URINARY ELIMINATION
Physiology of urinary elimination:
Renal physiology deals with the study
of structure and functions of the excretory system. The parts included in this
system are
1.
Pair of kidneys
2.
Pair of ureters
3.
Urinary bladder and
4.
Urethra
The primary function of the renal
system is to maintain the constancy of the internal environment (Homeostasis)
KIDNEYS:
•
Situated on either
side of the spinal column
•
Behind the peritoneal
cavity (Retro-peritoneal)
•
Bean shaped organs
•
Extend from the level
of the 12th thoracic vertebra to the 3rd lumbar vertebra
•
Left kidney is higher
than the right because of the anatomical position of the liver.
•
11cm long, 6cm wide,
3cm thick
•
Weighs 150gm
•
Each kidney is
enclosed by a fibrous capsule and supported by a mass of adipose tissue.
•
The functional unit of
the kidney is called the nephron.
•
Each kidney has more
than 1,000,000 nephrons and each nephron is capable of forming urine.
The nephron consists of the glomerulus, Bowman's capsule, proximal
convoluted tubules, loop of Henle, distal tubule and collecting duct.
The glomerulus is a network of blood vessels, surrounded by Bowman's
capsule, where urine formation begins.
The tubules, loop of Henle and collecting duct are passageways that
permit urine to flow to the renal pelvis and then to the ureters.
Functions of Kidneys:
•
Formation of secretion
of urine
•
Production and
secretion of erythropoietin, that hormone that controls formation of red blood
cells
•
Production and
secretion of renin, an important enzyme in the control of blood pressure.
Renin is released from juxtaglomerular cells. Renin functions as an
enzyme to convert angiotensinogen (a substance synthesized by liver) into
Angiotensin I. Angiotensin I is converted to angiotensin II in the lungs.
Angiotensin II causes vasoconstriction and stimulates aldosterone release from
the adrenal cortex Aldosterone causes retention of water, which increases blood
volume
Ureters:
•
The ureters are
tubular structures that enter the urinary bladder
•
Once the urine is
formed in the kidneys, moves through the collecting duct to the
•
Calyces of kidneys and
from there to the ureters
•
The ureters are 25-30
cm in length and 1.25cm in diameter
•
The upper end is
attached to the kidneys and lower end is attached to the bladder.
Bladder:
•
Hollow, distensible,
muscular organ (detrusor muscle) that stores and excretes urine
•
When empty it lies in
the pelvic cavity behind the symphysis pubis.
•
In men the bladder
lies against the anterior wall of the rectum, and in women it rests against the
anterior walls of the uterus and vagina
Urethra:
•
Urine exists the
bladder through the urethra and passes out of the body through the urethral
meatus
•
In
female it is approximately 4 to 6.5cm (1.5 — 2.5 inches)
long
•
predisposes women
&girls get infection.
•
In
male it is 20cm (8 inches) long
•
The internal sphincter muscle situated in
the proximal urethra and the bladder neck is composed of smooth muscles and is
under involuntary control.
•
The external sphincter is under voluntary
control allows the individual to determine the time for urination.
Formation
of Urine:
Urine formation occurs by the process of filtration,
reabsorption and secretion
Filtration
The process of filtration begins at the glomerulus.
The renal arteries bring blood to the kidneys. Blood passes through the
glomerular capillaries, some constituents of the blood are actually filtered.
The RBC and the proteins are too large to be filtered
and remain in the capillary, but most remaining plasma constituents can be
filtered.
Reabsorption
The glomerular filtrate then enters the second segment
of the nephron, the tubule. Substances include varying amounts of water and
electrolytes as well as all glucose and amino acids.
Reabsorption occurs mostly in the Proximal Convoluted
Tubules but also in the distal & collecting tubules.
The tubules reabsorb almost 99% of the glomerular
filtrate. The 1% that remains unabsorbed forms the fluid waste called urine
Secretion
In addition to reabsorbing substances, they secrete H+
and K+ ions, as well as ammonia,
creatinine, uric acid and other metabolites
Micturition:
Micturition is the periodic complete emptying of
urinary bladder, which is normally under voluntary control in an adult. Bladder
capacity varies with the individual but generally ranges from 600 to 1000ml of
urine. Individual are able to sense the desire to urinate when the bladder
contains a smaller amount of urine (150 to 200 ml in an adult and 50 to 100ml
in a child).
The nerves supplying the bladder and urethra belong to
the autonomic and somatic nervous system respectively.
The autonomic nerves are the pelvic and hypogastric
whereas the somatic nerve is the pudic/ pudental.
The pelvic afferent carries the impulses from the
stretch receptors present in the wall of the bladder and sympathetic afferent
carries the pain impulses from the bladder.
At the time of voiding
(micturition)
•
Pelvic afferent
carries the impulses from the stretch receptors present in the wall of the
bladder.
•
Detrusor muscle which
is present in the wall of the bladder, gets the excitatory impulses through the pelvic nerve that is
stimulated by the impulses coming along the pelvic afferent due to the
stimulation of stretch receptors in the walls of the bladder.
•
When the muscle is
contracting the internal urethral sphincter relaxes and urine flows from the
bladder into the urethra
•
Entry of urine into
the urethra will stimulate the stretch receptors present in the posterior
urethra.
•
The somatic afferent
nerve fibers (pudic) carry impulses from here
•
Somatic afferent
impulses ultimately inhibit the efferent excitatory nerves supplying the
external urethral sphincter
•
So the external
urethral sphincter relaxes and the urine gets voided from the body.
•
There will be
reinforcing impulses coming from the pontine centers to the spinal centers,
which facilitate the complete voiding of the urine.
Composition and
characteristics of urine:
•
96% of water and 4%
solutes
•
The normal
constituents of urine will be Na+, K+, bicarbonate, urea, uric acid, and creatinine.
•
Abnormal constituents:
- glucose, proteins, blood, bile salts
In adults, the average amount of urine per void is
approximately 250 to 400mL. All but 5 to 10 ml of urine is typically emptied
from the bladder.
Catheterized clients should drain a minimum of 30ml of
urine per hour. Urine output of less than 30ml/hr may indicate inadequate blood
flow to the kidneys.
Color: - The color of urine ranges
from a light yellow, to a darker yellow, to a dark yellow brown called amber.
The client's state of hydration affects the color.
Bleeding from the kidney or ureters ---- dark red
urine
Bleeding from the bladder or urethra ----- bright red
urine
Various medications ---- bright orange
Clarity: - Normal urine appears
transparent at voiding.
Patients with renal disease urine appears cloudy or foamy
Bacteria and WBC's, contaminants such as prostatic
fluid, sperm or vaginal drianage ---- thick and cloudy
Odour
Urine has a characteristic odour. The more
concentrated the urine, stronger the odour.
Stagnant urine has an ammonia odour.
Sweet or fruity odour is seen in patients with
diabetic mellitus or starvation.
Some foods (e.g. asparagus) cause a musty odor, infected urine fedit odor
and medications can affect the odour of urine.
pH- freshly voided urine
somewhat acidic. High pH (alkaline) — UTI, diet high in fruits and vegetables.
Low pH (acidic) found in starvation, diarrhea, diet high in protein
Concentrated urine has a higher specific gravity;
diluted urine has a lower specific gravity
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