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Urinary System consists of 2 kidneys, 2 ureters, a urinary bladder and a urethra.

Nephrology is the study of kidneys.

Urology is a branch of medicine dealing with urinary system and male reproductive system.

FUNCTIONS OF KIDNEYS:

1. Blood ion regulation – Kidneys regulate Na+, Cl-, K+, Ca++, HPO4- -

2. Blood volume regulation – Kidneys eliminate or conserve water, depending on the body’s needs.

3. Blood pressure regulation – Kidneys secrete enzyme renin to control blood pressure.

4. Blood pH regulation – Kidneys excrete H+ and conserve HCO3-

5. RBC production – Kidneys produce hormone erythropoetin that regulates RBC production.

6. Vitamin D synthesis – Kidneys, skin and liver synthesize calcitriol (active form of vitamin D)

7. Elimination of waste and foreign substances – Ammonia, urea, creatinine, bilirubin, toxins & drugs

ANATOMY OF KIDNEYS –

Kidneys are organs shaped like kidney beans.

One kidney lies retroperitoneally (behind peritoneum) on either side of upper abdominal vertebra.

Right kidney is lower than left because of liver.

Kidneys measure about 5” X 3” X 1” in.

Ureter, renal artery, vein, lymphatics and nerves enter and exit the kidneys at the renal hilus.

A tough membrane called renal capsule covers and protects each kidney.

- Fat around the capsule cushions the kidneys and anchors it to the posterior abdominal wall.

Internal anatomy –

Renal cortex is the outer reddish part of the kidneys.

Renal medulla is the inner, striated, red-brown part.

Renal pyramids, 8-18 striated triangular structures in the medulla, consist of tubules & blood vessels.

Renal papillae, the tips of pyramids, face the renal pelvis.

Renal columns are the lighter bands of tissue between the pyramids.

Blood supply –

Kidneys get 1200 ml of blood per minute (25% of cardiac output at rest) via L. & R. renal arteries.

Blood circulates in the kidneys as follows:

Renal artery ? Segmental artery ? Interlobar artery ? Arcuate artery ? Interlobular artery ?

Afferent arteriole ? Glomerular capillaries ? Efferent arteriole ? Peritubular capillaries ?

Interlobular vein? Arcuate vein ? Interlobar vein ? Segemtnal vein ? Renal vein

Nephron –

Each kidney has 1 million microscopic nephrons (functional unit of kidney)

A Nephron has two parts –
1. Renal corpuscle (situated in renal cortex)– filters blood plasma
2. Renal tubule (situated in renal medulla) – transports and modifies glomerular filtrate

1. Renal corpuscle has two parts –
• Glomerulus – capillary network
• Bowman’s (glomerular) capsule – two walled epithelial cup surrounding glomerulus

-Capsular space is the space between the two walls of Bowman’s capsule.

Filtration membrane, across which transfer of water and solutes occurs, is formed by:
• Podocytes (cells of inner wall of Bowman’s capsule), which are adherent to
• Endothelial cells of glomerulus

Glomerular filtrate includes water, glucose, amino acids, small proteins, nitrogenous waste and ions

2. Renal tubule is composed of (proximal to distal):
• Proximal convoluted tubule (PCT)
• Loop of Henle (descending & ascending limb)
• Distal convoluted tubule (DCT)
• Collecting duct
• Papillary duct

Juxtaglomerular apparatus –

Juxtaglomerular apparatus is formed by:

• Macula densa – closely packed columnar cells of (distal) ascending limb of loop of Henle
• Juxtaglomerular cells – modified smooth muscle cells of afferent arteriole
FORMATION OF URINE –

Urine is formed in the nephron and collecting ducts by:

1. GLOMERULAR FILTRATION –

Blood pressure forces water and most small solutes from plasma to the Bowman’s capsule.

Most proteins and blood cells are too large to pass through filtration membrane.

Glomerular filtration rate (GFR) is the amount of filtrate formed in both kidneys (105-125ml/min)
(150 – 180 L / day)
A steady glomerular filtration rate is important for proper elimination and reabsorption of substances.

[Glomerular filtration rate is regulated by:

A) Autoregulation by kidneys – This is carried out by 2 mechanisms:

i) Contraction and relaxation of afferent arteriolar smooth muscle decreases or increases GFR

ii) Macula densa detects increased Na+, Cl- and H2O when GFR is raised.
-It sends feedback to contract afferent arterioles, reducing GFR.

B) Hormonal regulation – Angiotensin II decreases GFR by causing vasoconstriction.
- Atrial Natriuretic peptide increases GFR (secreted when ? blood volume stretches atria).

C) Nerve regulation – Sympathetic stimulation decreases GFR by constricting afferent arterioles ]

2. TUBULAR REABSORPTION –

Occurs in the cells of the renal tubules and collecting ducts.

99% of glomerular filtrate (tubular fluid) is reabsorbed into the blood.

From 150-180 liters of glomerular filtrate, only 1-2 liters of urine is excreted daily.

Most re-absorption occurs in the proximal convoluted tubules.

Na+ is actively reabsorbed in the proximal convoluted tubule.

