The midwife must have a sound knowledge of the anatomy of the structures of the urinary tract and the basics of normal renal physiology to then understand the changes that take place during pregnancy and how they may impact on the health and wellbeing of the childbearing woman.
OBJECTIVES
• provide an
overview of the anatomy and functions of the various structures of the urinary
system
• describe the
processes of excretion, elimination and homeostatic regulation of the volume
and solute concentration of blood plasma
• explain how
urine is produced and eliminated through the process of micturition
• provide an
overview of how the physiological effects of pregnancy and its hormonal
influences may impact on the functioning of the urinary tract
PARTS OF THE URINARY TRACT
The urinary system's function is to filter blood and create urine as a waste by-product. The parts of the urinary tract include the kidneys, ureters, bladder and urethra.
1. THE KIDNEYS
The
kidneys are excretory glands with both endocrine and exocrine functions. They
perform the excretory functions of the urinary system by removing metabolic
waste products from the circulation to produce urine. In addition to removing
waste products the urinary system has a broad range of other essential
homeostatic functions
Functions of the kidney
• Regulation of water balance
• Regulation of blood pressure (renin–angiotensin system)
• Regulation of pH (acid–base balance) and inorganic ion balance (potassium, sodium and calcium)
• Control of formation of red blood cells (via erythropoietin)
• Secretion of hormones – renin, erythropoietin, 1.25-dihydroxyvitamin D3 (1,25-dihydroxycholecatciferol (also called calcitriol) and prostaglandins
• Vitamin D activation and calcium balance
• Gluconeogenesis (formation of glucose from amino acids and other precursors)
• Excretion of metabolic and nitrogenous waste products (urea from protein, uric acid from nucleic acids, creatinine from muscle creatine and hemoglobin breakdown products)
• Removal of toxic chemicals (drugs, pesticides and food additives)
A
typical adult kidney is a bean-shaped reddish-brown organ. Each kidney is about
10 cm long, 6.5 cm wide, 3 cm thick and weighs about 100g. Although similar in
shape, the left kidney is a longer and more slender organ than the right
kidney.
Congenital absence of one or both kidneys, known as unilateral or bilateral renal agenesis, can occur (Jones 2012). Bilateral renal agenesis is uncommon but is a serious failure in the development of both kidneys in the fetus. It is one causative agent of the Potter sequence (also known as Potter’s syndrome). This absence of fetal kidneys causes oligohydramnios, a deficiency of amniotic fluid in a pregnant woman which can place extra pressure on the developing fetus and can cause further malformations.
Non-pregnant
adults with unilateral renal agenesis have a considerably higher risk of
developing hypertension, which will become even more pronounced during pregnancy.
Structure of the Kidney
Each kidney has a smooth surface covered by a tough fibrous capsule. There is a concave side facing medially. On this medial aspect is an opening called the hilum. The hilum is the point of entry for the renal artery and renal nerves, and the point of exit for the renal vein and the ureter. Internally the hilum is continuous with the renal sinus.
Each kidney is enclosed by a thick fibrous capsule and has two distinct layers: the reddish-brown renal cortex, which has a rich blood supply, and the inner renal medulla where the structural and functional units of the kidney are located (Coad and Dunstall 2011). The renal medulla lies below the renal cortex and consists of between 8 and 18 distinct cone-shaped structures called medullary or renal pyramids. Each renal pyramid (which is striped in appearance) together with the associated overlying renal cortex forms a renal lobe. The base of each pyramid is broad and faces the cortex, while the pointed apex (papilla) projects into a minor calyx. Several minor calyces open into each of two or three major calyces, which then open into the renal pelvis. The renal pelvis is a flat funnel-shaped tube that is continuous with the ureter. Urine produced by the kidney flows continuously from the renal pelvis into the ureter and then into the bladder for storage (Stables and Rankin 2010).
