Kidney: Structure, Function and Related Diseases

What is a Kidney?

Kidneys are organs of utmost importance to the human body for generating blood-filtering, clearing waste, and other homeostatic functions. They lie on either side of the vertebral column at the rib cage and are approximately fist-sized.

Location 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.

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.

Structure of the Kidney

The structure of the kidney includes:

External Anatomy

• The kidneys are two bean-shaped organs that have a convex border and a concave border.

• On the inner concave side, there is a notch called the hilum where the renal artery enters, and the renal vein and ureter leave.

• The outermost layer of the kidney is a tough capsule.

Internal Anatomy

• It distinguishes between an outer renal cortex from an inner renal medulla within the interior of the kidney.

• Extensions from the hilum extend into a funnel-like space, called renal pelvis, which has projections called calyces.

• Divided plies of medullary pyramids project into the calyces, and the cortex extends between these pyramids as a renal column, also called Columns of Bertini.

Nephrons – Functional Units

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. Each nephron is made up of the glomerulus and Bowman's capsule, the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule.

• 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.

• 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.

Functions of the Kidney

The functions of the kidney include:

Filtration of Blood

Each kidney filtrates blood to produce urine. Filtration takes place in the glomerulus under sufficiently high blood pressure, which forces water and solutes to pass through a membrane.

Reabsorption and Secretion

Reabsorption into the bloodstream of what is needed from the filtrate occurs in the kidneys, with simultaneous secretion of waste products directly into the urine. Mostly occurs in the PCT and loop of Henle, as well as the DCT.

Regulation of Blood Pressure

The kidneys maintain the blood pressure through the renin-angiotensin-aldosterone system, whereby they alter the level of blood volume and the degree of constriction of the blood vessels.

Electrolyte Balance

The levels of electrolytes in the body are maintained by the kidneys through the reabsorption or excretion of ions such as sodium, potassium, and calcium.

Excretion of Metabolic Wastes

Kidneys clear metabolic wastes like urea and creatinine from the blood.

Hormone Production

Kidneys produce hormones like erythropoietin, which stimulates red blood cell production, or calcitriol, which works to help in calcium absorption.

Disorders of Kidney

The common disorders related to kidneys are:

Chronic Kidney Disease (CKD)

Progressive loss of kidney function characterises CKD. Diabetes, hypertension, or another cause can be the basis for this situation. It can present with fatigue, swelling, and a change in urination. It may be treated with medication, lifestyle modifications, and eventually dialysis or transplantation.

Acute Kidney Injury (AKI)

AKI is a sudden decrease in kidney function, generally due to trauma, infection, or medicine. It requires immediate medical attention and can be reversible if treated promptly.

Kidney Stones

kidney stones are made up of mineral salts and deposits that form inside the kidneys. They may cause severe pain, nausea, and problems related to urinary flow. Treatment comprises medication, increased fluid intake, and sometimes even surgery or lithotripsy.

Glomerulonephritis

This is the inflammation of the glomeruli. It produces pink urine, swelling, and an increase in blood pressure. Treatment differs according to cause and severity.

Polycystic Kidney Disease (PKD)

It is a genetic disorder wherein cysts form inside the kidneys. It can cause renal failure and therefore needs management by medication and alteration in lifestyle.

Urinary Tract Infections (UTIs) and Pyelonephritis

UTIs can spread up to the kidneys and cause pyelonephritis. The symptoms are fever, backache and urinary complaints. It is treated with antibiotics.

Diagnosis of Kidney Diseases

The diagnostic methods for kidney disease include:

Laboratory Investigations

• Urinalysis

• Blood tests

• Imaging techniques like ultrasound, CT scan, MRI

• Renal biopsy

Treatment Modalities

Disorder related to kidney can be managed by:

• Lifestyle changes like proper diet, regular exercise and avoiding smoking and drinking alcohol.

• Symptomatic and progression-retarding drugs

• Mainly hemodialysis and peritoneal dialysis are done for the blood filtration process

• Kidney transplant in cases of end-stage kidney failure

Kidney NEET MCQs (With Answers & Explanations)

Important questions asked in NEET from this topic are:

• Functions of the kidney

• Kidney related disorders

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

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