The urinary system's function is to filter blood and create urine as a waste by-product.
The organs of the
urinary system include the kidneys, renal pelvis, ureters, bladder and urethra. The body takes nutrients
from food and converts them to energy.
The urinary system has many functions, including:
• Removing waste: Filters waste products and drugs from the blood and removes them from the
body in urine
• Regulating blood pressure: Releases hormones that help regulate blood pressure
• Regulating blood volume: Adjusts blood volume based on hydration levels
• Regulating electrolytes: Filters electrolytes from the blood, returning some back into circulation
and excreting excess into urine
• Regulating acid-base balance: Controls the loss of hydrogen ions and bicarbonate ions in urine
• Producing red blood cells: Releases erythropoietin, a hormone that stimulates the production of
red blood cells
• Helping with vitamin D production: Helps synthesize calcitriol, the active form of vitamin D .
The urinary system is the main excretory system and consists of the following structures:
• 2 kidneys, which secrete urine
• 2 ureters, which convey the urine from the kidneys to the urinary bladder
• the urinary bladder where urine collects and is temporarily stored
• the urethra through which the urine passes from the urinary bladder to the exterior.
The urinary system plays a vital part in maintaining homeostasis of water and electrolyte concentrations
within the body. The kidneys produce urine that contains metabolic waste products, including the
nitrogenous compounds urea and uric acid, excess ions and some drugs.
The main functions of the kidneys are:
formation and secretion of urine, which regulates total body water, electrolyte and acid–base
balance and enables excretion of waste products
production and secretion of erythropoietin, the hormone that stimulates formation of red blood
cells
production and secretion of renin, an important enzyme in the control of blood pressure
Kidneys are bean-shaped organs, As the kidneys lie on either side of the vertebral column, each is
associated with a different group of structures.
Right kidney
Superiorly – the right adrenal gland
Anteriorly – the right lobe of the liver, the duodenum and the hepatic flexure of the colon
Posteriorly – the diaphragm, and muscles of the posterior abdominal wall
Left kidney
Superiorly – the left adrenal gland
Anteriorly – the spleen, stomach, pancreas, jejunum and splenic flexure of the colon
Posteriorly – the diaphragm and muscles of the posterior abdominal wall
The kidney is composed of about 1–2 million functional units, the nephrons, and a smaller number of
collecting ducts
The nephron consists of a tubule closed at one end, the other end opening into a collecting tubule. The
closed or blind end is indented to form the cup-shaped glomerular capsule (Bowman’s capsule), which
almost completely encloses a network of tiny arterial capillaries, the glomerulus.
The three processes involved – filtration, selective reabsorption and tubular secretion.
Ureters: The ureters are the tubes that carry urine from the kidneys to the urinary bladder. The ureter is
continuous with the funnel-shaped renal pelvis.
The urinary bladder: is a reservoir for urine. It lies in the pelvic cavity and its size and position vary,
depending on the volume of urine it contains. When distended, the bladder rises into the abdominal
cavity.
The bladder is roughly pear shaped, but becomes more oval as it fills with urine. The posterior surface is
the base. The bladder opens into the urethra at its lowest point, the neck.
The urethra is a canal extending from the neck of the bladder to the exterior, at the external urethral
orifice. It is longer in the male than in the female.
The male urethra is associated with the urinary and the reproductive systems. The female urethra is
approximately 4 cm long and 6 mm in diameter. It runs downwards and forwards behind the symphysis
pubis and opens at the external urethral orifice just in front of the vagina.
When 300 to 400 ml of urine have accumulated in the bladder, afferent autonomic nerve fibres in the
bladder wall sensitive to stretch are stimulated.
The micturition reflex is stimulated but sensory impulses also pass upwards to the brain and there is
awareness of the need to pass urine.
COMMON CONDITIONS
Oliguria-Urine output less than 400 ml per day
Haematuria-Presence of blood in the urine. Leaky glomeruli allow red blood cells to escape from the
glomerular capillaries and they cannot be reabsorbed from the filtrate as they are too large. Bleeding in
the urinary tract also causes haematuria
Proteinuria-Presence of protein in the urine. This is abnormal and occurs when leaky glomeruli allow
plasma proteins to escape into the filtrate but they are too large to be reabsorbed
Anuria-Absence of urine
Dysuria-Pain on passing urine, often described as a burning sensation
Glycosuria-Presence of sugar in the urine. This is abnormal and occurs in diabetes mellitus
Ketonuria-Presence of ketones in the urine. This is abnormal and occurs in, e.g., starvation, diabetes
mellitus
Nocturia-Passing urine during the night
Chronic renal failure: This occurs when the renal reserve is lost and there is irreversible damage to about
75% of nephrons. Onset is usually slow and asymptomatic, progressing over several years. The main
causes are diabetes mellitus, glomerulonephritis and hypertension
End-stage renal failure: When death is likely without renal replacement therapy, such as haemodialysis,
peritoneal dialysis or a kidney transplant, the condition is referred to as end-stage renal failure. The
excretory functions of the kidneys are lost, acid–base balance cannot be maintained and endocrine
functions of the kidney are disrupted.
Ureteritis: Inflammation of a ureter is usually due to the upward spread of infection in cystitis.
Cystitis: This is inflammation of the bladder
Urethritis: This is inflammation of the urethra
Acute renal failure: There is a sudden and severe reduction in the glomerular filtration rate and kidney
function that is often reversible over days or weeks if treated.
