The nephron is the functional unit of the kidney.
Each nephron consists of:
- A renal tubule
- Its glomerulus
Each human kidney contains about 1 million nephrons.
Figure 37–1 shows the structure of the nephron.
The glomerulus is about 200 µm in diameter.
It is formed by a tuft of capillaries inside the blind, dilated end of the nephron called Bowman’s capsule.
The glomerulus receives blood through the afferent arteriole.
Blood leaves through the efferent arteriole.
Glomerular filtrate is formed in the glomerulus.
The afferent arteriole has a larger diameter than the efferent arteriole.
Two cellular layers separate the blood from the glomerular filtrate:
- Fenestrated capillary endothelium
- Specialized epithelium of Bowman’s capsule
The glomerular capillary endothelium contains fenestrations (pores) measuring 70–90 nm.
The capillaries are surrounded by the glomerular basement membrane.
Specialized epithelial cells called podocytes cover the basement membrane.
Podocytes have many foot processes (pseudopodia) that interlock.
These form filtration slits along the capillary wall.
Each filtration slit is about 25 nm wide.
Each slit is covered by a thin membrane.
The glomerular basement membrane does not contain visible pores or gaps.
Mesangial cells are located between the capillary endothelium and the basement membrane.
They are similar to pericytes found around other capillaries.
Mesangial cells are especially common between adjacent capillaries.
They are contractile and help regulate glomerular filtration.
They also:
- Secrete the extracellular matrix
- Remove immune complexes
- Participate in the progression of glomerular disease
The glomerular filtration membrane:
- Allows free passage of neutral molecules up to 4 nm in diameter
- Almost completely blocks molecules larger than 8 nm
Both molecular size and electrical charge influence filtration.
The total glomerular filtration surface area in humans is about 0.8 m².
The renal tubules are lined by different types of epithelial cells.
Their structural differences are related to their specific functions.
The proximal convoluted tubule (PCT):
- Is about 15 mm long
- Has a diameter of about 55 µm
- Is lined by a single layer of epithelial cells
- Cells are joined by tight junctions
- Contains lateral intercellular spaces
- Has a brush border made of numerous microvilli
After the PCT, the tubule continues as the Loop of Henle.
The descending limb and thin ascending limb:
- Contain thin, permeable cells
The thick ascending limb:
- Contains large cells rich in mitochondria
There are two types of nephrons:
- Cortical nephrons
  - Have short loops of Henle
- Juxtamedullary nephrons
  - Have long loops extending deep into the medulla
Only about 15% of human nephrons are juxtamedullary nephrons.
The thick ascending limb returns close to its own glomerulus.
Specialized cells here form the macula densa.
The macula densa, lacis cells, and renin-secreting granular cells together form the juxtaglomerular apparatus.
The distal convoluted tubule (DCT):
- Begins at the macula densa
- Is about 5 mm long
- Has lower epithelial cells
- Contains few microvilli
- Has no brush border
The distal tubules join to form collecting ducts.
Collecting ducts are about 20 mm long.
They pass through the renal cortex and medulla.
They open into the renal pelvis at the apex of the medullary pyramids.
Collecting ducts contain two main cell types:
- Principal (P) cells
  - More numerous
  - Relatively tall
  - Contain few organelles
  - Responsible for:
    - Na⁺ reabsorption
    - Vasopressin (ADH)-stimulated water reabsorption
- Intercalated (I) cells
  - Fewer in number
  - Also present in the DCT
  - Contain more:
    - Microvilli
    - Cytoplasmic vesicles
    - Mitochondria
  - Responsible for:
    - Acid (H⁺) secretion
    - HCO₃⁻ transport
The total length of a nephron, including the collecting duct, is about 45–65 mm.
The kidney also contains secretory cells:
- Granular (juxtaglomerular) cells
- Renal medullary interstitial cells (RMICs)
RMICs:
- Are specialized fibroblast-like cells
- Contain lipid droplets
- Express COX-2 and prostaglandin E synthase (PGES)
RMICs produce prostaglandin E₂ (PGE₂).
PGE₂ is an important local (paracrine) regulator of salt and water balance.
PGE₂ is secreted by:
- RMICs
- Macula densa
- Collecting duct cells
Prostacyclin (PGI₂) and other prostaglandins are secreted by the glomeruli and renal arterioles.

