Anatomy and Physiology of Kidney

N. Loganayaki

Assistant Nursing Superintendent, Kauvery Hospital, Hosur, Tamil Nadu

Introduction

The Kidneys lie on the posterior abdominal wall, one on each side of the vertebral column, behind the peritoneum and below the diaphragm. They extend from the level of the 12th thoracic vertebra, receiving some protection from the rib cage. The right kidney slightly lower than the left kidney, probably because of the considerable space occupied by the liver.

Anatomy of the Kidney

Kidneys are bean shaped organs, approximately about 11 cm long, 6cm wide, 3cm thick and weigh 150 g.

Organs associated with Kidneys

Right Kidney

Superiorly – the right adrenal gland

Interiorly – the right lobe of the liver, the duodenum

Posteriorly- the diaphragm, and muscles of the posterior abdominal wall.

Left Kidney

Superiorly – the left adrenal gland

Interiorly – the spleen, stomach pancreas, jejunum and splenic flexure of the colon.

Posteriorly- the diaphragm, and muscles of the posterior abdominal wall.

Blood and Nerve Supply to Kidneys

The kidneys receive blood from the paired renal arteries which branch directly from the abdominal aorta, Venous return through the renal veins into the inferior vena cava. The kidneys are also innervated by the renal plexus, a network of nerves that regulate blood flow and kidney function

Parts of the Kidney

Kidney capsule (renal capsule)

The renal capsule consists of three layers of connective tissue or fat that cover kidneys. It protects kidneys from injury, increases their stability and connects kidneys to surrounding tissues.

Renal artery

The renal artery is a large blood vessel that controls blood flow into kidneys

Renal cortex

The outer layer of kidney, where the nephrons (blood-filtering units) begin. The renal cortex also creates the hormone erythropoietin (EPO), which helps make red blood cells in bone marrow.

Renal medulla

The renal medulla is the inner part of kidney. It contains most of the nephrons with their glomeruli and renal tubules. The renal tubules carry urine to the renal pelvis.

Renal papilla

These pyramid-shaped structures transfer urine to the ureters. Dehydration and certain medications — especially) NSAID — may damage your renal papilla.

Renal pelvis

This funnel-shaped structure collects urine and passes it down two ureters. Urine travels from the ureters to the bladder.

Renal vein

This vein is the main blood vessel that carries filtered blood out of your kidneys and back to your heart. Each of your kidneys has a renal vein.

Microscopic structure of the Kidney

The kidney is composed of about 1-2 million functional units, nephrons and smaller number of collecting ducts. The collecting ducts transport urine through the pyramids to the calyces and renal pelvis. The connecting ducts are supported by a small amount of connective tissue, containing blood vessels, nerves and lymph vessels.

Nephron Structure

Functions of the Kidney

1. Filtration and Waste Removal

Filtering Blood: Kidneys act as filters, removing waste products and excess fluid from the blood.

Urine Production: The filtered waste and excess fluid are excreted from the body as urine.

Removing Toxins: Kidneys remove toxins and other harmful substances from the blood.

2. Fluid and Electrolyte Balance

Maintaining Water Levels: Kidneys help regulate the amount of water in the body.

Balancing Electrolytes: They maintain the correct levels of essential electrolytes like sodium, potassium, and calcium.

Acid-Base Balance: Kidneys help maintain the body’s acid-base balance, ensuring the blood remains in a neutral pH range.

3. Hormone Production

Erythropoietin: Kidneys produce erythropoietin, a hormone that stimulates the production of red blood cells in the bone marrow.

Renin: They release renin, an enzyme that helps regulate blood pressure.

Calcitriol: Kidneys convert an inactive form of vitamin D into its active form, calcitriol, which is crucial for calcium absorption and bone health.

4. Blood Pressure Regulation

Fluid Balance: Kidneys play a crucial role in regulating blood pressure by controlling fluid and electrolyte balance.

Hormonal Control: They produce hormones like renin that help regulate blood pressure.

5. Bone Health

Calcium and Phosphorus Regulation

Kidneys help regulate calcium and phosphorus levels, which are essential for strong bones.

Vitamin D Activation

They activate vitamin D, which is necessary for calcium absorption from the intestines.

Formation of Urine

The kidneys form urine, which passes through the ureters to the bladder for storage prior to excretion. The composition of urine reflects exchange of substance between the nephron and blood in the renal capillaries. Waste products of protein metabolism are excreted, electrolyte levels are controlled and PH acid base balance is maintained by excretion of hydrogen ions.

Process involved in formation of urine

  • Infiltration
  • Selective Re absorption
  • Secretion

Infiltration

This takes place through the semipermeable wall of the glomerulus and glomerular capsule. Water and other small molecules pass through, although some are reabsorbed later. Blood cells, Plasma proteins and other large molecules are too large to filter through therefore remain in the capillaries.

Selective Re-absorption

Some ions eg. Sodium and chloride can be absorbed by both active and passive mechanisms depending on the site in the nephron. Some constituents of glomerular filtrate (eg. Glucose, aminoacids), do not normally appear in urine, because they completely reabsorbed unless blood levels are excessive. Reabsorption of nitrogenous waste products, such as urea, uric acid and creatinine is very limited.

Secretion

  • Parathyroid Hormone
  • Antidiuretic hormone
  • Aldosterone
  • Atrial natriuretic peptide

Water balance and urine output

The source of most body water is dietary food and fluid and a small amount called (metabolic water) is formed by metabolic processes. Water is main constituent of urine, in expired air, faeces and through the skin is sweat. The amount lost in expired air and faeces is fairly constant, the amount of sweat produced is associated with environmental and body temperature.

The balance between fluid intake and output is controlled by kidneys. Urinary volume in excess of this is controlled mainly by antidiuretic hormone (ADH) released in to the blood by the posterior lobe of the pituitary gland.

Electrolyte Balance

  • Sodium and Potassium Balance
  • Renin- Angiotensin- Aldosterone system
  • Calcium Balance

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