Location
The kidneys are a pair of organs found along the posterior
muscular wall of the abdominal cavity. The left kidney is located slightly more
superior than the right kidney due to the larger size of the liver on the right
side of the body. Unlike the other abdominal organs, the kidneys lie behind the
peritoneum that lines the abdominal cavity and are thus considered to be
retroperitoneal organs. The ribs and muscles of the back protect the kidneys
from external damage. Adipose tissue known as perirenal fat surrounds the
kidneys and acts as protective padding.
Structure
The kidneys are bean-shaped with the convex side of each
organ located laterally and the concave side medial. The indentation on the
concave side of the kidney, known as the renal hilus, provides a space for the
renal artery, renal vein, and ureter to enter the kidney.
A thin layer of fibrous connective tissue forms the renal
capsule surrounding each kidney. The renal capsule provides a stiff outer shell
to maintain the shape of the soft inner tissues.
Deep to the renal capsule is the soft, dense, vascular renal
cortex. Seven cone-shaped renal pyramids form the renal medulla deep to the
renal cortex. The renal pyramids are aligned with their bases facing outward
toward the renal cortex and their apexes point inward toward the center of the
kidney.
Each apex connects to a minor calyx, a small hollow tube
that collects urine. The minor calyces merge to form 3 larger major calyces,
which further merge to form the hollow renal pelvis at the center of the kidney.
The renal pelvis exits the kidney at the renal hilus, where urine drains into
the ureter.
Blood Supply
1. The renal arteries branch directly from the abdominal
aorta and enter the kidneys through the renal hilus.
2. Inside our
kidneys, the renal arteries diverge into the smaller afferent arterioles of the
kidneys.
3. Each afferent
arteriole carries blood into the renal cortex, where it separates into a bundle
of capillaries known as a glomerulus.
4. From the
glomerulus, the blood recollects into smaller efferent arterioles that descend
into the renal medulla.
5. The efferent
arterioles separate into the peritubular capillaries that surround the renal
tubules.
6. Next, the
peritubular capillaries merge to form veins that merge again to form the large
renal vein.
7. Finally, the renal
vein exits the kidney and joins with the inferior vena cava, which carries
blood back to the heart.
Each kidney contains around 1 million individual nephrons,
the kidneys’ microscopic functional units that filter blood to produce urine.
The nephron is made of 2 main parts: the renal corpuscle and the renal tubule.
Responsible for filtering the blood, our renal corpuscle is
formed by the capillaries of the glomerulus and the glomerular capsule (also
known as Bowman’s capsule). The glomerulus is a bundled network of capillaries
that increases the surface area of blood in contact the blood vessel walls.
Surrounding the glomerulus is the glomerular capsule, a cup-shaped double layer
of simple squamous epithelium with a hollow space between the layers. Special
epithelial cells known as podocytes form the layer of the glomerular capsule
surrounding the capillaries of the glomerulus. Podocytes work with the
endothelium of the capillaries to form a thin filter to separate urine from
blood passing through the glomerulus. The outer layer of the glomerular capsule
holds the urine separated from the blood within the capsule. At the far end of
the glomerular capsule, opposite the glomerulus, is the mouth of the renal
tubule.
A series of tubes called the renal tubule concentrate urine
and recover non-waste solutes from the urine. The renal tubule carries urine
from the glomerular capsule to the renal pelvis.
The curvy first
section of the renal tubule is known as the proximal convoluted tubule. The
tubule cells that line the proximal convoluted tubule reabsorb much of the
water and nutrients initially filtered into the urine.
Urine next passes
through the loop of Henle, a long straight tubule that carries urine into the
renal medulla before making a hairpin turn and returning to the renal cortex.
Following the loop
of Henle is the distal convoluted tubule.
Finally, urine
from the distal convoluted tubules of several nephrons enters the collecting
duct, which carries the concentrated urine through the renal medulla and into
the renal pelvis.
From the renal
pelvis urine from many collecting ducts combines and flows out of the kidneys
and into the ureters.
Physiology of the Kidneys
Excretion of Wastes
The primary function of the kidneys is the excretion of
waste products resulting from protein metabolism and muscle contraction. The
liver metabolizes dietary proteins to produce energy and produces toxic ammonia
as a waste product. The liver is able to convert most of this ammonia into uric
acid and urea, which are less toxic to the body. Meanwhile, the muscles of our
body use creatine as an energy source and, in the process, produce the waste product
creatinine. Ammonia, uric acid, urea, and creatinine all accumulate in the body
over time and need to be removed from circulation to maintain homeostasis.
The glomerulus in the kidneys filter all four of these waste
products out of the bloodstream, allowing us to excrete them out of our bodies
in urine. Around 50% of the urea found in the blood is reabsorbed by the tubule
cells of the nephron and returned to the blood supply. Urea in the blood helps
to concentrate other more toxic waste products in urine by maintaining the
osmotic balance between urine and blood in the renal medulla.
