How is effective filtration pressure calculated

This pressure acting to draw water into the glomerulus is called blood colloid osmotic pressure. The absence of proteins in the glomerular space the lumen within the glomerular capsule results in a capsular osmotic pressure near zero. Glomerular filtration occurs when glomerular blood hydrostatic pressure exceeds the hydrostatic pressure of the glomerular capsule and the blood colloid osmotic pressure.

The sum of all of the influences, both osmotic and hydrostatic, results in a net filtration pressure NFP. Glomerular hydrostatic pressure is typically about 55 mmHg pushing fluid into the glomerular capsule.

This outward pressure is countered by a typical capsular hydrostatic pressure of about 15 mmHg and a blood colloid osmotic pressure of 30 mmHg.

To calculate the value of NFP:. A proper concentration of solutes in the blood is important in maintaining osmotic pressure both in the glomerulus and systemically. 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. This is due to two internal autoregulatory mechanisms that operate without outside influence: the myogenic mechanism and the tubuloglomerular feedback mechanism.

The myogenic mechanism regulating blood flow within the kidney depends upon a characteristic shared by most smooth muscle cells of the body. When you stretch a smooth muscle cell, it contracts; when you stop, it relaxes, restoring its resting length. This mechanism works in the afferent arteriole that supplies the glomerulus and can regulate the blood flow into the glomerulus. When blood pressure increases, smooth muscle cells in the wall of the arteriole are stretched and respond by contracting to resist the pressure, resulting in little change in flow.

This vasoconstriction of the afferent arteriole acts to reduce excess filtrate formation, maintaining normal NFP and GFR. Reducing the glomerular pressure also functions to protect the fragile capillaries of the glomerulus. When blood pressure drops, the same smooth muscle cells relax to lower resistance, increasing blood flow.

The vasodilation of the afferent arteriole acts to increase the declining filtrate formation, bringing NFP and GFR back up to normal levels. The tubuloglomerular feedback mechanism involves the juxtaglomerular JG cells, or granular cells, from the juxtaglomerular apparatus JGA and a paracrine signaling mechanism utilizing ATP and adenosine.

These juxtaglomerular cells are modified, smooth muscle cells lining the afferent arteriole that can contract or relax in response to the paracrine secretions released by the macula densa. This mechanism stimulates either contraction or relaxation of afferent arteriolar smooth muscle cells, which regulates blood flow to the glomerulus Table Recall that the DCT is in intimate contact with the afferent and efferent arterioles of the glomerulus.

The increased fluid movement more strongly deflects single nonmotile cilia on macula densa cells. This increased osmolarity of the filtrate, and the greater flow rate within the DCT, activates macula densa cells to respond by releasing ATP and adenosine a metabolite of ATP. ATP and adenosine act locally as paracrine factors to stimulate the myogenic juxtaglomerular cells of the afferent arteriole to constrict, slowing blood flow into the glomerulus. This vasoconstriction causes less plasma to be filtered, which decreases the glomerular filtration rate GFR , which gives the tubule more time for NaCl reabsorption.

Conversely, when GFR decreases, less NaCl is in the filtrate, and most will be reabsorbed before reaching the macula densa, which will result in decreased ATP and adenosine, allowing the afferent arteriole to dilate and increase GFR. This vasodilation causes more plasma to be filtered, which increase the glomerular filtration rate GFR , which gives the tubule less time for NaCl reabsorption increasing the amount of NaCl in the filtrate. Blood pressure forces water and dissolved blood components through the pores of the capillaries, basement membrane, and on through the slit membranes between pedicels.

The resulting fluid that enters the capsular space is called glomerular filtrate. In the glomerulus, blood filtering depends on 3 main pressures, one that promotes filtration and two that oppose filtration:. Glomerular blood hydrostatic pressure GBHP promotes filtration - it pushes water and solutes in blood plasma through the glomerular filter.

GBHP is the blood pressure in glomerular capillaries, which is about 55mm Hg. Capsular hydrostatic pressure CHP is a back-pressure that opposes filtration. As the filtrate is forced into the capsular space, it meets 2 forms of resistance: the wall of the capsule and the fluid that has already filled the renal tubule.

As a result, some filtrate is pushed back into the capillary. The amount of back-pressure is the CHP, about 15mm Hg. The blood colloid osmotic pressure BCOP is the 2nd force opposing filtration. It is mainly due to the presence of proteins eg albumin, globulins etc.