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
The primary function of the renal system is to maintain the constancy of the internal environment (Homeostasis)
• 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
• 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.
• 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
• 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
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.
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
In addition to reabsorbing substances, they secrete H+ and K+ ions, as well as ammonia, creatinine, uric acid and other metabolites
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
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