Figure 37–1

Shows the overall structure of the nephron.
Includes:
- Glomerulus
- Bowman’s capsule
- Proximal convoluted tubule
- Loop of Henle
- Distal convoluted tubule
- Collecting duct

Figure 37–2

Shows:
- Afferent arteriole
- Efferent arteriole
- Podocytes
- Filtration slits
- Glomerular basement membrane
- Mesangial cells
- Macula densa
- Juxtaglomerular apparatus

KEY CONCEPT

The nephron is the functional unit of the kidney. It consists of the glomerulus and renal tubule, where filtration, reabsorption, secretion, and urine formation occur. Specialized structures such as podocytes, mesangial cells, the macula densa, and the juxtaglomerular apparatus regulate filtration, while different tubular segments perform specific functions in maintaining fluid, electrolyte, and acid-base balance.

Nephron (Ganong Fig. 37-1) – Easiest & Most Conceptual Explanation

🎯 One-Line Concept

The nephron is the structural and functional unit of the kidney.

Its job is to filter blood, remove waste, and produce urine while conserving water and useful substances.

Think of a nephron as a water purification plant.

🩸 Blood enters dirty
🧹 Useful substances are recovered
🚽 Waste leaves as urine

Complete Nephron Flow

Blood
   ↓
Glomerulus
   ↓
Proximal Convoluted Tubule (PCT)
   ↓
Loop of Henle
   ↓
Distal Convoluted Tubule (DCT)
   ↓
Collecting Duct
   ↓
Urine

Big Picture

The nephron has 5 major parts.

Step	Structure	Main Job
1	Glomerulus	Filters blood
2	PCT	Reabsorbs most useful substances
3	Loop of Henle	Concentrates urine
4	DCT	Fine adjustment of salts and pH
5	Collecting Duct	Final concentration of urine

1. Glomerulus – The Filter

📍 First part of the nephron

The glomerulus is a ball of tiny capillaries.

Its function is simple:

Filter blood.

Think of it like a kitchen sieve.

Blood
 ↓
Glomerulus
 ↓
Filtrate

What passes through?

✅ Water

✅ Sodium

✅ Glucose

✅ Amino acids

✅ Urea

What does NOT pass?

❌ Red blood cells

❌ White blood cells

❌ Platelets

❌ Large proteins

These remain in the blood.

Easy Memory

Glomerulus = Blood Filter

2. Proximal Convoluted Tubule (PCT)

📍 The Recovery Center

After filtration,

almost everything useful is recovered here.

Approximately

65–70% of filtered fluid is reabsorbed.

PCT Reabsorbs

✅ Water

✅ Sodium

✅ Chloride

✅ Potassium

✅ Bicarbonate

✅ Glucose

✅ Amino acids

Think of PCT as

The “Saving Department.”

It asks,

“Is this useful?”

If YES,

it sends it back into the blood.

Why does PCT have a Brush Border?

Look at the figure.

The PCT cells have many tiny finger-like projections.

These are

Microvilli (Brush Border).

Purpose:

Increase surface area

↓

More absorption

Think of a sponge.

More surface area

↓

Absorbs more water.

3. Loop of Henle

The Loop of Henle has

Two limbs

Descending Limb
        ↓
Hairpin Turn
        ↑
Ascending Limb

Its main function is

Concentrating urine.

A. Thin Descending Limb

Main Rule

Water leaves.

Salt stays.

The descending limb is

Permeable to water

Not permeable to salt.

Therefore

Water OUT

Salt stays

Result

Tubular fluid becomes

More concentrated.

Easy Memory

Descending = Water Down

B. Thick Ascending Limb

Here,

the opposite happens.

Water cannot leave.

Instead,

Salt is pumped out.

Main ions transported

✅ Sodium

✅ Potassium

✅ Chloride

This is called

NKCC2 transporter.