The nephron
Each kidney has over 1 million nephrons, which are the functional units of the kidney. The nephron is approximately 3 cm long and is a tubule that is closed at one end and opens into the collecting duct at the other (Coad and Dunstall 2011). The nephron has five distinct regions, each of which is adapted to a specific function:
• Bowman’s capsule containing the glomerulus (renal corpuscle)
• the proximal convoluted tubule
• the loop of Henle
• the distal convoluted tubule and
• the collecting duct.
There are two types of nephrons: cortical nephrons and juxtomedullary nephrons. The majority are cortical nephrons (85–90%) and these have short loops of Henle. Their main function is to control plasma volume during normal conditions. The juxtamedullary nephrons have longer loops of Henle extending into the medulla. These nephrons facilitate increased water retention when there is restricted water available (Coad and Dunstall 2011).
Each nephron begins at the renal corpuscle, which comprises the Bowman’s capsule, which is a blind-ended cup-shaped chamber, and the glomerulus, a coiled arranged capillary network incorporated within the capsule.
Blood enters the renal corpuscle by way of the afferent arteriole which delivers blood to the glomerulus, with blood leaving by way of the efferent arteriole. This is the only place in the body where an artery collects blood from capillaries. The pressure within the glomerulus is increased because the afferent arteriole has a wider bore than the efferent arteriole and this factor forces the filtrate out of the capillaries into the capsule. At this stage any substance with a small molecular size will be filtered out.
The cup of the capsule is attached to the tubule of the nephron. The proximal convoluted tubule initially winds and twists through the cortex, then forms a straight loop of Henle that dips into the medulla (descending arm), rising up into the cortex again (ascending arm) to wind and turn as the distal convoluted tubule before joining the straight collecting tubule. The straight collecting tubule runs from the cortex to a medullary pyramid where it forms a medullary ray and receives urine from many nephrons along its length (Martini et al 2011).
The distal convoluted tubule returns to pass alongside granular cells (also known as juxtaglomerular cells) of the afferent arteriole and this part of the tubule is called the macula densa. The granular cells and macula densa are known as the juxtaglomerular apparatus. The granular cells secrete renin whereas the macula densa cells monitor the sodium chloride concentration of fluid passing through.
Position and relations
The
kidneys are situated in the posterior part of the abdominal cavity, one on
either side of the vertebral column between the eleventh thoracic vertebra
(T11) and the third lumbar vertebra (L3) (Jones 2012). The right kidney is
slightly lower than the left kidney owing to its relationship to the liver
(Coad and Dunstall 2011). The anterior and posterior surfaces of the kidneys
are related to numerous structures, some of which come into direct contact with
the kidneys whereas others are separated by a layer of peritoneum:
Posteriorly,
the kidneys are related to rib 12 and the diaphragm, psoas major, quadratus
lumborum and transversus abdominis muscles.
Anteriorly,
the right kidney is related to the liver, duodenum, ascending colon and small
intestine. The left kidney is related to the spleen, stomach, pancreas, descending
colon and small intestine.
The
triangular-shaped adrenal (suprarenal) glands are situated in the upper pole of
the kidneys (Coad and Dunstall 2011).
Supports of the kidney
The
kidneys are maintained in position within the abdominal cavity by the overlying
peritoneum, contact with adjacent visceral organs, such as the gastrointestinal
tract, and by supporting connective tissue.
Blood supply to the kidney
The
kidneys receive about 20–25% of the total cardiac output (Jones 2012). In
healthy individuals, about 1200 ml of blood flows through the kidneys each
minute.
This
is a phenomenal amount of blood for organs that have a combined weight of less
than 300 g to experience (Martini et al 2011).
Each
kidney receives blood through the renal artery, which originates from the
lateral surface of the descending abdominal aorta near the level of the
superior mesenteric artery. The artery enters at the renal hilum, transmitting numerous
branches into the cortex to form the glomerulus for each nephron. Blood is
collected up and returned via the renal vein.
Lymphatic drainage in the kidney
A
rich supply of lymph vessels lies under the cortex and around the urine-bearing
tubules. Lymph drains into large lymphatic ducts that emerge from the hilum and
lead to the aortic lymph glands.
