Download Urinary System

Lec.10
Medical Physiology
Z.H.Al-Zubaydi
Urinary System
The kidneys alone perform the function and manufacture urine in the
process. The other organs of the urinary system; the paired ureters and
the single urinary bladder and urethra, provide temporary storage for
urine or serve as transportation channels to carry it from one body
region to another.
The Kidney
The kidneys are perfect example of hemostatic organs, which maintain
the purity and constancy of internal fluids. They also regulate blood
volume, blood pressure and stimulate RBCs production in bone
marrow.
Nephrons
Each kidney contains over a million tiny structures called nephrons.
Nephrons are the structural and functional units of the kidneys and
responsible for forming urine.
Each nephron consists of two main structures: a glonmerulus, which is
a knot of capillaries, and a renal tubule. The closed end of the renal
tubule is enlarged and cup-shaped and completely surrounds the
glonmerulus. This portion of the renal tubule is called the glomerular
or Bowman's, capsule. The inner (visceral) layer of the capsule is
made up of highly modified octopus-like cells called podocytes.
The rest of the tubule is about 3 cm long. As it extends from the
glomerular capsule, it coils and twists before forming a hairpin loop and
then again becomes coiled and twisted before entering a collecting
tubule called the collecting duct. These different regions of the tubule
have specific names; in order from the glomerular capsule they are the
proximal convoluted tubule (PCT), the loop of Henle, and the
distal convoluted tubule (DCT).
Most nephrons are called cortical nephrons because they are
located almost entirely within the cortex. In a few cases, the nephrons
are called juxtamedullary nephrons because they are situated close
to the cortex-medulla junction, and their loops of Henle dip deep into
the medulla. The collecting ducts, each of which receives urine from
many nephrons, run downward through the medullary pyramids,
giving them their striped appearance. They deliver the final urine
product into the calyces and renal pelvis.
The glomerulus is both fed and drained by arterioles. The afferent
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arteriole, which arises from an interlobular artery, is the "feeder
vessel," and the efferent arteriole receives blood that has passed
through the glomerulus. The glomerulus, specialized for filtration, is
unlike any other capillary bed in the entire body. Because it is both
fed and drained by arterioles, which are high-resistance vessels, and
the afferent arteriole has a larger diameter than the efferent, blood
pressure in the glomerular capillaries is much higher than in other
capillary beds. This extremely high pressure forces fluid and solutes
(smaller than proteins) out of the blood into the glomerular capsule.
Most of this filtrate (99 percent) is eventually reclaimed by the renal
tubule cells and returned to the blood in the peritubular capillary
beds.
The second capillary bed, the peritubular capillaries, arises from
the efferent arteriole that drains the glomerulus. Unlike the highpressure glomerulus, these capillaries are low-pressure, porous
vessels that are adapted for absorption instead of filtration. They
cling closely to the whole length of the renal tubule, where they are
in an ideal position to receive solutes and water from the tubule cells
as these substances are reabsorbed from the filtrate percolating
through the tubule. The peritubular capillaries drain into
interlobular veins leaving the cortex.
Urine Formation
Urine formation is a result of three processes; filtration, tubular
reabsorption, and tubular secretion. Each of these processes is
illustrated in Figure 1.
Filtration is nonselective, passive process. The filtrate that is formed
is essentially blood plasma without blood proteins. Both proteins and
blood cells are normally too large to pass through the filtration
membrane (when either of these appear in the urine, there is some
problem with the glomerular filters). As long as the systemic blood
pressure is normal, filtrate will be formed. If arterial blood pressure
drops too low, the glomerular pressure becomes inadequate to force
substances out of the blood into the tubules, and filtrate formation
stops.
Tubular Reabsorption Besides wastes and excess ions that must be
removed from the blood, the filtrate contains many useful
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substances (including water, glucose, amino acids, and ions), which
must be reclaimed from the filtrate and returned to the blood.
Tubular reabsorption begins as soon as the filtrate enters the
proximal convoluted tubule.
The tubule cells are "transporters" taking up needed substances from
the filtrate and then passing them out their posterior aspect into the
extracellular space, from which they are absorbed into peritubular
capillary blood. Some re-absorption is done passively (for example,
water passes by osmosis), but the reabsorption of most substances
depends on active transport processes, which use membrane carriers
and are very selective.
Needed substances (for example, glucose and amino acids) are usually
entirely removed from the filtrate. Nitrogenous waste products are
poorly reabsorbed. These include urea, formed by the liver as an end
product of protein breakdown when amino acids are used to produce
energy; uric acid, released when nucleic acids are metabolized; and
creatinine, associated with creatine metabolism in muscle tissue.
Various ions are reabsorbed or allowed to go out in the urine, according
to what is needed at a particular time to maintain the proper pH and
electrolyte composition of the blood.
Tubular Secretion is essentially reabsorption in reverse. Some
substances, such as hydrogen and potassium ions and creatinine, also
move from the blood of the peritubular capillaries through the tubule
cells or from the tubule cells themselves into the filtrate to be eliminated
in urine. This process seems to be important for getting rid of
substances not already in the filtrate, such as certain drugs, or as an
additional means for controlling blood pH.
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Fig. 1 : Scheme of single large uncoiled nephron.
[ a: Filtration , b: Tubular Reabsorption and c: Tubular
Secretion]
Characteristics of Urine
In 24 hours, the marvelously complex kidneys filter some 150 to 180
liters of blood plasma through their glomeruli into the tubules, only
about 1.0 to 1.8 liters of urine are produced. Filtrate contains
everything that blood plasma does (except proteins), but by the time it
reaches the collecting ducts, the filtrate has lost most of its water and
just about all of its nutrients and necessary ions. What remains, urine,
contains nitrogenous wastes and unneeded substances.
Freshly voided urine is generally clear and pale to deep yellow. The
normal yellow color is due to urochrome, a pigment that results from
the body's destruction of hemoglobin.
Urine pH is usually slightly acid (pH 6), but changes in body
metabolism and certain food may cause it becomes more acidic or
basic.
Solutes normally found in urine include sodium and potassium
ions, urea, uric acid, creatinine, ammonia, bicarbonate ions, and
various other ions, depending on blood composition. With certain
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diseases, urine composition can change dramatically, and the
presence of abnormal substances in urine is often helpful in
diagnosing the problem. This is why a routine urinalysis should
always be part of any good physical examination.
Substances not normally found in urine are glucose, blood
proteins, red blood cells, hemoglobin, white blood cells (pus), and
bile.
The ureters
The ureters are slender tubes running from each kidney to the
bladder. They conduct urine by peristalsis from kidney to bladder.
The bladder
The bladder is a muscular sac posterior to the pubic symphysis. It
has two inlets (ureters) and one outlet (urethra). In males, the
prostate gland surrounds its outlet. The function of the bladder is to
store urine.
The urethra
The urethra is a tube that leads urine from the bladder to the body
exterior. In females, it is 3 to 4 cm long and conducts only urine. In
males, it is 20 cm long and conducts both urine and sperm. The
internal sphincter of smooth muscle is at the bladder-urethra
junction. The external sphincter of skeletal muscle is located more
inferiorly.
Micturition is emptying of the bladder. The micturition reflex
causes the involuntary internal sphincter to open when stretch
receptors in the bladder wall are stimulated. Since the external
sphincter is voluntarily controlled, micturition can ordinarily be
temporarily delayed. Incontinence is the inability to control
micturition.
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