Result

The tubular fluid becomes

Dilute.

Easy Memory

Ascending = Salt Up

Why Is the Loop of Henle Important?

It creates the

Medullary Osmotic Gradient

This gradient allows the kidneys to produce

Concentrated urine (during dehydration)
Dilute urine (when excess water is present)

4. Distal Convoluted Tubule (DCT)

The Fine-Tuning Department

Most work has already been done.

Now the kidney makes

small adjustments.

DCT controls

Sodium

Potassium

Calcium

Hydrogen ions

Water

Hormones Acting Here

Aldosterone

↑ Sodium reabsorption

↓

Potassium secretion

Parathyroid Hormone (PTH)

↑ Calcium reabsorption

Think of DCT as

Final Editing Before Printing

5. Collecting Duct

Final Decision Point

This is where the kidney decides

How much water should be saved?

The hormone

ADH (Antidiuretic Hormone)

controls this.

When ADH is High

Water is reabsorbed.

Urine becomes

Small volume

Dark

Concentrated

When ADH is Low

Water remains in urine.

Urine becomes

Large volume

Pale

Dilute

Easy Memory

Collecting Duct = Water Saver

Cortex vs Medulla

The figure also shows three kidney regions.

Cortex

Contains

✅ Glomerulus

✅ PCT

✅ DCT

This is where filtration begins.

Outer Medulla

Contains

Loop of Henle

Collecting ducts

Inner Medulla

Contains

Long loops of Henle

Collecting ducts

This region creates the highest osmotic concentration, allowing maximum water reabsorption.

Why Do the Cells Look Different?

The figure also shows the microscopic appearance (histology) of each nephron segment.

Proximal Convoluted Tubule (PCT)

Tall cuboidal cells
Dense brush border (microvilli)
Many mitochondria
Built for maximum reabsorption

Thin Descending Limb

Thin, flat cells
Few mitochondria
Specialized for water movement

Thick Ascending Limb

Cuboidal cells
Many mitochondria
No brush border
Built for active salt transport

Distal Convoluted Tubule (DCT)

Cuboidal cells
Few microvilli
Fine-tunes electrolyte balance

Collecting Duct

Larger cells with clear borders
Responds to ADH
Determines the final concentration of urine

The Entire Nephron Story

Blood
   │
   ▼
Glomerulus
(Filters Blood)
   │
   ▼
PCT
(Reabsorbs most water,
glucose, amino acids,
and electrolytes)
   │
   ▼
Descending Limb
(Water leaves)
   │
   ▼
Ascending Limb
(Salt leaves,
water stays)
   │
   ▼
DCT
(Fine adjustment
under hormones)
   │
   ▼
Collecting Duct
(ADH controls
final water reabsorption)
   │
   ▼
Urine

Easy Analogy

Imagine a water purification factory:

🧺 Glomerulus = Entrance filter (separates fluid from blood).
♻️ PCT = Recovery department (returns valuable items like glucose and most water).
⚖️ Loop of Henle = Concentration machine (adjusts water and salt to create a concentration gradient).
🔧 DCT = Quality control section (fine-tunes electrolytes and pH under hormone control).
🚰 Collecting duct = Final packaging (decides how much water to keep before urine leaves).

MBBS High-Yield Summary

Part	Main Function	Memory Keyword
Glomerulus	Filters blood	Filter
PCT	Reabsorbs ~65–70% of filtrate	Recover
Descending Limb	Water leaves	Water Down
Ascending Limb	Salt leaves, water cannot	Salt Up
DCT	Hormonal fine-tuning	Adjust
Collecting Duct	ADH controls final water reabsorption	Save Water

🌟 Golden Flow to Remember

FILTER
   ↓
RECOVER
   ↓
CONCENTRATE
   ↓
ADJUST
   ↓
SAVE WATER
   ↓
URINE

💡 Golden Rule

The nephron filters blood in the glomerulus, reabsorbs useful substances in the tubules, secretes selected wastes into the tubular fluid, and finally produces urine in the collecting duct. Every segment has a specialized role, working together to maintain the body’s fluid, electrolyte, and acid–base balance.