Nerve supply to the kidney
The
kidneys are innervated by renal nerves. A renal nerve enters each kidney at the
hilum and follows tributaries of renal arteries to reach individual nephrons.
The sympathetic innervation adjusts rates of urine formation by changing blood
flow and blood pressure at the nephron and mobilizes the release of renin,
which ultimately restricts losses of water and salt in urine by stimulating re-absorption
at the nephron (Martini et al 2011).
Endocrine activity of the Kidney
The
kidney secretes two hormones: renin and erythropoietin. Renin is produced in
the afferent arteriole and is secreted when the blood supply to the kidneys is
reduced and in response to lowered sodium levels. It acts on angiotensinogen,
which is present in the blood, to form angiotensin, which raises blood pressure
and encourages sodium reabsorption. The kidneys produce the hormone
erythropoietin, in response to low oxygen levels that stimulate an increase in
the production of red blood cells from the bone marrow (Coad and Dunstall
2011).
URINE AND URINE FORMATION
Urine
is usually acid and contains no glucose or ketones, nor should it carry blood
cells or bacteria. The amber color is due to the bile pigment urobilin and the color
varies depending on the concentration. In the newborn baby, it is almost clear.
The volume and final concentration of urea and solutes depend on fluid intake.
An
adult can void between 1000 ml and 2000ml of urine daily. Urine has a
characteristic smell, which is not unpleasant when fresh. Strong odor or
cloudiness generally indicates a bacterial infection.
Women
are susceptible to urinary tract infection, usually due to ascending infection
acquired via the urethra. A colony bacterial count of more than 100000/ml is
considered to be pathologically significant and is often referred to as
bacteraemia (Coad and Dunstall 2011).
Characteristics of urine
|
Characteristics
|
Normal
range |
|
pH
|
4.5–8.0 (average 6.0) |
|
Specific gravity |
1.010–1.030 |
|
Osmotic
concentration (osmolarity) |
855–1335
mOsmol/l |
|
Water content |
93–97% |
|
Volume |
Varies
depending on intake but usually 1000–1500 ml/day |
|
Color |
Clear pale straw (dilute) Dark brown (very concentrated) Clear (in babies) |
|
Odor
|
Varies
with composition |
|
Bacterial content |
None (sterile) |
The production of urine
The
production of urine takes place in three stages: filtration, selective
reabsorption and secretion.
Filtration
Filtration
is a largely passive, non-selective process that occurs through the
semipermeable walls of the glomerulus and glomerular capsule. Fluids and
solutes are forced through the membrane by hydrostatic pressure. The passage of
water and solutes across the filtration membrane of the glomerulus is similar
to that in other capillary beds: moving down a pressure gradient. However, the glomerular
filtration membrane is thousands of times more permeable to water and solutes,
and glomerular pressure is much higher than normal capillary blood pressure
(Stables and Rankin 2010). Water and small molecules such as glucose, amino
acids and vitamins escape through the filter as the filtrate and enter the
nephron, whereas blood cells, plasma proteins and other large molecules are
usually retained in the blood. The content of the Bowman’s capsule is referred
to as the glomerular filtrate (GF) and the rate at which this is formed is
referred to as the glomerular filtration rate (GFR). The kidneys form about 180
l of dilute filtrate each day (125 ml/min). Most of this is selectively
reabsorbed so that the final volume of urine produced daily is about 1000–1500
ml/day (Coad and Dunstall 2011).
Selective Reabsorption
Substances
from the glomerular filtrate are reabsorbed from the rest of the nephron into
the surrounding capillaries. Some substances, such as amino acids and glucose, are
completely reabsorbed and are not normally present in urine. The reabsorption
of other substances is under the regulation of several hormones. Water balance
is mainly regulated by the antidiuretic hormone (ADH)
produced by the posterior pituitary gland. This is regulated through a negative
feedback loop.
