Urinary System - Allied Guru

Table of Contents

Introduction

Urinary system consists of:
- A pair of kidneys
- A pair of ureters
- Urinary bladder
- Urethra
Kidneys produce urine.
Urine is conveyed by ureters to the urinary bladder.
Urinary bladder stores urine temporarily.
Urine is expelled through the urethra during micturition.

Kidney

General Features

Each kidney is a large bean-shaped organ situated in the posterior abdominal wall behind the peritoneum.
It measures:
- Length: 7.5 cm
- Breadth: 5 cm
- Thickness: 2.5 cm
The medial concave border has a hilum, through which renal vessels and nerves pass.

Functions of Kidney

The chemical composition of urine reflects the important functions performed by the kidney:

Controls water and electrolyte balance in the body
Maintains acid–base balance
Excretes toxic metabolic waste products (nitrogen, urea, and creatinine)
Maintains blood pressure by the renin–angiotensin–aldosterone mechanism
Stimulates RBC production in bone marrow through erythropoietin

Macroscopic Features

A vertical section through the kidney reveals its gross internal features:

The hilum leads to a space called the renal sinus, which contains:
- Branches of renal vessels and nerves
- Pelvis of the ureter
- Major calyces (2–3)
- Minor calyces (8–12)
The substance of the kidney is divided into:
- An outer dark granular (reddish-brown) zone called the cortex
- An inner pale striated zone called the medulla

Medulla

The medulla is formed by 8–12 renal pyramids.
The bases of the pyramids face the cortex.
The apices (renal papillae) are directed toward the renal sinus.
Each renal papilla is cupped by a minor calyx.
The tip of each papilla is pierced by many papillary ducts (ducts of Bellini) opening into the minor calyx.

Cortex

From the base of the pyramids, medullary tissue extends into the cortex as medullary rays.
Cortical tissue extends between adjacent pyramids as the renal columns of Bertin.

Microscopic structure of kidney

Kidney is composed of numerous uriniferous (renal) tubules.
Tubules are embedded in a vascular interstitium.
Each uriniferous tubule has two parts:
- Nephron – produces urine.
- Collecting tubule – concentrates urine.

Uriniferous tubule

Nephron:
- Derived from metanephric blastema.
- Concerned with urine formation.
Collecting tubule:
- Derived from ureteric bud of mesonephric duct.
- Responsible for urine concentration (hypertonicity).

Nephron

Structural and functional unit of kidney.
Number: 1–4 million per kidney.
Parts:
- Renal corpuscle.
- Proximal convoluted tubule (pct).
- Loop of henle.
- Distal convoluted tubule (dct).

Types of nephrons

Cortical nephrons:
- Located in cortex.
- Have short loops of henle.
- Extend up to outer medulla.
Juxtamedullary nephrons:
- Located at corticomedullary junction.
- Have long loops of henle with vasa recta.
- Create hypertonic medulla.
- Essential for formation of concentrated urine.

Renal corpuscle (malpighian corpuscle)

Site of blood filtration.
Located in cortex.
Diameter: approximately 200 µm.
Components:
- Bowman’s capsule.
- Glomerulus.

Bowman’s capsule

Blind, cup-shaped end of nephron.
Formed by invagination of glomerulus.
Consists of two layers:
- Parietal layer:
  - Lined by simple squamous epithelium.
  - Continuous with pct at urinary pole.
- Visceral layer:
  - Composed of podocytes.
Space between layers:
- Bowman’s (urinary) space.
Poles:
- Vascular pole – entry and exit of arterioles.
- Urinary pole – origin of pct.

Glomerulus

Tuft of anastomosing capillaries.
Supplied by afferent arteriole (wider).
Drained by efferent arteriole (narrower).
Pressure gradient produces filtration.
Capillaries lined by fenestrated endothelium.
Supported by mesangial cells with contractile and phagocytic functions.
Nuclei seen belong to endothelial cells, mesangial cells, and podocytes.