Structural Details of the Glomerulus (Ganong Fig. 37-2) – Easiest & Most Conceptual Explanation

🎯 One-Line Concept

The glomerulus is the kidney’s microscopic filter that allows water and small molecules to pass into Bowman’s capsule while preventing blood cells and proteins from leaving the blood.

Think of it as a high-security water filter.

✅ Water and small molecules pass.
❌ Blood cells and large proteins are blocked.

Big Picture

The figure has 4 parts (A–D), each showing the glomerulus in greater detail.

A → Complete glomerulus
        ↓
B → Relationship of cells
        ↓
C → Filtration barrier
        ↓
D → Magnified filtration slit

PART A – Whole Glomerulus

This is the overall structure of the renal corpuscle.

Components

Afferent Arteriole
        ↓
   Glomerulus
        ↓
Bowman's Capsule
        ↓
Proximal Tubule

1. Afferent Arteriole

Function

Brings blood into the glomerulus.

Think of it as

🚰 The inlet pipe.

2. Glomerular Capillaries

These are tiny capillary loops where blood filtration occurs.

Blood pressure inside these capillaries forces fluid outward.

3. Efferent Arteriole

Carries blood away from the glomerulus.

Think of it as

🚰 The outlet pipe.

Easy Memory

Afferent = Arrives

Efferent = Exits

4. Bowman’s Capsule

A cup-shaped structure surrounding the glomerulus.

It collects the filtered fluid.

Think of it as

🥣 A collecting bowl placed under a filter.

5. Bowman’s Space

The small gap between

Glomerular capillaries
Bowman’s capsule

This is where the filtrate first collects.

6. Proximal Tubule

The filtrate enters here after leaving Bowman’s space.

This is the beginning of tubular reabsorption.

7. Distal Tubule & Macula Densa

The distal tubule passes close to the glomerulus.

Here lies the

Macula Densa

Its function:

Measures sodium chloride in tubular fluid.

If sodium delivery changes,

the macula densa helps adjust the glomerular filtration rate (GFR) through tubuloglomerular feedback.

Think of it as the kidney’s quality-control sensor.

8. Granular (Juxtaglomerular) Cells

Located in the wall of the afferent arteriole.

They secrete

Renin

Renin is released when:

Blood pressure falls.
Renal blood flow decreases.
Sodium delivery to the macula densa falls.

Renin activates the Renin–Angiotensin–Aldosterone System (RAAS) to help restore blood pressure.

9. Mesangial Cells

These cells lie between the capillary loops.

Functions

Provide structural support.
Remove debris by phagocytosis.
Contract to alter the surface area available for filtration, helping regulate GFR.

Think of them as the support and cleaning staff of the glomerulus.

PART B – Relationship Between Cells

This panel shows the close arrangement of the filtration barrier.

Main structures

Glomerular capillary
Endothelial cell
Basement membrane (basal lamina)
Podocyte
Mesangial cell

Together, they form the filtration apparatus.

What is a Podocyte?

Podocytes are specialized epithelial cells covering the outer surface of the glomerular capillaries.

They have many finger-like extensions called

Foot Processes (Pedicels)

These wrap around the capillaries.

Think of them as

🖐 Hands gently holding a tube.

PART C – Filtration Barrier

This panel shows how the filtration barrier is organized.

The filter has three layers.

Layer 1 – Fenestrated Endothelium

The capillary wall contains many tiny pores called

Fenestrations

These allow

✅ Water

✅ Electrolytes

✅ Glucose

to pass easily.

However,

blood cells are too large to pass.

Layer 2 – Glomerular Basement Membrane (GBM)

This is the main filter.

It acts as both

A size barrier
A charge barrier

It prevents

❌ Large proteins (e.g., albumin)

from passing into the filtrate.

Layer 3 – Podocyte Foot Processes

The podocyte processes interlock, leaving narrow gaps called

Filtration Slits

These slits provide the final level of filtration before fluid enters Bowman’s space.

