KIDNEY PHYSIOLOGY One of the more important aspects of kidney function is the GFR or glomerular filtration rate. Roughly five liters of blood get pumped per minute by the heart with about one liter or 20 percent going through the kidneys per minute. Only about 110 milliliters per minute ends up as filtrate. This is referred to as the glomerular filtration rate or GFR. The range is about 80-140 milliliters per minute. This would be a huge amount of urine produced (about 150 to 180 liters per day) if it weren’t concentrated. Fortunately, most of the filtrate gets reabsorbed so only 12 liters of urine are produced per day. There are many things that influence the GFR. The first is the hydrostatic pressure or the pressure of blood against the glomerular capillaries. The second is the hydrostatic pressure of filtrate acting in the opposite direction. The pressure is higher in the capillaries so the net osmosis of water travels from the capillaries to the lumen of the Bowman’s capsule. This is why the capillaries need to be under such high pressure. There is also a colloidal osmotic pressure (the pressure of the non-filtered blood products). This would tend to keep the water in the capillaries and is what the hydrostatic pressure must overcome to filter the plasma. The sum of the influences of hydrostatic pressure and osmotic pressure is called the net filtration pressure or NFP. This is about 10 millimeters of mercury. It is this small amount of pressure that propels a steady amount of water into the filtrate. There is autoregulation of the blood flow reaching the glomerulus so that the amount of pressure influencing the net filtration pressure stays roughly the same. Most of it involves smooth muscle contraction in the afferent capillaries. They contract under high blood pressure situations (to limit kidney blood flow) and relax under low pressure situations (to increase blood flow in the glomerulus). If the mean blood pressure is above 60 mm Hg, there will be adequate GFR. If this is lower than that, the individual is said to be in “shock” and the GFR will drop. The GFR is a tool used to assess the excretory function of the kidneys. If the GFR is down, there is an increase in the potential toxicity of certain drugs. Some drugs must be limited in
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