Understanding Kidney Structure and Functions

Introduction

There are two Kidneys. Kidneys are located retroperitoneal in the upper dorsal region of the abdominal cavity on either side of the vertebral column. The kidney is a vital organ that performs numerous functions, including the excretion of metabolic waste, maintaining homeostatic balance, regulating body fluid volume, controlling electrolyte concentration in body fluids, and regulating acid-base balance, as well as participating in endocrine and metabolic functions.

Location:

There are two Kidneys. Kidneys are located retroperitoneal in the upper dorsal region of the abdominal cavity on either side of the vertebral column.

The position of the left Kidney corresponds to the levels of T 12 to L3. The right Kidney is slightly lower than the left Kidney due to the presence of the liver. The 11th and 12th ribs partially protect the kidneys.

The kidneys are retroperitoneal (located behind the peritoneum).

Size and shape of the Kidneys

A kidney is a bean-shaped structure with convex and concave borders. There is a recess in the concave wall known as the renal hilum.

Each adult Kidney weighs about 150 grams and approximately 10 cm long x5cm wide,and2.5cm thick.(lxwt) 10 cm roughly the size of a fist.

Macroscopic Structure of Kidney:

In the vertical section of the Kidney, there are two layers:

Ø Red outer cortex, and

Ø Pale inner medulla.

There are 10 to 15 pyramids in the medulla that terminate medially in the renal papilla and project into the minor calyces.

The minor calyces join to form two major calyces, which open into the renal pelvis. The renal pelvis continues as a ureter and ends in the urinary bladder.

Structures passing through the hilus of the Kidney are :

The ureter, blood vessels, lymphatics, and nerves.

Microscopic structure

In each Kidney, there are about 1 to 1.5 million nephrons. Nephrons are tiny, only 45 to 65 mm in length.

Different parts of the nephron

1. Bowman’s capsule

2. Glomerulus: a tuft of capillaries

3. Proximal convoluted tubule

4. Loop of Henle

Descending part

A thin loop of Henle, and

The thick ascending portion of the loop of Henle

5. Distal convoluted tubule, and

6. Collecting tubule.

Bowman’s capsule and the Glomerulus

Bowman’s capsule is the initial dilated part of the nephron. In addition, a tuft of capillaries, known as the glomerulus, is present in the Bowman’s capsule. The tuft is supplied by afferent arterioles and drained by efferent arterioles.

[Renal corpuscles= Bowman’s capsule with the glomerulus.]

Bowman’s capsule and the glomerulus are known as Malpighian corpuscles.

Bowman’s space is present between the visceral and parietal layers of the Bowman’s capsule.

Structures between the blood of the capillary loop and Bowman’s space form the ‘glomerular capillary wall’ or ‘glomerular membrane.’

What is a glomerular membrane?

The glomerular membrane is a highly specialized structure composed of five layers. From outside to inside, they are

1. Foot processes of podocytes: The Visceral epithelial cell layer covering the capillaries gives out pedicles upon the capillary surface to form filtration sites.

2. Lamina rara externa or outer cement layer overlying these are the foot processes of podocytes.

3. Lamina densa is the basement membrane

4. Lamina rara interna or inner cement layer provides a bed for the capillary endothelium and

5. The endothelial cell layer of the glomerular capillaries is fenestrated.

What is ultrafiltration?

Filtration under pressure is known as ultrafiltration. As a result, small molecules are filtered through the membrane, while large particles are retained. Ultrafiltration is also known as hydrostatic filtration.

Proximal convoluted tubule

The proximal convoluted tubule is continuous with that of the Bowman’s capsule.

The proximal convoluted tubule is divided into two parts :

1. Pars convoluta is continuous with the lumen of the Bowman’s capsule.
2. Pars recta are straight parts and are continuous with the loop of Henle.

Loop of Henle

1. Descending limb of Henle’s loop

Descending limb of Henle’s loop is thick and continues above with the proximal convoluted tubule and on the lower side with the thin portion of the loop of Henle.

2. Thin portion of the loop of Henle.

3. The thick ascending limb

The thick ascending limb is continuous with the distal convoluted tubule.

The cortical nephrons’ loop of Henle is small, while in Juxt-Medullary nephrons, they are large.

Distal convoluted tubule

The distal convoluted tubule is continuous with the thick ascending limb of the loop of Henle, which is characterized by low cuboidal epithelial cells with few mitochondria. The cells are closely packed, adjacent to the afferent and efferent arterioles of the glomerulus. At this site, the proximal convoluted tubule’s cuboidal cells modify into columanar. This part of the distal convoluted tubule is called ‘Macula densa.

What is ‘Macula densa?

The macula densa is part of the distal convoluted tubule, which is very close to the afferent and efferent arterioles of the same glomerulus.

Juxtaglomerular apparatus

The macula densa and the adjacent Juxtaglomerular part of the afferent arteriolar wall are functionally associated and known as the Juxtaglomerular apparatus.

The juxtaglomerular apparatus is the macula densa and the adjacent afferent arteriolar wall.

Collecting Tubule

The distal convoluted tubule is continuous with the collecting tubule and passes through the renal cortex and medulla, and empties into the Kidney’s pelvis.

Two types of cuboidal cells line the collecting tubule:

1. P-Cells, or principal cells, have two functions

Ø Increases the pore size of aquaporin-2 and increases the permeability of the collecting tubule to water in the presence of ADH.

Ø Increases sodium reabsorption.

2. I cells or intercalated cells perform two functions:

Ø Acidic urine formation.

Ø Secret acid is hydrogen and

Ø Transport of bicarbonate ions.

Blood Supply of the Kidney

The renal artery, a branch of the aorta, supplies each Kidney. The renal vein drains each Kidney.

Nerve supply

Afferent and efferent nerves innervate the kidneys.

Afferent nerves:

J Parasympathetic innervation is from the vagus, but its function is unclear.

J Sympathetic innervation causes a fall in renal blood flow, which increases reabsorption of sodium ions and water in the renal tubule.

Efferent nerves

Efferent nerves convey pain sensation along with the sympathetic nerves.

Functions of the Kidney

1. Excretion of metabolic waste products

2. Homeostatic function regulation of volume, the concentration of body fluids, and regulation of acid-base balance

3. Endocrine functions –Renin, erythropoietin,1,25-DHCC (dihydroxy cholecalciferol), and

4. Gluconeogenesis.

And above (upper pole ) the kidneys, there are two suprarenal glands.

The functional unit of the Kidney is the nephron, that of the CNS is the neuron, and that of the respiratory system is the alveolus.

Types of nephrons

There are two types of nephrons

Ø Cortical nephron, and

Ø Juxta-medullary nephrons

1. Location:
2. Size and shape of the Kidneys
3. Macroscopic Structure of Kidney:
4. Microscopic structure
   1. Different parts of the nephron
5. Bowman’s capsule and the Glomerulus
   1. What is a glomerular membrane?
   2. What is ultrafiltration?
6. Proximal convoluted tubule
7. Loop of Henle
   1. 1. Descending limb of Henle’s loop
   2. 2. Thin portion of the loop of Henle.
   3. 3. The thick ascending limb
8. Distal convoluted tubule
   1. What is ‘Macula densa?
   2. Juxtaglomerular apparatus
9. Collecting Tubule
10. Blood Supply of the Kidney
11. Nerve supply
    1. Afferent nerves:
    2. Efferent nerves
12. Functions of the Kidney
    1. Types of nephrons