Podocytes

Specialized epithelial cells of visceral layer.
Possess primary processes.
Primary processes give rise to secondary foot processes (pedicels).
Pedicels interdigitate forming filtration slits (~25 nm).
Filtration slits are bridged by slit membrane.
Cell bodies and primary processes do not touch the basement membrane.

Glomerular basement membrane

Thickness: approximately 100 nm.
Formed by fused basal laminae of endothelium and podocytes.
Acts as a selective filtration barrier.

Glomerular filtration barrier

Consists of three layers:

Fenestrated endothelium – prevents passage of blood cells.
Glomerular basement membrane – blocks particles >10 nm and negatively charged proteins.
Filtration slits and slit membrane – final filtration barrier.

Proximal convoluted tubule

Proximal convoluted tubule (pct) starts at the urinary pole of the renal corpuscle.
It is longer and more convoluted than distal convoluted tubule.
It forms the bulk of the renal cortex.
It is lined by simple cuboidal epithelium.
Luminal surface has microvilli forming a brush border.
Basal surface shows striations due to:
- Infoldings of plasma membrane
- Longitudinally arranged mitochondria
About 75% of water and electrolytes (sodium, potassium, chloride) are reabsorbed by selective reabsorption.
It also secretes certain metabolites, dyes, and drugs.

Loop of henle

Loop of henle arises from pct in the cortex.
It descends into medulla as descending limb.
It turns and ascends as ascending limb.
It becomes continuous with dct at corticomedullary junction.

Segments of loop of henle

Descending limb:
- Short thick segment (straight part of pct)
- Long thin segment
Ascending limb:
- Short thin segment
- Long thick segment (straight part of dct)

Histology

Thick segments:
- Lined by cuboidal epithelium
- Impermeable to water
Thin segments:
- Lined by simple squamous epithelium
- Permeable to water and sodium

Functional mechanism

Descending limb:
- Passive removal of water
Ascending limb:
- Active transport of sodium and chloride
Tubular fluid becomes:
- Hypertonic initially
- Isotonic later

Vascular association

Loops of henle are closely related to vasa recta.
Vasa recta maintain osmotic gradient of medullary interstitium.

Collecting tubule and collecting duct

Collecting tubule begins in the medullary ray.
It is a continuation of distal convoluted tubule.
On entering the medulla, several collecting tubules join together.
They form a larger duct called duct of bellini or papillary duct.
Papillary ducts open at the apex of renal pyramid (renal papilla).

Histology

Collecting tubules:
- Lined by simple cuboidal epithelium
- Cells have distinct boundaries
- Cytoplasm is clear and pale
Collecting ducts (papillary ducts):
- Larger and wider
- Lined by tall columnar epithelium
- Cytoplasm is pale-staining

Ureter

General features

Ureters are muscular tubes.
They conduct urine from renal pelvis to urinary bladder.
Transport of urine occurs by peristaltic contractions.
Peristalsis is produced by smooth muscle in the ureteric wall.

Structure of ureter

Wall of ureter is composed of three coats:
1. Mucosa
2. Muscle coat
3. Adventitia

Mucosa

Lined by transitional epithelium.
Supported by lamina propria rich in elastic fibers.
Mucosa forms longitudinal folds.
Folds give a star-shaped lumen in cross section.

Muscle coat

Composed of smooth muscle fibers.
Upper two-thirds of ureter have two layers:
- Inner longitudinal layer
- Outer circular layer
Lower one-third of ureter has three layers:
- Inner longitudinal layer
- Middle circular layer
- Outer longitudinal layer

Adventitia

Outermost coat of ureter.
Made of loose connective tissue.
Contains blood vessels, lymphatics, and nerves.

Urinary bladder

General features

Urinary bladder is a muscular sac.
It temporarily stores urine.
Urine is expelled through urethra during micturition.
Empty bladder lies in pelvis behind pubic symphysis.
Shape of empty bladder is four-sided pyramidal.
Normal capacity is about 200–300 ml.

Structure of urinary bladder

Wall of urinary bladder has three coats:
1. Mucosa
2. Muscle coat
3. Adventitia / serosa

Mucosa

Lined by transitional epithelium (urothelium).
Supported by lamina propria.
Urothelium is present only in urinary system.
Lines passages from minor calyx to upper urethra.
Specially adapted for stretching without damage.