PART D – Magnified Filtration Slit

This is a close-up of the filtration barrier.

Fluid must pass through three filters in sequence:

Blood
   │
   ▼
Fenestrated Endothelium
   │
   ▼
Glomerular Basement Membrane
   │
   ▼
Filtration Slit (Between Podocyte Foot Processes)
   │
   ▼
Bowman's Space

Only after crossing all three layers does the fluid become glomerular filtrate.

What Passes Through the Filter?

✅ Can Pass

Water
Sodium
Potassium
Chloride
Glucose
Amino acids
Urea
Creatinine
Bicarbonate

❌ Cannot Pass

Red blood cells
White blood cells
Platelets
Most plasma proteins (especially albumin)

Easy Analogy

Imagine making tea with a tea strainer.

🍵 Water and dissolved sugar pass through.
🍃 Tea leaves stay behind.

Similarly:

Water and small molecules pass.
Blood cells and proteins remain in the circulation.

The Complete Filtration Journey

Blood enters
(Afferent arteriole)
        │
        ▼
Glomerular Capillaries
        │
        ▼
Fenestrated Endothelium
        │
        ▼
Glomerular Basement Membrane
        │
        ▼
Filtration Slits
(Podocyte Foot Processes)
        │
        ▼
Bowman's Space
        │
        ▼
Proximal Tubule

High-Yield Functions of the Main Cells

Structure	Main Function	Easy Memory
Afferent arteriole	Brings blood to the glomerulus	Arrives
Efferent arteriole	Carries blood away	Exits
Mesangial cells	Support capillaries, remove debris, regulate filtration surface area	Support & Clean
Podocytes	Form filtration slits	Final Filter
Basement membrane	Main size and charge barrier	Main Filter
Macula densa	Senses tubular NaCl and helps regulate GFR	Salt Sensor
Granular (JG) cells	Release renin	Blood Pressure Controller

MBBS High-Yield Points

✅ Glomerular filtration barrier has three layers: fenestrated endothelium, glomerular basement membrane, and podocyte filtration slits.
✅ Podocyte foot processes create the filtration slits.
✅ Mesangial cells provide support, clear debris, and help regulate filtration.
✅ Macula densa + juxtaglomerular (granular) cells form part of the juxtaglomerular apparatus, which regulates GFR and blood pressure.
✅ Albumin and blood cells normally do not appear in urine because the filtration barrier prevents their passage.

🌟 Super Memory Summary

Blood
   │
Afferent Arteriole
   │
Glomerular Capillaries
   │
Fenestrated Endothelium
   │
Basement Membrane
   │
Podocyte Filtration Slits
   │
Bowman's Space
   │
Proximal Tubule

Golden Rule

The glomerulus acts as a highly selective three-layer filter. It allows water and small solutes to enter Bowman’s space while retaining blood cells and most plasma proteins within the bloodstream, ensuring efficient filtration without losing essential blood components.

BLOOD VESSELS

The renal blood circulation is shown in Figure 37–3.
The afferent arterioles arise from the interlobular arteries.
They are short and straight blood vessels.
Each afferent arteriole divides into many capillaries to form the glomerulus.
The glomerular capillaries then join together to form the efferent arteriole.
The efferent arteriole divides again into peritubular capillaries, which surround the renal tubules.
These capillaries finally drain into the interlobular veins.
The blood vessels between the glomerulus and renal tubules form a portal system.
The glomerular capillaries are unique because they are the only capillaries in the body that drain into an arteriole (the efferent arteriole).
The efferent arteriole contains relatively little smooth muscle.
In cortical nephrons:
- The efferent arteriole forms a peritubular capillary network.
In juxtamedullary nephrons:
- The efferent arteriole forms:
  - Peritubular capillaries
  - Vasa recta
The vasa recta are hairpin-shaped blood vessels.
They descend into the renal medulla alongside the loops of Henle.
The descending vasa recta:
- Have nonfenestrated endothelium
- Contain facilitated urea transporters
The ascending vasa recta:
- Have fenestrated endothelium
- Help conserve solutes
One efferent arteriole supplies capillaries around several different nephrons.
Therefore, a nephron does not always receive blood only from its own efferent arteriole.
In humans:
- The total surface area of the renal capillaries is about 12 m².
- The total surface area of the renal tubules is also about 12 m².
At any moment, the renal capillaries contain about 30–40 mL of blood.

