all filtration takes place in the kidneys at glomerulus.
where does all filtration take place in the kidneys?
glomerulus
bullet::::- in the kidneys, angiotensin ii constricts glomerular arterioles, having a greater effect on efferent arterioles than afferent. as with most other capillary beds in the body, the constriction of afferent arterioles increases the arteriolar resistance, raising systemic arterial blood pressure and decreasing the blood flow. however, the kidneys must continue to filter enough blood despite this drop in blood flow, necessitating mechanisms to keep glomerular blood pressure up. to do this, angiotensin ii constricts efferent arterioles, which forces blood to build up in the glomerulus, increasing glomerular pressure. the glomerular filtration rate (gfr) is thus maintained, and blood filtration can continue despite lowered overall kidney blood flow. because the filtration fraction has increased, there is less plasma fluid in the downstream peritubular capillaries. this in turn leads to a decreased hydrostatic pressure and increased oncotic pressure (due to unfiltered plasma proteins) in the peritubular capillaries. the effect of decreased hydrostatic pressure and increased oncotic pressure in the peritubular capillaries will facilitate increased reabsorption of tubular fluid.
renin–angiotensin system
once the kidneys receive blood, filtration of substances from the blood into urine takes place. the glomerular filtration rate of single nephrons in birds is low because avian glomeruli have small surface area. through the process of reabsorption, the majority of the fluid volume and solutes are transported from the urine to the blood. next, secretion of materials from the renal epithelia into the urine occurs. finally, urine as the end product travels to the ureters to be excreted. the kidneys of a common raven filter about eleven times its total body water daily, and more than 95% of the filtered water is reabsorbed. urine of birds is typically concentrated to an osmolarity that is two to three times the osmolarity of plasma. glomerular filtration only accounts for 10% to 20% of urinary urate. greater than 90% of urate excreted by the kidneys is derived from the process of secretion.
common raven physiology