Water, glucose, amino acids, lactic acid, Cl-, K+, HCO3, Ca++, Mg++ & HPO4 passively follow Na+.
ADH (released by posterior pituitary) causes water re-absorption in the DCT and collecting ducts.

Diuretics (lasix, caffeine, alcohol) inhibit ADH secretion, preventing water re-absorption in kidneys.

3. TUBULAR SECRETION –

Cells of the renal tubules and collecting ducts secrete the following in the urine:
K+,H+, NH4+, creatinine, urea, hormones and drugs
Tubular secretion of H+ maintains blood pH between 7.35-7.45 (and urine pH around 6.0)

Dialysis –

Dialysis is removal of waste products by artificial means, in a person whose kidneys have failed.

Hemodialysis is the filtering of blood by an artificial kidney machine.

- In the machine, a selectively permeable membrane separates blood from dialysis solution.

- Waste products and excess solutes diffuse from the blood into the dialysis solution.

-Cleaned blood is then pumped back into the body.

Continuous ambulatory peritoneal dialysis (CAPD) uses peritoneum as the dialysis membrane.

It is performed on a patient as he goes about his business.

-Sterile dialysis solution is introduced in the peritoneal cavity by gravity.

-Used dialysis solution containing wastes and excess solutes is drained and discarded.

TRANSPORTATION, STORAGE & ELIMINATION OF URINE –

Urine drains as follows –

(Blood in glomerulus filters into capsular space) ? Proximal convoluted tubule ? Loop of Henle

(descending & ascending limbs) ? Distal convoluted tubule ? Collecting duct ? Papillary duct ?

Minor calyx ? Major calyx ? Renal pelvis ? Ureter ?Urinary bladder ? urethra ? elimination

Ureters –

Ureter has 3 layers (inside to outside):

1. Mucous membrane – Mucus secreted by this layer prevents damage by acidic urine.
2. Smooth muscle – Peristalsis, pressure of urine and gravity assist urine passage.
3. Serosa – Outer layer of areolar connective tissue, blood vessels, lymphatics & nerves.

Ureter transport urine from the renal pelvis to the urinary bladder.

It pass behind the bladder and enters lower part of the bladder.

-Thus, pressure of a full bladder prevents backflow of urine into the kidneys (physiological valve)

Urinary bladder –

It is a hollow muscular organ, posterior to symphysis pubis. It stores urine.
Folds of peritoneum keep it in place in front of rectum (males) or vagina (females)

When full, it rises into the abdominal cavity.

It can accommodate about 700-800 ml of urine.

Trigone is an upside down triangle formed at the base of the bladder.

-Openings of Ureters form the two points of the base of this triangle.

-Internal urethral meatus (orifice) forms the apex of this triangle.

Transitional epithelium & rugae in bladder mucous membrane stretch to accommodate more urine.

Detrusor, the bladder muscle, is made of three layers of smooth muscle.

Leakage of urine is prevented by:
• Internal urethral sphincter- a valve of involuntary smooth muscle at the bladder neck
• External urethral sphincter- a valve of voluntary skeletal muscle in the pelvic floor

Outer bladder wall is made of fibrous connective tissue, which is superiorly covered by peritoneum.

Micturition (voiding, urination) occurs as follows:

1. 200-400 ml of urine in the bladder stretches receptors in the bladder wall.

2. Impulse is transmited to the spinal cord.

3. Micturition reflex is activated.

4. This causes detrusor muscle contraction & internal sphincter relaxation via parasympathetic nerves

5. At the same time, external urethral sphincter is relaxed via somatic motor neurons.

6. Voiding of urine occurs.

Voluntary control of urination by cerebral cortex is learned in early childhood.

Incontinence is the lack of voluntary control over micturition.

-It occurs in infants, during pregnancy, some smokers and paralysed patients.

Stress incontinence occurs when abdominal pressure rises (cough, sneeze, exercise)

Urethra –

It extends from the internal urethral orifice to external urethral orifice.

It is longer in males than females (1.5”)
In females, it passes along the anterior vaginal wall and opens between the clitoris and vulva.

Urinalysis –

It is the measurement of urine volume and its’ physical, chemical and microscopic contents.

A normal adult passes about 1-2 liters of urine daily.

Normal urine pH is 4.6 – 8.0 (average 6.0)

Normal urine specific gravity is 1.001 – 1.035

Urine output depends on:

• General health (diabetes increases UOP)
• Amount of fluid intake
• Climate
• Stress
• Drug intake (diuretics, caffeine, alcohol)
• Diet (salt retains water)

Normal urine contains:

• 95% water
• 5% are solutes (urea, uric acid, creatinine, hormones, water soluble vitamins, enzymes, ions)

Abnormal constituents in urine include:

• Albumin (renal disease)
• Glucose (diabetes, stress)
• RBCs, WBCs (injury, infection, tumor, menstruation)
• Ketones (diabetes, starvation)
• Bilirubin (jaundice)
• Stones (excess ingestion of minerals, dehydration, hyperparathyroidism)
• Microbes (bacteria, candida, trichomonas vaginalis)

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