Changes with bladder state

When bladder is empty:
- Mucosa shows folds.
- Epithelium is thick (5–6 cell layers).
- Superficial cells are rounded and bulge into lumen.
- Superficial cells may be binucleate or polyploid.
- Plasma membrane of superficial cells is thickened.
- Acts as osmotic barrier against toxic urine.
When bladder is distended:
- Mucosal folds disappear.
- Epithelium becomes thin (3–4 cell layers).
- Superficial cells become flattened.

Muscle coat

Made of smooth muscle fibers.
Arranged in three ill-defined layers:
- Inner longitudinal
- Middle circular
- Outer longitudinal
Collectively forms detrusor muscle.
Muscle fibers around internal urethral orifice form internal sphincter.

Adventitia / serosa

Adventitia is composed of fibroelastic connective tissue.
Contains blood vessels, nerves, and lymphatics.
Superior surface of bladder is covered by peritoneum.
This surface forms serosa instead of adventitia.

MCQs

Urinary system consists of
a. Kidneys and ureters only
b. Kidneys, ureters, bladder, and urethra
c. Kidneys and bladder only
d. Bladder and urethra only
Answer: b
Urine is produced by
a. Ureter
b. Urinary bladder
c. Kidney
d. Urethra
Answer: c
Temporary storage of urine occurs in
a. Kidney
b. Ureter
c. Urinary bladder
d. Urethra
Answer: c
Urine is expelled through
a. Ureter
b. Renal pelvis
c. Urethra
d. Minor calyx
Answer: c
Kidneys are located
a. In pelvic cavity
b. In thoracic cavity
c. On posterior abdominal wall
d. In peritoneal cavity
Answer: c
Kidneys are situated
a. In front of peritoneum
b. Within peritoneum
c. Behind peritoneum
d. Inside pelvis
Answer: c
Shape of kidney is
a. Oval
b. Round
c. Bean-shaped
d. Pyramidal
Answer: c
Length of kidney is approximately
a. 5 cm
b. 7.5 cm
c. 10 cm
d. 12 cm
Answer: b
Medial border of kidney contains
a. Capsule
b. Papilla
c. Hilum
d. Pyramid
Answer: c
Renal vessels and nerves pass through
a. Cortex
b. Medulla
c. Hilum
d. Renal pelvis
Answer: c
Kidney maintains blood pressure by
a. Adh
b. Aldosterone only
c. Renin–angiotensin–aldosterone mechanism
d. Erythropoietin
Answer: c
Hormone secreted by kidney for rbc production is
a. Renin
b. Aldosterone
c. Adh
d. Erythropoietin
Answer: d
Outer fibrous covering of kidney is
a. Renal fascia
b. Perinephric fat
c. Fibrous capsule
d. Peritoneum
Answer: c
Fat surrounding kidney is called
a. Mesenteric fat
b. Perinephric fat
c. Subcutaneous fat
d. Retroperitoneal fat
Answer: b
Renal fascia is also known as
a. Fascia lata
b. Fascia of gerota
c. Camper’s fascia
d. Scarpa’s fascia
Answer: b
Renal sinus contains
a. Cortex and medulla
b. Renal pyramids
c. Pelvis and calyces
d. Nephrons
Answer: c
Number of major calyces is
a. 1
b. 2–3
c. 6–8
d. 10–12
Answer: b
Number of minor calyces is
a. 2–3
b. 4–6
c. 8–12
d. 15–20
Answer: c
Outer zone of kidney is
a. Medulla
b. Renal sinus
c. Cortex
d. Pelvis
Answer: c
Inner pale striated zone of kidney is
a. Cortex
b. Medulla
c. Capsule
d. Hilum
Answer: b
Renal pyramids are present in
a. Cortex
b. Medulla
c. Pelvis
d. Capsule
Answer: b
Apex of renal pyramid is called
a. Renal column
b. Medullary ray
c. Renal papilla
d. Minor calyx
Answer: c
Papillary ducts open into
