Transcript Blood Supply of a Nephron

By Matthew M. Green and Carlos
Krumpach
The purpose of the urinary system is to
 remove certain salts and nitrogenous wastes from the body,
 maintain normal concentrations of water and electrolytes
within body fluids,
 regulate the pH and volume of body fluids,
 and help control red blood cell production and blood volume.
The Urinary System consists of
 a pair of kidneys,
 a pair of tubular ureters,
 a saclike urinary bladder,
 and a tubular urethra
A kidney is a reddish-brown, bean-shaped organ, with a smooth
surface. (Adult kidneys are 12 centimeters long, 6 centimeters
wide, and 3 centimeters thick.) They are enclosed in a strong,
fibrous capsule.
The kidneys lie on either side of the vertebral column in a
depression high on the posterior wall of the abdominal cavity. The
upper and lower borders of the kidneys are usually at the levels of
the twelfth thoracic and third lumbar vertebrae. (The left kidney is
1.5-2.0 centimeters higher than the right kidney.) The kidneys are
positioned retroperitoneally, or behind the parietal peritoneum and
against the deep muscles of the back. (The kidneys are held in
place by connective tissue and adipose tissue.)
The kidneys later surface is convex, while its medial side is concave.
The resulting medial depression leads to the renal sinus, a hallow
chamber. The entrance to the renal sinus is called the hilium, this
where blood vessels, nerves, lymphatic vessels and the ureter enter the
kidney. The superior end of the ureter expands and forms a funnelshaped sac called the renal pelvis. The renal pelvis subdivides into two
or three major calyces, which subdivide into several minor calyces.
Renal papillae project into the renal sinus from its wall, and contain
openings that lead into a minor calyx. The inside of the kidney is the
renal medulla, which is composed of renal pyramids and appears
striated. The renal cortex forms a shell around the medulla and dips in
between renal pyramids, forming renal columns. Within in the renal
cortex are nephrons, or the kidneys functional units.
The kidneys help maintain homeostasis by regulating the
composition, volume, and pH of extracellular fluid. They do this
by forming, and excreting urine, a combination of metabolic
wastes, (from the blood), water, and electrolytes. The kidneys also
secrete the hormone erythropoietin, which helps control red blood
cell production, they help activate vitamin D, and they help
maintain blood volume and pressure by releasing the enzyme
renin.
Renal arteries supply blood to the kidneys. When a person is at rest
these arteries usually carry 15-30% of the total cardiac out put into
the kidneys. Renal arteries give off several branches called
interlobar arteries, which pass between renal pyramids. Arcuate
arteries at the junction between the medulla and cortex give rise to
interlobular arteries. Afferent arterioles, the final branches of
interlobular arteries, lead to nephrons. Venous blood returns in
vessels that correspond to arterial pathways, which then lead to the
renal vein. The renal vein leads the blood back to the inferior vena
cava.
Nephrons consist of a renal corpuscle and a renal tubule. Fluid
flows through the tubules on its way out of the body. Renal
corpuscles are made of tangled clusters of blood capillaries called
glomerulus. Glomerular capillaries filter fluid, which is the first
step in urine formation. The glomerular capsule, a thin-walled,
saclike structure surrounds the glomerulus. The glomerular
capsule, an expansion at the proximal end of a renal tubule,
receives the fluid the glomerulus filters. The renal tubule leads
away from the capsule and becomes tightly coiled, this coiled
portion is the proximal convoluted tubule. This tubule dips towards
the renal pelvis, and becomes the descending limb of the nephron
loop. The tubule then curves back towards the renal corpuscle and
forms the ascending limb of the nephron loop. This tubule returns
to the region of the renal corpuscle, where it is tightly coiled and is
called the distal convoluted tubule. These tubules merge in the
renal cortex to form collecting ducts, which passes through the
renal medulla and is joined by other tubules. The resulting tube
empties into a minor calyx through an opening in a renal papilla.
Blood Supply of a Nephron
The capillaries that form a glomerulus all come from arterioles.
Blood, after passing through the glomerular capillaries, enters an
efferent arteriole, (that is smaller than the afferent vessel). The
arteriole resists blood flow, which raises blood pressure in the
glomular capillary. The efferent arteriole branches off into a
freely interconnecting network of capillaries called the
peritubular capillary system, that surrounds the renal tubule.
After flowing through the peritubular capillary system, the blood
rejoins from other branches and enters the venous system of the
kidney.
Juxtaglomerular Apparatus
The structure of densely packed epithelial cells at the point of
contact between the distal convoluted tubule and efferent and
afferent arterioles makes up the macula densa. Within the walls
of arterioles are juxtaglomerular cells, that with the cells of the
macula densa form the juxtaglomerular apparatus. This apparatus
controls the secretion of renin.
Urine formation begins with glomerular filtration, which is the
process of the glomerular capillaries filtrating plasma. Tubular
reabsorption, which moves substances from tubular fluid back into
the blood within the peritubular capillary, and tubular secretion,
which is the reverse process of tubular absorption, also aid in
forming urine.
Urine formation begins with water and certain dissolved
substances being filtered out. The resulting glomerular filtrate is
sent into glomerular capsules.
The hydrostatic pressure of blood and the osmotic pressure of
plasma influence the pressure of glomerular capillaries, and
capsules. The net filtration pressure is the net pressure for forcing
substances out of the glomerulus. The net filtration pressure is
responsible for the kidney’s filtration rate.
The glomerular filtration rate is an example of a negative feedback
mechanism. The filtration rate can also be regulated by
sympathetic nervous system reflexes. Juxtaglomerular cells also
control the rate by releasing renin, which reacts with the plasma
protein angiotensinogen to form angiotensin I. A second enzyme
called angiotensin converting enzyme, converts angiotensin I to
angiotensin II. Angiotensin II is a plasma protein that helps
maintain sodium balance, water balance, and blood pressure in the
body.
Tubular reabsorption is the process in which filtered substances are
returned to the blood stream. Sodium and water can be reabsorbed
multiple times making them more concentrated. Tubular secretion
is when substances move from the bloods plasma in the peritubular
capillary to fluid in the renal tubule.
Aldosterone and antidiuretic hormone may stimulate the additional
reabsorption of sodium and water. These changes in sodium and
water excretion in response to these hormones are the final
adjustments the kidney makes before urination.
Urea is a by-product of amino acid catabolism, uric acid is a result
of the metabolism of organic bases in nucleic acids. Both of these
by-products are excreted in urine. Urine composition reflects the
amount of water and solutes that the kidney must eliminate or
retain.
From the kidneys, urine passes through the renal pelvis, and a
ureter conveys it to the urinary bladder and reference plate. The
urethra then excretes urine to the outside.
The ureters are 25 centimeters long, they begin as the renal pelvis,
and connect to the urinary bladder. The urinary bladder is a hollow,
distensible, muscular organ that stores urine and forces it out into
the urethra. The detrusor muscle is a muscle that prevents the
bladder from emptying until a certain pressure is reached.
Micturition is the process that conveys urine from the urinary
bladder to the outside of the body.
Dysuria is a condition that causes painful urination.
Nephrolithiasis, which is a kidney stone.
Cystis, which is an inflammation of the urinary bladder.
Butler, Jackie. Lewis, Ricki. Shier, David, ed. Hole’s Essentials of
Human Anatomy and Physiology. New York: McGrawHill.2006. 454-471. Print.