Urinary System

The key functions of the urinary system-

1) Excretion of waste (urea, creatinine, uric acid).

2) Regulation of blood volume and pressure.
3) Electrolyte balance (sodium, potassium, calcium).
4) Acid-base balance (pH regulation).
5) Hormone production (erythropoietin, renin).
Detoxification.
Parts/Components of urinary system

1. Two Kidneys -Filter blood to

produce urine, regulating waste
excretion and fluid balance.
2. Two Ureters - Transport urine
from the kidneys to the bladder.
3. Urinary Bladder - Stores urine
until it is ready to be excreted.
4. Urethra - Conveys urine from the
bladder to the outside of the body.
 Shape - bean shaped
Colour - reddish brown
Location - Retroperitoneal ( T12 to L3 )
Kidney Weight - 120-170 gram
Size - 11 cm long , 6 cm wide , 3 cm thik
Right kidney sites slightly lower due to livers position
Structure of Kidney

Here are the points about the kidney structure:

Hilum: Notch on the inner concave surface; entry point

for ureter, blood vessels, and nerves.
Renal Pelvis: Broad funnel-shaped space located inner
to the hilum.
Calyces: Projections from the renal pelvis.
Outer Layer: Tough capsule surrounding the kidney.
Cortex: Outer region of the kidney.
Medulla: Inner region, consisting of medullary pyramids
and renal columns known as Columns of Bertini.
Functions of the Kidney
Excretion: Removal of metabolic wastes such as urea, uric acid, and creatinine.
Regulation:
Fluid and Electrolyte Balance: Maintains the body's water content, sodium, potassium, and
calcium and chloride levels.
Acid-Base Balance: Helps maintain a pH of 7.35-7.45 by excreting hydrogen ions and
reabsorbing bicarbonate.
Blood Pressure: Via the renin-angiotensin-aldosterone system (RAAS), the kidneys regulate
blood pressure.
Erythropoiesis: The kidneys produce erythropoietin, which stimulates red blood cell production
in response to low oxygen levels.
 Nephrones
The nephron is the functional and
structural unit of the kidney,
responsible for the formation of
urine and the regulation of water and
electrolytes.
Each kidney contains approximately
one million nephron
30–55 mm (1.2–2.2 inches) long tune
like structure
Structure of nephron in short
Bowman's Capsule: Surrounds the glomerulus, where filtration begins.
Glomerulus: A network of capillaries where blood is filtered.
Proximal Convoluted Tubule (PCT): Reabsorbs water, ions, and nutrients.
Loop of Henle: Concentrates urine by absorbing water and salts.
Distal Convoluted Tubule (DCT): Regulates pH and ionic balance.
Collecting Duct: Collects urine from multiple nephrons and transports it to the renal pelvis.

Functions of the Nephron:

