There are disorders in which too much protein passes through the filtration slits into the kidney filtrate. This excess protein in the filtrate leads to a deficiency of circulating plasma proteins. Together, blood colloid osmotic pressure decreases, resulting in an increase in urine volume potentially causing dehydration. As you can see, there is a low net pressure across the filtration membrane. Intuitively, you should realize that minor changes in osmolarity of the blood or changes in capillary blood pressure result in major changes in the amount of filtrate formed at any given point in time.
The kidney is able to cope with a wide range of blood pressures. In large part, this is due to the autoregulatory nature of smooth muscle. When you stretch it, it contracts. Thus, when blood pressure goes up, smooth muscle in the afferent arterioles contracts to limit any increase in blood flow and filtration rate. When blood pressure drops, the same capillaries relax to maintain blood flow and filtration rate. The net result is a relatively steady flow of blood into the glomerulus and a relatively steady filtration rate in spite of significant systemic blood pressure changes.
One third of this is 10, and when you add this to the diastolic pressure of 80, you arrive at a calculated mean arterial pressure of 90 mm Hg. Therefore, if you use mean arterial pressure for the GBHP in the formula for calculating NFP, you can determine that as long as mean arterial pressure is above approximately 60 mm Hg, the pressure will be adequate to maintain glomerular filtration.
Blood pressures below this level will impair renal function and cause systemic disorders that are severe enough to threaten survival. This condition is called shock. It is vital that the flow of blood through the kidney be at a suitable rate to allow for filtration and yet not too fast to overwhelm the reabsorbing potential of the nephron tubule. This rate determines how much solute is retained or discarded, how much water is retained or discarded, and ultimately, the osmolarity of blood and the blood pressure of the body.
Glomerular filtration has to be carefully and thoroughly controlled because the simple act of filtrate production can have huge impacts on body fluid homeostasis and systemic blood pressure. Due to these two very distinct physiological needs, the body employs two very different mechanisms to regulate GFR. The kidney can control itself locally through intrinsic controls, also called renal autoregulation.
These intrinsic control mechanisms maintain filtrate production so that the body can maintain fluid, electrolyte, and acid-base balance and also remove wastes and toxins from the body. There are also control mechanisms that originate outside of the kidney, the nervous and endocrine systems, and are called extrinsic controls.
The nervous system and hormones released by the endocrine systems function to control systemic blood pressure by increasing or decreasing GFR to change systemic blood pressure by changing the fluid lost from the body. The kidneys are very effective at regulating the rate of blood flow over a wide range of blood pressures. Your blood pressure will decrease when you are relaxed or sleeping. It will increase when exercising. Yet, despite these changes, the filtration rate through the kidney will change very little.
Bulk reabsorption, which is not under hormonal control, occurs largely in the proximal tubule. In addition, many important solutes glucose, amino acids, bicarbonate are actively transported out of the proximal tubule such that their concentrations are normally extremely low in the remaining fluid.
Further bulk reabsorption of sodium occurs in the loop of Henle. Regulated reabsorption, in which hormones control the rate of transport of sodium and water depending on systemic conditions, takes place in the distal tubule and collecting duct.
Specifically, it estimates how much blood passes through the glomeruli each minute. Glomeruli are the tiny filters in the kidneys that filter waste from the blood. The blood sample is sent to a lab. There, the creatinine level in the blood sample is tested. Creatinine is a chemical waste product of creatine. Creatine is a chemical the body makes to supply energy, mainly to muscles.
The lab specialist combines your blood creatinine level with several other factors to estimate your GFR. Different formulas are used for adults and children.
The formula includes some or all of the following:. The creatinine clearance test , which involves a hour urine collection, can also provide an estimate of kidney function.
Your health care provider may ask you to temporarily stop any medicines that may affect the test results. These include antibiotics and stomach acid medicines.
Be sure to tell your provider about all the medicines you take. AccessBiomedical Science. AccessEmergency Medicine. Case Files Collection.
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