Figure 37–3

Shows the pathway of renal blood flow:
- Interlobular artery
- Afferent arteriole
- Glomerulus
- Efferent arteriole
- Peritubular capillaries
- Vasa recta (juxtamedullary nephrons)
- Interlobular vein

KEY CONCEPT

Blood enters the glomerulus through the afferent arteriole, is filtered, and leaves through the efferent arteriole. The efferent arteriole forms peritubular capillaries in cortical nephrons and vasa recta in juxtamedullary nephrons. The glomerular capillaries are unique because they drain into an arteriole, creating a portal system that supports filtration and tubular exchange.

Renal Circulation (GANONG Fig. 37-3) – Easiest & Most Conceptual Explanation

🎯 One-Line Concept

Renal circulation is the pathway by which blood enters the kidney, gets filtered in the glomerulus, nourishes the kidney tubules, and then returns to the heart.

Think of it as a two-step blood circulation:

Blood is filtered.
Filtered blood then nourishes the kidney tissue.

This is unique because the kidney has two capillary beds connected in series.

Big Picture

Heart
   ↓
Renal Artery
   ↓
Interlobar Artery
   ↓
Arcuate Artery
   ↓
Interlobular Artery
   ↓
Afferent Arteriole
   ↓
Glomerulus ⭐
   ↓
Efferent Arteriole
   ↓
Peritubular Capillaries / Vasa Recta
   ↓
Interlobular Vein
   ↓
Arcuate Vein
   ↓
Interlobar Vein
   ↓
Renal Vein
   ↓
Heart

Step 1. Blood Enters the Kidney

Blood enters through the

Renal Artery

This artery brings oxygen-rich blood to the kidney.

It divides into smaller branches.

Renal artery
      ↓
Interlobar arteries

Step 2. Interlobar Arteries

These arteries run between the renal pyramids.

Think of them as

🛣️ Main highways inside the kidney.

They then divide into

Arcuate Arteries

Step 3. Arcuate Arteries

These arteries run along the boundary between

Cortex
Medulla

Think of them as

🌉 Border roads separating the cortex and medulla.

They give rise to

Interlobular Arteries

Step 4. Interlobular Arteries

These arteries extend upward into the cortex.

Their job is to supply every nephron.

Each interlobular artery gives rise to

Afferent Arterioles

Step 5. Afferent Arteriole

Function

Brings blood into the glomerulus.

Easy Memory

Afferent = Arrives

Think of it as

🚰 The inlet pipe of the filter.

Step 6. Glomerulus ⭐

This is the first capillary bed.

Its function is

Filter blood.

Blood pressure forces water and small molecules into Bowman’s capsule.

What is filtered?

✅ Water

✅ Sodium

✅ Glucose

✅ Amino acids

✅ Urea

What stays in blood?

❌ RBCs

❌ WBCs

❌ Platelets

❌ Large proteins

Step 7. Efferent Arteriole

After filtration,

blood leaves through the

Efferent Arteriole

Easy Memory

Efferent = Exits

Unlike most organs,

the glomerular capillaries drain into another arteriole, not a venule.

This helps maintain pressure for efficient filtration.

Why Is This Unique?

Most body organs have:

Artery
   ↓
Capillary
   ↓
Vein

But the kidney has:

Artery
   ↓
Capillary (Glomerulus)
   ↓
Arteriole
   ↓
Second Capillary Bed
   ↓
Vein

This arrangement is unique.

Step 8. Second Capillary Bed

After leaving the efferent arteriole,

blood enters one of two capillary networks.

A. Peritubular Capillaries

These surround the renal tubules in the cortex.

Function

Reabsorb water
Reabsorb electrolytes
Reabsorb nutrients
Secrete some substances into the tubules

Think of them as

♻️ The recovery network.