Filtration, Reabsorption, and Secretion
The kidneys filter
blood as it passes through the capillaries that form the glomerulus. Blood
pressure forces most of the blood plasma through the lining of the capillaries
and into the glomerular capsule. Blood cells are too large to pass through the
capillary lining and so remain within the capillaries along with some residual
plasma. The filtered plasma, now known as tubular fluid, begins to flow out of
the glomerular capsule and into the proximal convoluted tubule.
At the same time,
the concentrated blood that remains inside the capillaries of the glomerulus
moves into the efferent arterioles and on to the peritubular capillaries
surrounding the proximal convoluted tubule. Epithelial cells lining the tubule
actively reabsorb valuable molecules of glucose, amino acids, and ions from the
filtrate and deposit them back into the blood. These cells also absorb any
waste products remaining in the blood (such as ammonia and creatinine) and
secrete these chemicals into the filtrate. While these solutes are being
exchanged, osmotic pressure pushes water from the dilute, hypotonic filtrate
back into the concentrated, hypertonic blood.
From the proximal
convoluted tubule, the tubular fluid next enters the loop of Henle, where water
and ions are reabsorbed. The descending limb of the loop of Henle is permeable
to water and carries the filtrate deep into the medulla of the kidney. Tissues
in the medulla surrounding the tubule contain a high concentration of ions and
very little water compared to the filtrate. Osmotic pressure between the
hypotonic filtrate and hypertonic medullary cells pushes water out of the
filtrate and into the cells. The cells of the medulla return this water to the
blood flowing through nearby capillaries.
Filtrate next
passes through the ascending limb of the loop of Henle as it exits the medulla.
The tissues surrounding the ascending limb are not permeable to water but are
permeable to ions. The filtrate is very concentrated after passing through the
descending limb, so ions easily diffuse out of the filtrate and into the cells
lining the ascending limb. These ions are returned to the blood flowing through
nearby capillaries.
Tubular fluid
exiting the loop of Henle next passes through the distal convoluted tubule and
the collecting duct of the nephron. These tubules continue to reabsorb small
amounts of water and ions that are still left in the filtrate. The tissues
surrounding the collecting duct actively absorb excess potassium and hydrogen
ions from the nearby capillaries and secrete these excess ions as waste into
the filtrate.
When filtrate
reaches the end of the collecting duct, almost all of the valuable nutrients,
ions, and water have been returned to the blood supply while waste products and
a small amount of water are left to form urine. The urine exits the collecting
duct and joins with urine from other collecting ducts in the renal pelvis.
Water Homeostasis
The kidneys are able to control the volume of water in the
body by changing the reabsorption of water by the tubules of the nephron. Under
normal conditions, the tubule cells of the nephron tubules reabsorb (via
osmosis) nearly all of the water that is filtered into urine by the glomerulus.
Water reabsorption leads to very concentrated urine and the
conservation of water in the body. The hormones antidiuretic hormone (ADH) and
aldosterone both increase the reabsorption of water until almost 100% of the
water filtered by the nephron is returned to the blood. ADH stimulates the
formation of water channel proteins in the collecting ducts of the nephrons
that permit water to pass from urine into the tubule cells and on to the blood.
Aldosterone functions by increasing the reabsorption of Na+ and Cl- ions,
causing more water to move into the blood via osmosis.
In situations where there is too much water present in the
blood, our heart secretes the hormone atrial natriuretic peptide (ANP) in order
to increase the excretion of Na+ and Cl- ions. Increased concentration of Na+
and Cl- in urine draws water into the urine via osmosis, increasing the volume
of urine produced.
Acid/Base Homeostasis
The kidneys regulate the pH level of the blood by
controlling the excretion of hydrogen ions (H+) and bicarbonate ions (HCO3-).
Hydrogen ions accumulate when proteins are metabolized in the liver and when
carbon dioxide in the blood reacts with water to form carbonic acid (H2CO3).
Carbonic acid is a weak acid that partially dissociates in water to form
hydrogen ions and bicarbonate ions. Both ions are filtered out of the blood in
the glomerulus of the kidney, but the tubule cells lining the nephron selectively
reabsorb bicarbonate ions while leaving hydrogen ions as a waste product in
urine. The tubule cells may also actively secrete additional hydrogen ions into
the urine when the blood becomes extremely acidic.
The reabsorbed bicarbonate ions enter the bloodstream where
they can neutralize hydrogen ions by forming new molecules of carbonic acid.
Carbonic acid passing through the capillaries of the lungs dissociates into
carbon dioxide and water, allowing us to exhale the carbon dioxide.
Electrolyte Homeostasis
The kidneys maintain the homeostasis of important
electrolytes by controlling their excretion into urine.
Sodium (Na+):
Sodium is a vital electrolyte for muscle function, neuron function, blood
pressure regulation, and blood volume regulation. Over 99% of the sodium ions
passing through the kidneys are reabsorbed into the blood from tubular
filtrate. Most of the reabsorption of sodium takes place in the proximal
convoluted tubule and ascending loop of Henle.
Potassium (K+):
Just like sodium, potassium is a vital electrolyte for muscle function, neuron
function, and blood volume regulation. Unlike sodium, however, only about 60 to
80% of the potassium ions passing through the kidneys are reabsorbed. Most of
the reabsorption of potassium occurs in the proximal convoluted tubule and
ascending loop of Henle.
