where it increases the permeability of the collecting duct

  • more water can be reabsorbed into the blood

• if blood plasma becomes too little salt

  • osmoreceptors in the hypothalamus send a signal to the pituitary gland to decrease the release ADH

  • kidneys reabsorb less water