The
secretion of ADH is initiated by an increase in plasma osmolality, by a decrease
in circulating blood volume and by lowered blood pressure (e.g. through reduced
fluid intake or sweating). The action of ADH is to increase permeability of the
renal tubular cells. More water is reabsorbed, resulting in reduced volume of
more concentrated urine. When the body has sufficient fluid intake and
physiological parameters are within normal range then the production of ADH is
inhibited and urine increases in volume and is more dilute. One exception to note
relates to the consumption of alcohol, which inhibits the effect of ADH on the
kidneys, thereby inducing diuresis that is out of proportion to the volume of
fluid ingested (Weise et al 2000). Newborn babies have poor ability to
concentrate and dilute their urine and this is even more so for preterm
infants. For this reason, they are unable to tolerate wide variations in their
fluid intake.
Minerals
are selected according to the body’s needs. Calcitonin increases calcium
excretion and parathyroid hormone enhances reabsorption of calcium from the
renal tubules (Coad and Dunstall 2011). The reabsorption of sodium is
controlled by aldosterone, which is produced in the cortex of the suprarenal
gland. The interaction of aldosterone and ADH maintains water and sodium balance.
It is vital that the pH of the blood is controlled in the body and if it is
tending towards acidity then acids will be excreted in urine. However, if the
opposite situation arises then alkaline urine will be produced. Often this is
the result of an intake of an alkaline substance. A diet high in meat and
cranberry juice will keep the urine acidic whilst a diet rich in citrus fruit,
most vegetables and legumes will keep the urine alkaline. Bacteria causing a urinary
tract infection or bacterial contamination will also produce alkaline urine.
Secretion
Tubular
secretion is an important mechanism in clearing the blood of unwanted
substances. Secreted substances into the urine include hydrogen ions, ammonia,
creatinine, drugs and toxins.
2. THE URETERS
The
ureters are hollow muscular tubes. The upper end is funnel-shaped and merges
into the renal pelvis, where urine is received from the renal tubules.
Function of the ureters
The
ureters transport urine from the kidneys to the bladder by waves of
peristalsis. About every 30 seconds a peristaltic contraction begins at the
renal pelvis and sweeps along the ureter, forcing urine towards the urinary
bladder (Martini et al 2011).
Structure of the
Each
ureter is about 0.3 cm in diameter and 25–30 cm long, running from the renal
hilum to the posterior wall of the bladder. The ureters extend inferiorly and
medially, passing over the anterior surfaces of the psoas major muscle and are firmly
attached to the posterior abdominal wall. At the pelvic brim the ureters
descend along the side walls of the pelvis to the level of the ischial spines
and then turn forwards to pass beside the uterine cervix and enter the bladder
from behind. The ureters penetrate the posterior wall of the urinary bladder
without entering the peritoneal cavity. They pass through the bladder wall at
an oblique angle, and the ureteral openings are slit-like rather than rounded.
This shape helps prevent the backflow of urine toward the ureter and kidneys
when the urinary bladder contracts (Martini et al 2011).
Layers of the
The
ureters are composed of three layers: an inner lining, a middle muscular layer
and an outer coat (Martini et al 2011). The inner lining comprises of
transitional epithelium arranged in longitudinal folds. This type of epithelium
consists of several layers of pear-shaped cells and makes an elastic and
waterproof inner coat.
The
middle muscular layer is made up of longitudinal and circular bands of smooth
muscle. The outer coat comprises of fibrous connective tissue that is
continuous with the fibrous capsule of the kidney.
Blood supply to the ureter
The
blood supply to the upper part of the ureter is similar to that of the kidney.
In its pelvic portion, it derives blood from the common iliac and internal
iliac arteries and from the uterine and vesical arteries, according to its
proximity to the different organs. Venous return is along corresponding veins.
Lymphatic drainage in the ureter
Lymph
drains into the internal, external and common iliac nodes.
Nerve supply to the ureter
The
nerve supply is from the renal, aortic, superior and inferior hypogastric plexuses.
3. THE BLADDER
The
bladder is a distensible, hollow, muscular, pelvic organ that functions as a
temporary reservoir for the storage of urine until it is convenient for it to
be voided.
Pregnancy
and childbirth can affect bladder control and thus midwives need to be familiar
with the anatomy and physiology of the bladder.