a. Major calyx
b. Renal pelvis
c. Minor calyx
d. Ureter
Answer: c
Ducts of bellini open at
a. Cortex
b. Renal sinus
c. Renal papilla
d. Hilum
Answer: c
Medullary rays extend from
a. Cortex to medulla
b. Medulla to cortex
c. Pelvis to cortex
d. Capsule to cortex
Answer: b
Renal columns of bertin are
a. Medullary tissue in cortex
b. Cortical tissue between pyramids
c. Collecting ducts
d. Renal tubules
Answer: b
A renal lobe consists of
a. Pyramid only
b. Cortex only
c. Pyramid with overlying cortex
d. Medullary ray only
Answer: c
Renal lobule consists of
a. Pyramid and papilla
b. Medullary ray with surrounding cortex
c. Cortex only
d. Medulla only
Answer: b
Renal artery divides first into
a. Interlobular arteries
b. Arcuate arteries
c. Segmental arteries
d. Afferent arterioles
Answer: c
Arcuate arteries are located at
a. Hilum
b. Cortex
c. Medulla
d. Corticomedullary junction
Answer: d
Interlobular arteries give rise to
a. Efferent arterioles
b. Vasa recta
c. Afferent arterioles
d. Renal veins
Answer: c
Efferent arteriole of cortical nephron forms
a. Vasa recta
b. Peritubular capillary plexus
c. Sinusoids
d. Portal system
Answer: b
Efferent arteriole of juxtamedullary nephron forms
a. Peritubular plexus
b. Sinusoids
c. Vasa recta
d. Arcuate veins
Answer: c
Vasa recta help in
a. Filtration
b. Hormone secretion
c. Maintaining medullary osmotic gradient
d. Urine storage
Answer: c
Renal vein exits kidney through
a. Cortex
b. Medulla
c. Pelvis
d. Hilum
Answer: d
Structural and functional unit of kidney is
a. Renal corpuscle
b. Collecting duct
c. Nephron
d. Renal pyramid
Answer: c
Nephron is derived from
a. Ureteric bud
b. Mesonephric duct
c. Metanephric blastema
d. Pronephros
Answer: c
Collecting tubule is derived from
a. Metanephric blastema
b. Ureteric bud
c. Cloaca
d. Allantois
Answer: b
Number of nephrons per kidney is
a. 10,000
b. 50,000
c. 1–4 million
d. 10 million
Answer: c
Cortical nephrons have
a. Long loop of henle
b. Vasa recta
c. Short loop of henle
d. No loop of henle
Answer: c
Juxtamedullary nephrons are located at
a. Cortex only
b. Medulla only
c. Corticomedullary junction
d. Renal pelvis
Answer: c
Renal corpuscle is present in
a. Medulla
b. Cortex
c. Pelvis
d. Papilla
Answer: b
Diameter of renal corpuscle is about
a. 50 µm
b. 100 µm
c. 200 µm
d. 500 µm
Answer: c
Bowman’s capsule has
a. One layer
b. Two layers
c. Three layers
d. Four layers
Answer: b
Visceral layer of bowman’s capsule is made of
a. Mesangial cells
b. Endothelial cells
c. Podocytes
d. Fibroblasts
Answer: c
Filtration slits are formed between
a. Endothelial cells
b. Podocyte pedicels
c. Mesangial cells
d. Basement membrane
Answer: b
Thickness of glomerular basement membrane is about
a. 25 nm
b. 50 nm
c. 75 nm
d. 100 nm
Answer: d
Glomerular filtrate is similar to plasma except for
a. Glucose
b. Electrolytes
c. Plasma proteins
d. Water
Answer: c
Normal urine output per day is approximately
a. 500 ml
b. 1000 ml
c. 1500 ml
d. 3000 ml
Answer: c
Ureters conduct urine by
a. Gravity
b. Diffusion
c. Peristaltic contraction
d. Osmosis
Answer: c

Introduction

Kidney

Ureter

Urinary bladder

MCQs

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