The kidneys are located just above the pelvis on either side of the body. Directly above then are
the adrenal glands. Each kidney is connected to the bladder by a ureter. The kidney receives
blood from the renal artery while the renal vein carries blood from the kidneys back to the heart.
In cross section you see that the kidney is composed of an outer region called the renal cortex
and an inner region called the renal medulla. Furthermore, the kidney consists of repeating
segments called renal pyramids. Within each renal pyramid there are numerous nephrons (the
functional unit of the kidney). Fluid (urine) from the collecting ducts of nephrons within any
given renal pyramid drains into a minor calyx. The urine then flows into the central region of the
kidney (the renal pelvis) before entering the ureter which drains urine from the kidney to the
The functional unit of the kidney is the nephron. The majority of nephrons are found in the renal
cortex and are called cortical nephrons. However, the collecting ducts from the cortical nephrons
extend into the renal medulla before emptying into the minor calyx. Some nephrons are
contained primarily within the medulla (although part of them is in the cortex). These are called
juxtamedullary nephrons and they play an important role in establishing the conditions required
for the formation of a concentrated urine.
A nephron begins with a dense capillary bed called the glomerulus which is surrounded by
Bowman’s capsule. The glomerulus receives blood from the afferent glomerular arteriole and is
drained by the efferent glomerular arteriole. Together, the glomerulus and Bowman’s capsule are
called the renal corpuscle.
From Bowman’s capsule, fluid flows into the proximal convoluted tubule and then into the
proximal straight tubule. From there fluid enters the descending Loop of Henle and then the thin
and thick ascending Loop of Henle. Fluid leaves the Loop of Henle and enters the collecting
tube. The collecting tubes from many nephrons connect to a collecting duct which drains into the
The function of the glomerulus and Bowman’s capsule is to filter the plasma to create the initial
fluid that will become urine. The proximal tubule is the primary site of bulk reabsorption of
water, ions, glucose, etc that occurs without any hormonal regulation. The Loop of Henle
(particularly of the juxtaglomerular nephrons) is involved in creating the conditions necessary for
the production of a concentrated urine. The distal convoluted tubule is a site of regulated
ion/water reabsorption. The collecting tube and collecting duct are the site of regulated water
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
apical membrane into the tubular cell then across the basolateral membrane, then across the
basement membrane into the peritubular (extracellular) fluid before crossing the capillary
endothelial cell and entering the blood.
The vast majority of all the substances that are filtered are reabsorbed. The table in the power
point slides indicates that over 99% of the water that is filtered is reabsorbed. 100% of all glucose
that is filtered is reabsorbed and other ions are reabsorbed at rates of approximately 86 to 99% of
the filtered amount.
3. Tubular Secretion
In some cases the kidney may wish to get rid of a greater amount of a substance than is filtered at
the glomerulus during glomerular filtration. In this case, substances can be secreted from the
blood into the tubular fluid. This occurs primarily in the proximal tubule. Tubular secretion
involves the movement of a substance out of the blood into the peritubular space and then across
the basement membrane. From their the substance crosses the basolateral membrane and enters
the tubular epithelial cell before crossing the apical membrane and entering the tubular fluid.
Secretion involves both active transport and facilitated diffusion. The active transport can take
place either on the apical membrane or the basolateral membrane (depending on the substance in
question). Facilitated diffusion will occur on which ever of these two membranes that active
transport does not occur.
Excretion refers to the removal of a substance, from the body, in the urine. The amount excreted
is equal to the amount filtered plus the amount secreted minus the amount reabsorbed.
If excretion is greater than filtration then a substance must have been secreted from the blood into
the tubular fluid.
If excretion is less than filtration then a substance must have been reabsorbed from the tubular
fluid back into the blood.
The voiding of urine is referred to as urination or micturition. Fluid leaves the collecting tube,
enters the minor calyx, flows into the renal pelvis and then moves from the kidney to the bladder
in the ureter. During urination, fluid leaves the bladder and is voided through the urethra.
The bladder is surrounded by a muscle called the detrusor muscle. At the site where the bladder
joins the urethra there is a sphincter (ring of muscle) called the internal urethral sphincter that,
when closed, helps to keep urine within the bladder. Closer to the external opening of the urethra,
The kidneys are located just above the pelvis on either side of the body. Each kidney is connected to the bladder by a ureter. The kidney receives blood from the renal artery while the renal vein carries blood from the kidneys back to the heart. In cross section you see that the kidney is composed of an outer region called the renal cortex and an inner region called the renal medulla. Furthermore, the kidney consists of repeating segments called renal pyramids. Within each renal pyramid there are numerous nephrons (the functional unit of the kidney). Fluid (urine) from the collecting ducts of nephrons within any given renal pyramid drains into a minor calyx. The urine then flows into the central region of the kidney (the renal pelvis) before entering the ureter which drains urine from the kidney to the bladder. The functional unit of the kidney is the nephron.