B. Vasa Recta

These are present mainly in juxtamedullary nephrons.

They run alongside the Loop of Henle.

Function

Supply oxygen to the medulla
Maintain the medullary osmotic gradient by acting as a countercurrent exchanger
Help produce concentrated urine

Think of them as

💧 The water-saving blood vessels.

Two Types of Nephrons in the Figure

1. Superficial (Cortical) Nephrons

Located in the outer cortex.

Features

Short loops of Henle
Mainly associated with peritubular capillaries
Responsible for most filtration and reabsorption

About 85% of nephrons are cortical.

2. Juxtamedullary Nephrons

Located near the corticomedullary junction.

Features

Long loops of Henle extending deep into the medulla
Closely associated with the vasa recta
Essential for producing concentrated urine

About 15% of nephrons are juxtamedullary.

Step 9. Venous Drainage

After exchange in the capillaries,

blood returns through the veins.

Peritubular capillaries
        ↓
Interlobular vein
        ↓
Arcuate vein
        ↓
Interlobar vein
        ↓
Renal vein
        ↓
Heart

Cortex vs Medulla

Renal Cortex

Contains

Glomeruli
PCT
DCT
Peritubular capillaries

This is where filtration begins.

Renal Medulla

Contains

Loop of Henle
Collecting ducts
Vasa recta

This region is responsible for urine concentration.

Why Are There Two Capillary Beds?

This is the most important concept.

First Capillary Bed (Glomerulus)

Purpose

➡ Filtration

Blood pressure is high, allowing fluid to be filtered into Bowman’s capsule.

Second Capillary Bed (Peritubular Capillaries/Vasa Recta)

Purpose

➡ Reabsorption and secretion

Blood pressure is low, making it easier to reabsorb water and solutes back into the blood.

Easy Analogy

Imagine a water treatment plant.

High-Yield Flow Chart

Renal Artery
      │
      ▼
Interlobar Artery
      │
      ▼
Arcuate Artery
      │
      ▼
Interlobular Artery
      │
      ▼
Afferent Arteriole
      │
      ▼
Glomerulus
(Filtration)
      │
      ▼
Efferent Arteriole
      │
      ├──────────────► Peritubular Capillaries
      │                 (Reabsorption & Secretion)
      │
      └──────────────► Vasa Recta
                        (Maintains Medullary Gradient)
      │
      ▼
Interlobular Vein
      │
      ▼
Arcuate Vein
      │
      ▼
Interlobar Vein
      │
      ▼
Renal Vein

MBBS High-Yield Summary Table

Structure	Main Function	Memory Keyword
Renal artery	Brings blood to kidney	Entry
Interlobar artery	Travels between pyramids	Main Highway
Arcuate artery	Runs along corticomedullary junction	Border Road
Interlobular artery	Supplies nephrons	Branch Road
Afferent arteriole	Delivers blood to glomerulus	Arrives
Glomerulus	Filters blood	Filter
Efferent arteriole	Carries blood away	Exits
Peritubular capillaries	Reabsorption and secretion	Recovery Network
Vasa recta	Maintains medullary osmotic gradient	Water Saver
Renal vein	Returns blood to circulation	Exit

⭐ Super Memory Summary

Blood Enters Kidney
        │
Renal Artery
        │
Interlobar Artery
        │
Arcuate Artery
        │
Interlobular Artery
        │
Afferent Arteriole
        │
Glomerulus
⭐ FILTERS BLOOD
        │
Efferent Arteriole
        │
Peritubular Capillaries
(Reabsorb Useful Substances)
        │
OR
        ▼
Vasa Recta
(Maintains Medullary Gradient)
        │
Veins
        │
Renal Vein
        │
Heart

💡 Golden Rule

The kidney is unique because blood passes through two capillary beds in series. The first capillary bed (glomerulus) filters plasma under high pressure, while the second capillary bed (peritubular capillaries or vasa recta) operates under lower pressure to reabsorb water and solutes, nourish the kidney tissue, and help concentrate urine.

