Kidney structure and function:-

Kidneys are complex organs, they are important for maintaining normal body functions. The most important function is excretion. It is the process by which unwanted, metabolic wastes are eliminated.

During metabolism a lot of waste materials, residues of food, heavy metals and toxic substances are also present. All these should be eliminated either by the digestive system, lungs, skin, liver, and various systems are involved in excretion but their capacity is limited.

The Renal system is having maximum excretory function capacity. A Human being's survival rests on function and process done by the kidneys. The kidneys are a pair of bean-shaped organs present in all vertebrates. They remove waste products, maintain electrolyte balance and regulate blood pressure. So these are the most important organs in the body. 

In this article, we look at the structure and function of the kidneys, and their physiology.

Anatomy of Kidney

Kidneys are reddish, bean shaped organs. They are located just above the waist between the peritoneum and posterior wall of the stomach. So kidneys are said to be retroperitoneal organs as they are present posterior to the peritoneum of the stomach.

These are present between the levels of the last thoracic and third lumbar vertebrae (protected by eleven & twelve pairs of ribs. The right kidney is generally slightly lower than the left kidney because the liver occupies considerable space on the right superior kidney. 

Each kidney is about the size of a clenched fist. It is 3 centimetres(cm) thick, 6 cm wide, and 12 cm long. In males, the average weight is 129 grams(g) for the right kidney and 137 g for the left one. In females, the average weight is 108 g for the right kidney and 116 g for the left kidney.

Structure of Kidney

The kidneys are relatively small organs, but they receive 20–25% of the heart’s output.

Parts of Kidney

Kidneys consist of several parts, and each of them are discussed below.

01. Renal Capsule 

It is the outer membrane that surrounds the kidney. It is made up of thin, fibrous connective tissue. It helps the glomerulus to filter the blood.

02. Renal cortex

This is the outer , reddish region after the renal capsule of the kidney.

It contains several nephrons which have a filter called glomerulus and a tubule. 

03. Renal Medulla 

This is the inner region of the kidney. It contains 8- 12 renal pyramids or Malpighian pyramids. They look striated as they contain straight tubular structures and blood vessels. The main function of medulla is to regulate the concentration of urine.

04. Renal Pelvis

It is the shallow funnel-like structure in the central region of the kidney. The Renal pelvis is a large cavity that collects urine . It is located in the renal sinus extended to the ureter.

05. Calyx

The periphery of the renal pelvis is interrupted by cuplike projections called calyces. This chamber helps to pass urine. Minor calyces are present on the apex of renal pyramids, urine which is produced in kidney flow through renal papillary at the apex of minor calyx.

Two or three minor calyces are combined to form major calyces, which is helpful to pass urine to the ureter before it enters to the renal pelvis.

06. Medullary Pyramids

The medulla is separated into 8-18 conical regions which are called renal pyramids. They look striped because they contain millions of nephrons (functional unit of the kidney). Nephrons form urine and drain into small collecting ducts within the medullary pyramids. Renal papilla is formed by the connection of collecting ducts at the base of the pyramids.

07. Ureters 

These are two thin tubes 25-30 cm long. They are helpful for collecting the filtrate and urine from the renal pelvis and carry to the urinary bladder . Ureter muscles are tightened and relaxed to force urine down and away from the kidneys. Small amounts of urine flow from the ureters into the bladder about every 10 to 15 seconds.

08. Renal Arteries 

These are two larger blood vessels that help to carry oxygenated blood from heart to kidneys. The right renal artery supplies blood to the right kidney while the left renal artery supplies to the left kidney.

09. Renal Veins

These are also paired blood vessels which carry deoxygenated blood from kidney to inferior vena cava. It is located closer to the right kidney so the right vein is short than the left vein.

10. Renal Sinus

Renal sinus contains the following structures:

• Renal pelvis: Upper expanded part of ureter
• Subdivisions of pelvis: 8 minor calyces, 2 or 3 major calyces
• Unleash connective tissue and fat
• Branches of nerves, arteries and tributaries of veins.

Functions or Physiology of Kidney

Kidney performs several important functions like: Formation of urine, Maintains internal setting, manages fluid levels and electrolytes balance. Kidney also has some other roles which are explained further.

01. Role in Homeostasis

During the formation of urine, kidneys regulate various activities in the body, which are concerned with physiological states.

Homeostasis is maintained by the following,

• Removal of Waste Product: Which are formed in metabolic activities like: Urea - that is created by the breakdown of protein, Uric acid - last product in nucleic acid metabolism, Creatinine - end product of metabolism in muscles, Bilirubin - formed by the effect of haemoglobin degradation, Kidneys also egest foreign materials i.e., toxins, heavy metals, pesticides etc.
• Maintenance of Water Balance: Kidneys maintain the water balance within the body by preserving once it's decreased and excreting water when it is excess within the body. This is a very important process for homeostasis.
• Maintenance of Electrolyte Balance: Maintenance of electrolyte balance, especially sodium in relation to water balance. When the osmolarity of body water decreases, kidneys retain sodium and remove sodium when osmolarity increases.Osmolality is a measure of the body’s electrolyte-water balance,it is the ratio between fluids and minerals in the body. Dehydration is a primary cause of electrolyte imbalance.
• Maintenance of Acid-Base Balance: Kidneys help to keep PH of blood and body fluids within the narrow range. Normally in humans the acceptable pH range is 7.35–7.45. At levels below or above this range, the body enters a state of acidemia and alkaline. Kidneys play a significant role in preventing acidosis when compared to other organs like lungs, blood buffers. In fact kidneys are the only organ capable of eliminating metabolic acids like phosphoric acid and sulfuric acid etc.

02. Role in Hematopoiesis

Kidneys stimulate the production of erythrocytes, thrombocytes which are important for secreting erythropoietin (glycoprotein), thrombopoietin helpful for erythropoiesis.

03. Endocrine Role

The hormones secreted by kidneys are: Erythropoietin, Thrombopoietin, Renin, 1,25-dihydroxycholecalciferol (calcitriol), and Prostaglandins.

04. Role in BP Regulation

Kidneys play an important role in the regulation of arterial blood pressure. Kidneys regulates blood pressure by 2 ways:

• By regulating the amount of extracellular fluid.
• By the renin-angiotensin mechanism. 

They also reabsorb other products to help maintain homeostasis. Reabsorbed products like glucose, amino acids, bicarbonate, water, phosphate, chloride, sodium, magnesium, and potassium ions.