Filtration: In the glomerulus
Reabsorption: Of water, electrolytes, glucose, and nutrients.
Secretion: Removal of waste products and excess ions.
Renal Corpuscle
The renal corpuscle consists of two main components:
Glomerulus:
A tuft of capillaries surrounded by the Bowman's capsule.
Its walls are made up of fenestrated endothelial cells that allow for the filtration of blood.
Blood enters the glomerulus via the afferent arterioles and exits through the efferent arterioles.
Bowman’s Capsule:
A double-walled cup structure that surrounds the glomerulus.
It has two layers:
Parietal layer: A simple squamous epithelium.
Visceral layer: Composed of specialized cells called podocytes that wrap around the capillaries,
forming filtration slits to facilitate the filtration process.
Proximal Convoluted Tubule (PCT)
The PCT is the first segment of the renal tubule, located just after the
Bowman’s capsule.
Structure:
Lined by simple cuboidal epithelium with microvilli (brush border),
increasing the surface area for reabsorption.
Cells contain numerous mitochondria, indicating high metabolic activity.
Function:
Reabsorbs approximately 65-70% of the filtered water, electrolytes (like
sodium and potassium), and nutrients (like glucose and amino acids).
Secretes substances such as hydrogen ions and certain drugs.
Loop of Henle
The loop of Henle has two limbs:
Descending Limb:
Lined with simple squamous epithelium.
Permeable to water but not to solutes, allowing for water reabsorption.
Ascending Limb:
Has a thick segment lined with cuboidal cells that are impermeable to water.
Actively transports sodium, potassium, and chloride ions out of the filtrate,
contributing to the concentration gradient in the medulla.
Distal Convoluted Tubule (DCT)
Located after the loop of Henle and before the collecting duct.
Structure:
Lined with simple cuboidal epithelium, but with fewer microvilli
compared to the PCT.
Contains specialized cells for hormone action.
Function:
Reabsorbs sodium and calcium ions, and secretes potassium and
hydrogen ions.
Regulated by hormones such as aldosterone and parathyroid hormone.
Collecting Duct
The collecting duct receives urine from multiple nephrons.
Structure:
Formed by cuboidal epithelial cells that become columnar as the
duct progresses.
Function:
Reabsorbs water and sodium, regulated by antidiuretic hormone
(ADH) and aldosterone.
Plays a key role in urine concentration and volume.
Juxtaglomerular Apparatus (JGA)
A specialized structure formed by the DCT and the afferent arteriole.
Components:
Macula Densa: A group of specialized cells in the DCT that detect
sodium concentration and regulate blood flow.
Juxtaglomerular Cells: Smooth muscle cells in the wall of the afferent
arterioles that secrete renin in response to low blood pressure.
Mesangial Cells: Support cells located between the glomerular capillaries,
providing structural support and regulating blood flow.
Ureters:
These are two thin tubes that carry urine from each kidney to the urinary bladder. Each ureter is about 25-30 cm long
and made up of smooth muscles, allowing urine to flow through peristaltic contractions.

Urinary Bladder:
A hollow, muscular organ that stores urine until it is ready to be expelled. The bladder can hold about 400-600 ml of
urine and is lined with transitional epithelium, allowing it to stretch as it fills.

Urethra:
This tube carries urine from the bladder to the outside of the body. In males, the urethra is longer and passes through
the penis, serving both urinary and reproductive functions. In females, the urethra is shorter and located in front of the
vaginal opening, solely for urination.

Internal and External Urethral Sphincters:

