Kidney and Nephron – Diagram, Definition, Function, Structure, Facts

What Is the Kidney? (Definition & Importance)

The kidneys do the major work of the urinary system. The other parts of the system are mainly passageways and storage areas. The paired kidneys are reddish, kidney bean–shaped organs located between the peritoneum and the posterior wall of the abdomen. The right kidney is slightly lower than the left because the liver occupies considerable space on the right side superior to the kidney.

Structure of Human Kidney

The anatomy of the kidney is discussed below:

Shape and Size

The kidneys are paired, bean-shaped organs, each being about 11-14 cm long, 6 cm wide and 4 cm in thickness. They weigh about 150 grams in adults.

Position

One on either side of the spine in the abdominal cavity, the kidneys lie directly below the rib cage. A little lower than the left, the right kidney is normally located to provide space for the bulky liver.

Renal Cortex

The outer part of the kidney, having renal corpuscles and the bulk of renal tubules. It is said to be the real machine for the filtration of blood and the place of urine formation.

Renal Medulla

It is the inner portion that contains the renal pyramids. The renal columns are the visible portions of tissue between the pyramids. The renal pyramids are made of the Henle loops and collecting ducts, elements that are responsible for concentrating the urine and transporting it.

Renal Pelvis

The hollow spelt in the centre of each kidney collects urine from medullary pyramids and leads it into the ureter. So, it works as a funnel passage of urine to the bladder.

Nephron – Functional Unit of Kidney

Nephrons are the functional units of the kidneys. Each nephron consists of two parts: a renal corpuscle, where blood plasma is filtered, and a renal tubule into which the filtered fluid passes.

Renal Corpuscle

• Glomerulus: Ball of capillaries where filtration of blood starts. The pressure there of the capillaries pushes water, ions and small molecules out into Bowman's capsule.

• Bowman's Capsule: A cup-shaped chamber surrounding that glomerulus which gathers the filtrate from the glomerulus.

Renal Tubule

• Proximal Convoluted Tubule (PCT): This is the first section of the renal tubule of the nephron. Here, approximately 65% of filtered water and 65% of the filtered ions are reabsorbed.

• Loop of Henle: This U-shaped loop projects into the medulla. Water and salt are reabsorbed there, concentrating the urine. It is composed of two limbs. There is a downrunning limb, which is permeable to water, and an uprunning limb, which is permeable to salts.

• Distal Convoluted Tubule (DCT): This is followed by additional fine-tuning of the filtrate due to differential reabsorptions and secretion—chiefly under aldosterone influence.

• Collecting Duct: It is formed by a collection of many nephrons; this at last completes the concentration of the urine and hence carries it out to the renal pelvis.

Types of Nephrons

The basic functional unit of the kidney is the nephron, which filters the blood of waste. The different types of nephron are:

Cortical Nephron

This type of nephron mostly lies in the renal cortex. They have been found with their shorter loops of Henle which lead to most of the renal bulk filtration and absorption.

Juxtamedullary Nephron

They too have long loops that dip deep into the medulla from a position close to the junction of the cortex and medulla.

Mechanism of Urine Formation (Three Steps)

The net process of urine formation comprises filtration, reabsorption, and secretion. This occurs in the kidneys.

Glomerular Filtration

• Blood enters the glomerulus water and solutes get filtered into Bowman's capsule.

• Filtration is driven by blood pressure.

Tubular Reabsorption

• Essential substances like glucose, amino acids, and ions get reabsorbed into the blood.

• Basically in the proximal convoluted tubule.

• Plays a significant role in the reabsorption process of important nutrients and water balance.

Tubular Secretion

• The secretion of waste products hydrogen ions, potassium, and some drugs into the tubule.

• This contributes to excess unwanted compounds that are eliminated from the blood.

Disorders of the Excretory System

Many disorders of the excretory system, which inhibits the proper clearance of wastes and continued homeostasis.

Kidney Stones

• These are caused by low fluid intake, high salt consumption, and genetics.

• Symptoms include severe pain, blood in urine, and frequent urination.

• Treatment is increased fluid intake, medications, and surgery.

Renal Failure

• The kidneys lose their ability to filter blood effectively, causing waste products and toxins to accumulate.

• Fluid, electrolyte, and acid–base balance become disturbed, affecting multiple organ systems.

• Severe cases may require dialysis or kidney transplantation to sustain life.

GFR Abnormalities

• Stages 1 to 5, based on GFR

• Caused by diabetes, blood pressure, glomerulonephritis.

• Symptoms include fatigue, swelling, nausea, and decreased urine output.

• Treatment options include behavioural changes, medications, dialysis, and kidney transplants.

Kidney & Nephron NEET MCQs (With Answers & Explanations)

Important questions asked in NEET from this topic are:

• Major organs in excretory system

• Mechanism of urine formation

Practice Questions for NEET

Q1. Middle protective layer seen in kidneys of human is

1. Renal capsule

2. Renal fascia

3. Renal peritoneum

4. Adipose layer

Correct answer: 4) Adipose layer

Explanation:

The layers surrounding the kidney are three.
a) Renal fascia (outermost) - This is the dense connective tissue layer that provides support and protection to the kidneys and adrenal glands.
b) Adipose capsule (intermediate) - This is known as perirenal fat; it surrounds the kidneys and serves to cushion and insulate them.
c) Renal capsule (innermost) - A thick fibrous layer that directly encases the kidney, providing further protection and maintaining its shape.

Hence, the correct answer is option 4) Adipose layer

Q2. Columns of Bertini in the kidneys of mammals are formed as extensions of

1. Cortex in the medulla

2. Cortex in pelvis

3. Medulla in pelvis

4. Pelvis in ureter

Correct answer: 1) Cortex in the medulla

Explanation:

Columns of Bertini-The renal cortex extends between the renal pyramids in the kidney to form the columns of Bertini, also referred to as renal columns. They consist of fibrous material, urine tubes, and blood arteries. The cortex is anchored in place by the Bertini columns.

Hence, the correct answer is option 1) Cortex in the medulla.

Q3. Skin can eliminate certain substances like

1. Sweat

2. NaCl

3. Urea

4. All of these

Correct answer: 4) All of these

Explanation:

The sweat and sebaceous glands in the skin can eliminate certain substances through their secretions. The sweat produced by the sweat glands is a watery fluid containing NaCl, small amounts of urea, lactic acid, etc. This procedure aids in both thermoregulation and the body's removal of trace amounts of waste.

Hence, the correct answer is option 4)All of these.

Also Read:

• MCQs on Human Excretory System

• Ultrafiltration

• Counter Current Mechanism

Recommended Video for Kidney and Nephron