Chloride (Cl-):
Chloride is the most important anion (negatively charged ion) in the body.
Chloride is vital to the regulation of factors such as pH and cellular fluid
balance and helps to establish the electrical potential of neurons and muscle
cells. The proximal convoluted tubule and ascending loop of Henle reabsorb
about 90% of the chloride ions filtered by the kidneys.
Calcium (Ca2+):
Calcium is not only one of the most important minerals in the body that
composes the bones and teeth, but is also a vital electrolyte. Functioning as
an electrolyte, calcium is essential for the contraction of muscle tissue, the
release of neurotransmitters by neurons, and the stimulation of cardiac muscle
tissue in the heart. The proximal convoluted tubule and the ascending loop of
Henle reabsorb most of the calcium in tubular filtrate into the blood.
Parathyroid hormone increases the reabsorption of calcium in the kidneys when
blood calcium levels become too low.
Magnesium (Mg2+):
Magnesium ion is an essential electrolyte for the proper function of enzymes
that work with phosphate compounds like ATP, DNA, and RNA. The proximal
convoluted tubule and loop of Henle reabsorb most of the magnesium that passes
through the kidney.
Blood Pressure Homeostasis
The kidneys help to control blood pressure in the body by
regulating the excretion of sodium ions and water and by producing the enzyme
renin. Because blood is mostly made of water, an increased volume of water in
the body results in an increase in the volume of blood in the blood vessels.
Increased blood volume means that the heart has to pump harder than usual to
push blood into vessels that are crowded with excess blood. Thus, increased
blood volume leads to increased blood pressure. On the other hand, when the
body is dehydrated, the volume of blood and blood pressure decrease.
The kidneys are able to control blood pressure by either
reabsorbing water to maintain blood pressure or by allowing more water than
usual to be excreted into urine and thus reduce blood volume and pressure.
Sodium ions in the body help to manage the body’s osmotic pressure by drawing
water towards areas of high sodium concentration. To lower blood pressure, the
kidneys can excrete extra sodium ions that draw water out of the body with
them. Conversely, the kidneys may reabsorb additional sodium ions to help
retain water in the body.
Finally, the kidneys produce the enzyme renin to prevent the
body’s blood pressure from becoming too low. The kidneys rely on a certain
amount of blood pressure to force blood plasma through the capillaries in the
glomerulus. If blood pressure becomes too low, cells of the kidneys release
renin into the blood. Renin starts a complex process that results in the
release of the hormone aldosterone by the adrenal glands. Aldosterone
stimulates the cells of the kidney to increase their reabsorption of sodium and
water to maintain blood volume and pressure.
Hormones
The kidneys maintain a small but important endocrine
function by producing the hormones calcitriol and erythropoietin.
Calcitriol is the
active form of vitamin D in the body. Tubule cells of the proximal convoluted
tubule produce calcitriol from inactive vitamin D molecules. At that point,
calcitriol travels from the kidneys through the bloodstream to the intestines,
where it increases the absorption of calcium from food in the intestinal lumen.
Erythropoietin
(EPO) is a hormone produced by cells of the peritubular capillaries in response
to hypoxia (a low level of oxygen in the blood). EPO stimulates the cells of
red bone marrow to increase their output of red blood cells. Oxygen levels in
the blood increase as more red blood cells mature and enter the bloodstream.
Once oxygen levels return to normal, the cells of the peritubular capillaries
stop producing EPO.
Several hormones produced elsewhere in the body help to
control the function of the kidneys.
Antidiuretic
hormone (ADH), also known as vasopressin, is a hormone produced by
neurosecretory cells in the brain’s hypothalamus. These cells extend into the
posterior pituitary, which stores and releases ADH. ADH production is
stimulated by a decrease in blood volume and increased blood osmolarity. ADH
helps the body retain water by increasing the number of water channels in the
cells of the collecting ducts of the kidneys. These water channels allow water
remaining in urine to be reabsorbed into the blood, resulting in extremely
concentrated urine.
Angiotensin II is
a hormone made in the liver and activated by the enzymes renin and
angiotensin-converting enzyme. Once activated, angiotensin II increases the
reabsorption of sodium and chloride ions in the proximal convoluted tubule,
leading to an increased reabsorption of water as well.
Aldosterone is a
hormone produced in the adrenal cortex in response to Angiotensin II.
Aldosterone binds to target cells in the walls of the nephron’s collecting ducts.
These cells reabsorb additional sodium and chloride ions that would have been
excreted as urine. The target cells also remove potassium ions from the blood
and excrete it into urine.
Atrial natriuretic
peptide (ANP) is a hormone produced by cardiac muscle cells in the atria of the
heart. These cells produce ANP in response to high levels of sodium in the
blood or increased blood pressure. In the kidneys, ANP increases the glomerular
filtration rate so that more blood plasma is forced into the glomerular capsule
and into the renal tubules. ANP also removes some solutes from the cells of the
renal medulla, making the loop of Henle less efficient in reabsorbing water and
ions from the filtrate. The net result of ANP is that more sodium and water end
up being excreted into urine, blood volume decreases, and blood pressure
decreases as well.
