Juxtaglomerular cells, located between the distal tubule and the renal corpuscle release a
substance called renin. Renin activates the renin-angiotensin system which is involved in
ion/water regulation and blood pressure regulation (to raise blood pressure).
Macula Densa cells, located in the wall of the distal tubule as it passes by the glomerulus, release
as of yet unidentified paracrine substances that either dilate (vasodilators) or constrict
(vasoconstrictors) the afferent and efferent glomerular arteriole. By constricting the afferent
glomerular arteriole you reduce blood flow to the glomerulus and lower glomerular filtration
rate. By dilating the afferent glomerular arteriole you increase blood flow to the glomerulus and
raise glomerular filtration rate.
Cortical nephrons receive blood from the peritubular capillaries. These capillaries are a
continuation of the efferent glomerular arteriole that drains blood away from the glomerulus. The
vasa recta is a capillary bed that surrounds the Loop of Henle. In juxtamedullary nephrons the
vasa recta plays a key role in the formation of concentrated urine.
Exchange Processes Within the Nephron
1. Glomerular Filtration
Glomerular filtration refers to the filtration of the plasma at the glomerulus with water and small
substances such as ions, glucose and amino acids moving from inside the glomerular capillaries
into Bowman’s capsule. Proteins are to big to be filtered as are red/white blood cells. Therefore,
the fluid that enters Bowman’s capsule is similar to plasma but without proteins (remember that
plasma is blood without red/white blood cells).
There are two main forces that drive glomerular filtration. The most important of these is the
hydrostatic pressure within the glomerulus. Hydrostatic pressure simply refers to pressure exerted
to the presence of fluid. Hydrostatic pressure in the glomerulus is the blood pressure in that
capillary bed. Glomerular hydrostatic pressure forces fluid (and small substances) from the
glomerulus into Bowman’s capsule. The other force is oncotic pressure but we won’t discuss that
in this course.
2. Tubular Reabsorption
Tubular reabsorption refers to the movement of water or small substances such as ions, glucose
and amino acids from the fluid within the kidney tubule (i.e., within the nephron) back in to the
blood. The kidney’s basic approach is to filter everything that it can at the glomerulus (anything
small enough to be filtered) and then take back (reabsorb) what it does not want to get rid of
The cells lining the kidney tubules (nephron) are called tubular cells. The membrane that faces
the lumen of the kidney tubule is called the apical membrane while the membrane that faces the
blood is called the basolateral membrane. To be reabsorbed, substances must travel across the