Position,
shape and size of the
The
empty bladder lies in the pelvic cavity and is described as being pyramidal
with its triangular base resting on the upper half of the vagina and its apex
directed towards the symphysis pubis. However, as it fills with urine it rises
up out of the pelvic cavity becoming an abdominal organ and more globular in
shape as its walls are distended. It can be palpated above the symphysis pubis
when full. During labor the bladder is an abdominal organ, as it is displaced
by the fetus as it descends into the pelvic cavity.
The empty bladder is of similar size to the uterus, but when full of urine it becomes much larger. The normal capacity of the bladder is approximately 600 ml although the capacity in individuals does vary between 500 ml (Stables and Rankin 2010) and 1000 ml (Martini et al 2011).
Structure
The base of the bladder is termed the trigone. It is situated at the back of the bladder, resting against the vagina. Its three angles are the exit of the urethra below and the two slit-like openings of the ureters above. The apex of the trigone is thus at its lowest point, which is also termed the neck.
The anterior part of the bladder lies close to the symphysis pubis and is termed the apex of the bladder. From the apex of the bladder, the urachus runs up the anterior abdominal wall to the umbilicus. In fetal life, the urachus is the remains of the yolk sac but in the adult is simply a fibrous band.
Layers of the bladder
The lining of the bladder, like that of the ureter, is formed of transitional epithelium, which helps to allow the distension of the bladder without losing its water-holding effect. The lining, except over the trigone, is thrown into rugae, which flatten out as the bladder expands and fills.
The mucous membrane lining lies on a submucous layer of areolar tissue that carries blood vessels, lymph vessels and nerves.
The epithelium over the trigone is smooth and firmly attached to the underlying muscle. The musculature of the bladder consists chiefly of the large detrusor muscle whose function is to expel urine. This muscle has three coats of smooth muscle: an inner longitudinal, a middle circular and an outer longitudinal layer. Around the neck of the bladder, the circular muscle is thickened to form the internal urethral sphincter (Stables and Rankin 2010). The general elasticity of the numerous muscle fibers around the bladder neck tends to keep the urethra closed (Standring 2009). In the trigone, the muscles are somewhat differently arranged. A band of muscle between the ureteric openings forms the interureteric bar. The urethral dilator muscle lies in the ventral part of the bladder neck and the walls of the urethra and it is thought to be of significance in overcoming urethral resistance to micturition (Standring 2009).
The outer layer of the bladder is formed of visceral pelvic fascia, except on its superior surface, which is covered with peritoneum.
Relations to the bladder
•
Anterior to the bladder is the symphysis pubis, which is separated from it by a
space filled with fatty tissue called the Cave of Retzius.
•
Posterior to the bladder is the cervix and ureters.
•
Laterally are the lateral ligaments of the bladder and the side walls of the
pelvis.
•
Superiorly lie the intestines and peritoneal cavity. In the non-pregnant female,
the anteverted, ante-flexed uterus lies partially over the bladder.
• Inferior to the bladder is the urethra and the muscular diaphragm of the pelvic floor, which forms its main support, and on which its function partly depends.
Supports of the bladder
There
are five ligaments attached to the bladder (Stables and Rankin 2010). A fibrous
band called the urachus extends from the apex of the bladder to the umbilicus. Two
lateral ligaments extend from the bladder to the side walls of the pelvis. Two
pubovesical ligaments attach from the bladder neck anteriorly to the symphysis
pubis and they also form part of the pubocervical ligaments of theuterus.
Blood supply to of the bladder
Blood
supply is from the superior and inferior vesical arteries and drainage is by
the corresponding veins.
Lymphatic drainage in the bladder
Lymph
drains into the internal iliac and the obturator nodes.
Nerve supply to the bladder
The
nerve supply is parasympathetic and sympathetic and comes via the Lee–Frankenhauser
pelvic plexus in the pouch of Douglas. The stimulation of sympathetic nerves
causes the internal urethral sphincter to contract and the detrusor muscle to
relax, whereas the parasympathetic nerve fibers cause the sphincter to relax
and the bladder to empty.