LYMPHATICS

The kidneys have a rich (abundant) lymphatic supply.
The renal lymphatic vessels drain into the thoracic duct.
The thoracic duct carries the lymph back into the venous circulation in the thorax.

KEY CONCEPT

The kidneys have an abundant lymphatic network. Renal lymph drains through the thoracic duct and finally returns to the venous circulation in the thorax.

CAPSULE

The renal capsule is thin but strong (tough).
If the kidney becomes edematous (swollen with fluid):
- The renal capsule limits the swelling because it cannot stretch much.
As a result:
- Renal interstitial pressure increases.
The increased renal interstitial pressure decreases the glomerular filtration rate (GFR).
A reduced GFR may enhance and prolong anuria in acute kidney injury (AKI).

Easy Concept

Imagine the kidney is a balloon inside a tight plastic cover.

Normal Kidney

Kidney 😊
Inside a loose capsule

↓

Normal GFR

Swollen Kidney (Edema)

Kidney 💧 Swells

↓

Tight capsule prevents expansion

↓

Pressure inside kidney ↑

↓

GFR ↓

↓

Less urine (Anuria)

KEY CONCEPT

The renal capsule is a tough covering that limits kidney swelling. During kidney edema, this increases renal interstitial pressure, reduces the glomerular filtration rate (GFR), and can worsen or prolong anuria in acute kidney injury (AKI).

INNERVATION OF THE RENAL VESSELS

The renal nerves travel along the renal blood vessels as they enter the kidney.
These nerves contain:
- Many postganglionic sympathetic efferent fibers
- A few afferent (sensory) fibers
The kidneys may also receive cholinergic nerve fibers through the vagus nerve.
However, the function of the vagal innervation is still uncertain.
The sympathetic preganglionic fibers arise mainly from the:
- Lower thoracic spinal cord
- Upper lumbar spinal cord
The cell bodies of the postganglionic sympathetic neurons are located in:
- Sympathetic chain ganglia
- Superior mesenteric ganglion
- Ganglia along the renal artery
Sympathetic nerve fibers are mainly distributed to:
- Afferent arterioles
- Efferent arterioles
- Proximal tubules
- Distal tubules
- Juxtaglomerular apparatus
There is also a dense noradrenergic nerve supply to the:
- Thick ascending limb of the Loop of Henle
Pain (nociceptive) afferent fibers from the kidney:
- Carry pain signals during kidney disease
- Travel alongside the sympathetic nerves
- Enter the spinal cord through the:
  - Thoracic dorsal roots
  - Upper lumbar dorsal roots
Other renal afferent fibers are involved in the renorenal reflex.
In this reflex:
- Increased ureteral pressure in one kidney
- Causes decreased sympathetic efferent activity to the opposite kidney
- This allows the opposite kidney to:
  - Increase sodium (Na⁺) excretion
  - Increase water excretion

Figure 37–2

Shows:
- Afferent arteriole
- Efferent arteriole
- Juxtaglomerular apparatus
- Macula densa
- Podocytes
- Mesangial cells
- Glomerular filtration barrier

Easy Concept

Think of the kidney nerves as communication wires.

Sympathetic Nerves

Brain & Spinal Cord
        │
        ▼
Kidney

They mainly control:

Blood vessels
Renal tubules
Juxtaglomerular apparatus

Pain Pathway

Kidney pain
      ↓
Sensory nerves
      ↓
Thoracic & Upper Lumbar Spinal Cord

Renorenal Reflex

Kidney 1

↑ Ureter Pressure

↓

Signal sent

↓

Kidney 2

↓

↓ Sympathetic activity

↓

↑ Na⁺ excretion

↑ Water excretion

KEY CONCEPT

The kidneys receive mainly sympathetic nerve supply, which regulates the renal blood vessels, renal tubules, and juxtaglomerular apparatus. Pain fibers travel with sympathetic nerves to the spinal cord, while renal sensory fibers also participate in the renorenal reflex, helping the opposite kidney increase sodium and water excretion when ureteral pressure rises in one kidney.