These sphincters control the release of urine from the bladder. The internal sphincter (involuntary) is located at the
junction of the bladder and urethra, while the external sphincter (voluntary) is located in the pelvic floor muscles,
allowing conscious control over urination.
Urine Formation Process
It occurs through three major steps/process –
1. Glomerular Filtration
2. Tubular Reabsorption
3. Tubular Secretion
Glomerular Filtration
This is the first step in urine formation and occurs in the glomerulus, which is a network of capillaries
surrounded by the Bowman’s capsule.
Location: Glomerulus within the Bowman’s capsule in the kidney cortex.
Process:
Blood enters the glomerulus through the afferent arteriole under high pressure.
The pressure forces water, ions (e.g., sodium, potassium, chloride), glucose, amino acids, urea, and other small
molecules through the glomerular capillary walls into the Bowman’s capsule, forming the glomerular filtrate.
Large molecules like proteins and blood cells are too large to pass through and are retained in the blood.
Filtration Rate: The rate of filtration is called the Glomerular Filtration Rate (GFR), which is typically 90-120
mL/min in healthy adults.
Composition of Filtrate: The filtrate is similar to blood plasma but does not contain proteins or blood cells.
It consists of:Water, electrolytes (sodium, potassium, chloride), glucose, amino acids, urea, creatinine, and
other small solutes.
Tubular Reabsorption
After the filtrate is formed, the kidneys selectively reabsorb useful substances like water, glucose, and ions back into the
bloodstream. This process occurs primarily in the proximal convoluted tubule (PCT), but it continues in other parts of the nephron.
Location:
Proximal Convoluted Tubule (PCT): The majority of reabsorption occurs here.
Loop of Henle: Responsible for water and salt balance.
Distal Convoluted Tubule (DCT) and Collecting Duct: Fine-tuning of reabsorption based on the body's needs.
Key Reabsorbed Substances:
Water: About 99% of water in the filtrate is reabsorbed. This occurs in the PCT, loop of Henle, DCT, and collecting duct.
Glucose: Normally, 100% of glucose is reabsorbed in the PCT. In conditions like diabetes mellitus, if blood glucose levels exceed the
renal threshold (~180 mg/dL), glucose can appear in urine (glycosuria).
Sodium and Chloride Ions: Sodium reabsorption occurs throughout the nephron, and it is under hormonal control (e.g.,
aldosterone). Normal sodium level in blood is 135-145 mEq/L.
Amino Acids: Like glucose, amino acids are reabsorbed entirely in the PCT.
Mechanisms of Reabsorption:
Active Transport: Uses energy to move substances like sodium, glucose, and amino acids against their concentration gradient.
Passive Transport: Water follows solutes (especially sodium) by osmosis due to the concentration gradient.
Tubular Secretion
This is the process by which waste products, excess ions, and certain drugs are actively transported from the
blood into the nephron for excretion. It helps to fine-tune the composition of blood and eliminate substances
not filtered by the glomerulus.
Location:
Mainly in the Distal Convoluted Tubule (DCT) and collecting duct, though some secretion occurs in the
PCT.
Substances Secreted:
Hydrogen Ions (H⁺): Regulate acid-base balance. The kidneys secrete H⁺ to lower blood acidity and maintain
a normal blood pH of 7.35-7.45.
Potassium Ions (K⁺): Under the control of aldosterone, excess potassium is secreted into the filtrate. Normal
serum potassium levels are 3.5-5.0 mEq/L.
Ammonia (NH₄⁺): Generated from protein metabolism and is toxic, so it is secreted into the nephron for
elimination.
Creatinine: A waste product from muscle metabolism, secreted by the kidney and used as a marker for
kidney function.
Concentration and Dilution of Urine
The concentration of urine is adjusted according to the body's hydration status,
controlled primarily by the hormone antidiuretic hormone (ADH).
ADH Mechanism:
Secreted by the posterior pituitary gland in response to increased blood osmolarity
(dehydration).
ADH increases the permeability of the collecting ducts to water, allowing more
water to be reabsorbed into the bloodstream, producing concentrated urine.
When blood volume is high or osmolarity is low (overhydration), ADH secretion
decreases, leading to diluted urine output.
Osmolarity: Normal plasma osmolarity is 275-295 mOsm/L. The kidneys adjust
urine osmolarity within a range of 50 to 1200 mOsm/L depending on hydration
levels.
Micturition (Urination) Process
The process of expelling urine from the bladder involves both voluntary and
involuntary control.
Bladder Filling: As the bladder fills with urine (typically around 300-400 mL),
stretch receptors in the bladder wall are activated.
Micturition Reflex:
Parasympathetic nerves cause contraction of the detrusor muscle (bladder wall)
and relaxation of the internal urethral sphincter.
The external urethral sphincter is under voluntary control. When relaxed, urine is
expelled through the urethra.
Inhibition of the Reflex: The brain can override the micturition reflex to delay
urination until an appropriate time.
Role of Renin-Angiotensin-Aldosterone System (RAS) in the Kidney
Renin: Secreted by juxtaglomerular cells in the kidney when blood pressure
or sodium levels are low.
Angiotensin II: Renin converts angiotensinogen to angiotensin I, which is
converted to angiotensin II in the lungs. Angiotensin II constricts blood
vessels and stimulates aldosterone release.
Aldosterone: Increases sodium and water reabsorption in the distal tubule,
raising blood pressure.
Effects:
Vasoconstriction: Angiotensin II narrows blood vessels.
Blood Pressure Increase: Aldosterone raises blood pressure by increasing
sodium reabsorption.