4. THE URETHRA
In
the female the urethra is a narrow tube, about 4 cm long, that is embedded in
the lower half of the anterior vaginal wall. It passes from the internal meatus
of the bladder to the vestibule of the vulva, where it opens externally as the
urethral meatus. The internal sphincter surrounds the urethra as it leaves the
bladder. As the urethra passes between the levator ani muscles it is enclosed
by bands of striated muscle known as the membranous sphincter of the urethra,
which is under voluntary control (Stables and Rankin 2010). During labor, the
urethra becomes elongated as the bladder is drawn up into the abdomen,
extending several centimeters.
Structure of the Urethra
The
urethra forms the junction between the urinary tract and the external genitalia.
The epithelium of its lining reflects this. The upper half is lined with
transitional epithelium whereas the lower half is lined with squamous epithelium.
The lumen is normally closed unless urine is passing down it or a catheter is
in situ. When closed, it has small longitudinal folds. Small blind ducts called
urethral crypts (of which the two largest are the paraurethral glands or
Skene’s ducts) open into the urethra near the urethral meatus (Martini et al
2011).
The
submucous coat of the urethra is composed of epithelium, which lies on a bed of
vascular connective tissue. The musculature of the urethra is arranged as an
inner longitudinal layer, continuous with the inner muscle fibres of the
bladder, and an external circular layer. The inner muscle fibres help to open
the internal urethral sphincter during micturition.
The
outer layer of the urethra is continuous with the outer layer of the vagina and
is formed of connective tissue. At the lower end of the urethra, voluntary,
striated muscle fibres form the so-called membranous sphincter of the urethra.
This is not a true sphincter but it gives some voluntary control to the woman
when she desires to resist the urge to void urine. The powerful levator ani
muscles, which pass on either side of the uterus, also assist in controlling
continence of urine.
Blood supply to the Urethra
The
blood to the urethra is circulated by the inferior vesical and pudendal
arteries and veins.
Lymphatic drainage in the Urethra
Lymph
drains through the internal iliac glands.
Nerve supply to the Urethra
The
internal urethral sphincter is supplied by sympathetic and parasympathetic
nerves but the membranous sphincter is supplied by the pudendal nerve and is
under voluntary control.
MICTURITION
The
process of micturition (urination) is a coordinated response that is due to the
contraction of the muscular wall of the bladder, reflex relaxation of the
internal sphincter of the urethra and voluntary relaxation of the external
sphincter (Coad and Dunstall 2011). As the bladder fills with urine, stretch
receptors in the wall of the urinary bladder are stimulated which then relay
parasympathetic sensory nerve impulses to the brain generating awareness of
fluid pressure in the bladder. This usually occurs when the bladder contains
approximately 200–300 ml of urine (with increasing discomfort as the
volume
increases). The urge to micturate can be voluntarily resisted and postponed
until a suitable time. This is due to the conscious descending inhibition of
the reflex bladder contraction and relaxation of the external sphincter. If the
urge to micturate is not voluntarily resisted then the bladder will empty of
urine by the muscle wall contracting, the internal sphincter opening by the
action of Bell’s muscles and voluntary relaxation of the external sphincter.
This is assisted by the increased pressure in the pelvic cavity as the
diaphragm is lowered and the abdominal muscles contract. The tone of the external
sphincter is also affected by psychological stimuli (such as waking or leaving
home) and external stimuli (such as the sound of running water). Any factor
that raises the intra-abdominal and intra-vesicular pressures (such as laughter
or coughing) in excess of the urethral closing pressure can result in
incontinence (Coad and Dunstall 2011).
Infants
lack voluntary control over micturition because the necessary corticospinal
connections have yet to be established (Martini et al 2011). Cortical control
of micturition occurs from learned behavior and is usually achieved by about 2
years of age.
Female Urinary Tract Infection
Most urinary tract infections are bladder infections. A bladder infection usually is not serious if it is treated right away. If you do not take care of a bladder infection, it can spread to your kidneys. A kidney infection is serious and can cause permanent damage.
