Chapter 19 Glomerular Filtration
myogenic mechanism
An increase in blood pressure within the afferent arteriole will cause the vessel to constrict which in turn will cause glomerular filtration rate to decrease.
The hydrostatic pressure within the capsular space
Tends to oppose glomerular blood hydrostatic pressure, and so is called the capsular hydrostatic pressure (CHP). CHP is on the order of 15 mmHg.
Excretion rate
filtration rate - reabsorption rate + secretion rate
Tubuloglomerular feedback
An increase in glomerular filtration rate will result in constriction of the afferent arteriole so that glomerular filtration rate reduced. The sensory input to this reflex involves increase flow of water and NaCI to the macula densa which in turn leads to the release of a vasoconstrictory substance.
The 2 hormones that affect glomerular filtration rate
Angiotensin 2 and ANP. Angiotensin 2 causes both arterioles to constrict which results in a decrease in glomerular filtration rate. ANP causes glomerular filtration rate to increase by acting on mesangial cells causing them to relax and thereby increasing the capillary surface area available for filtration.
Tubular secretion
As fluid moves along the nephron, materials can be added to tubular fluid.
Why it is important that glomerular filtration rate be maintained at a relatively constant value
Because if glomerular filtration rate were to become too high, certain substances would pass through the nephron too fast and thus be lost in urine. On the other hand, if glomerular filtration rate were to become too low, nearly all the filtrate might be reabsorbed and so waste products will be retained.
Net filtration pressure (NFP)
Can be calculated by subtracting the factors that promote filtration from the factors that oppose filtration. Specifically the net filtration pressure is equal to glomerular blood hydrostatic pressure minus capsular hydrostatic pressure minus blood colloid osmotic pressure. Under typical conditions, net filtration pressure is positive on the order of 10 mmHg favoring filtration.
What glomerular filtration depends upon
Depends on 3 of the 4 Starling forces. Only interstitial fluid osmotic pressure is essentially zero because negligible amounts of protein are found in the capsular space (space of Bowman's capsule).
Filtration barrier
Excludes substances on the basis of size and charge. In general, small molecules like water, ions, glucose and amino acids freely pass the filtration barrier so that their concentration in plasma is equal to their concentration in the glomerular filtrate. By contrast, most proteins are excluded from the filtrate and their presence in urine indicates a problem in glomerular filtration.
What glomerular filtration rate is also subject to
Extrinsic regulation in the form of neural regulation and hormonal regulation.
The first step in urine production
Glomerular filtration involving the transfer of water and solutes from plasma to the space of Bowman's capsule (capsular space)
The 3 basic processes performed by the nephron and collecting ducts
Glomerular filtration, tubular reabsorption, tubular secretion.
Blood colloid osmotic pressure (BCOP)
Like other capillaries, filtration is opposed in glomerular capillarie by the blood colloid osmotic pressure, attributable to the osmotic pressures exerted by plasma proteins.
What happens as filtered solutes pass along the nephrons
Most of this material is returned to peritubular capillaries or vasa recta in a process called tubular reabsorption.
Renal regulation
Refers to the kidney's ability to maintain a constant renal blood flow and glomerular filtration rate despite changes in systemic arterial pressure. Renal autoregulation takes the form of 2 distinct mechanisms called myogenic mechanism and tubuloglomerular feedback.
Glomerular blood hydrostatic pressure (GBHP)
Relatively high compared to other capillaries between two high resistance arterioles. A typical value of glomerular blood hydrostatic pressure is 55 mmHg compared to 35 mmHg in most capillaries. Also, unlike most capillaries, the glomerular blood hydrostatic pressure does not vary along the capillary length.
The 3 basic mechanisms which help to maintain constant glomerular filtration rate
Renal autoregulation, neural regulation, and hormonal regulation.
Glomerular capillaries
Specialized for filtration by being more permeable than most capillaries and are subject to higher hydrostatic pressures than most capillaries.
At moderate levels of sympathetic stimulation
The afferent and efferent arterioles constrict to about the same extent so that glomerular filtration rate remains unchanged. With higher levels of sympathetic stimulation, the afferent arteriole constricts relatively more than the efferent so that golmerular filtration rate decreases.
Glomerular filtration rate (GFR)
The amount of filtrate formed in all renal corpuscles of both kidneys per unit time. A typical glomerular filtration rate is on the order of 190 L/day or when expressed per minute, 125mL/min.
What glomerular filtration rate depends on
The surface area for filtration, the filtration permeability, and net filtration pressure. With respect to the net filtration pressure, an increase in blood pressure will increase glomerular filtration rate whereas a decrease in blood pressure will decrease glomerular filtration rate.