Causes of Urinary tract infection
Usually, germs get into the system through the urethra, the tube that carries urine from the bladder to the outside of the body. The germs that usually cause these infections live in the large intestine and are found in the stool. If these germs get inside the urethra, they can travel up into the bladder and kidneys and cause an infection.
Women tend to get more bladder infections than men. This is probably because women have shorter urethras, so it is easier for the germs to move up to their bladders. Having sex can make it easier for germs to get into the urethra.
Symptoms of Urinary tract infection
A person with a urinary tract infection has any of these symptoms:
· feel pain or burning when urinating.
· feel the urge to urinate often, but not much urine comes out
· stomach feels tender or heavy.
· urine is cloudy or smells bad.
· have pain on one side of the back under the ribs. This is where the kidneys are. 0 have fever and chills.
· have nausea and vomiting.
Treatment of Urinary tract infection
Antibiotics will usually cure a bladder infection. It may help to drink lots of water and other fluids and to urinate often, emptying the bladder each time.
CHANGES TO THE URINARY TRACT IN PREGNANCY AND CHILDBIRTH
The
urinary system can be markedly stressed by pregnancy, mostly because of its
close proximity to the reproductive organs and the major changes in fluid
balance resulting in fluid retention during pregnancy (Coad and Dunstall 2011).
In pregnancy the enlarging uterus affects all the parts of the urinary tract at various times.
In
early pregnancy, bladder capacity is compromised by the growing uterus within
the pelvic cavity which is relieved once the uterus becomes an abdominal organ.
Once the presenting part engages through the pelvic brim in late pregnancy this
again restricts space available for bladder capacity. The hormones of pregnancy
also have an influence on the urinary tract. Under the influence of
progesterone, bladder capacity increases to about 1000 ml by late pregnancy and
the walls of the ureters relax, which allows them to dilate, bend or ‘kink’. If
this occurs in the ureters, then it tends to result in a slowing down or stasis
of urinary flow, causing women to be more at risk from infection.
During
pregnancy large amounts of urine are produced due to an increase in glomerular
filtration as this helps to eliminate the additional wastes created by maternal
and fetal metabolism. In labor, the urethra becomes elongated as the bladder is
drawn up into the abdomen. During the postnatal period there is a rapid and
sustained loss of sodium and a major diuresis occurs, especially on the 2nd to
5th postnatal day. A normal urine output for a woman during this time may be up
to 3000 ml/day with voiding of 500–1000 ml at any one micturition (Stables and
Rankin 2010).
CONCLUSION
The
kidneys are excretory glands with both endocrine and exocrine functions. Urine
produced by the kidney flows continuously from the renal pelvis into the ureter
and then into the bladder for storage. The three major functions are:
excretion, elimination and homeostatic regulation of the volume and solute
concentration of blood plasma. Water balance is mainly regulated by the antidiuretic
hormone (ADH) through a negative feedback loop.
During
pregnancy the urinary system can be markedly stressed, mostly because of its
close proximity to the reproductive organs, the major changes in fluid balance
and the hormones of pregnancy. It is therefore important that the midwife
recognizes what effects these can have on childbearing women to offer them
appropriate advice and support in relieving any discomfort.
REFERENCES
1.
Coad J, Dunstall M 2011 Anatomy and physiology for midwives, 3rd edn. Churchill
Livingstone Elsevier, Edinburgh
2.
Jones T L 2012 Crash course: renal and urinary system, 4th edn. Mosby Elsevier,
London
3.
Martini F H, Nath J L, Bartholomew E F 2011 Fundamentals of anatomy and physiology,
9th edn. Pearson International, London
4.
Stables D, Rankin J 2010 Physiology in childbearing with anatomy and related
biosciences, 3rd edn. Elsevier, Edinburgh
5.
Standring S 2009 Gray’s anatomy: the anatomical basis of clinical practice, 40th
edn. Churchill Livingstone, New York
6. Weise J G, Shlipak M G, Browner W S 2000 The alcohol hangover. Annals of Internal Medicine 132(11):897–902
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