Kidney; structure and function

Excretion is the process by which the unwanted substances and metabolic wastes are eliminated from the body. A large amount of waste materials and carbon dioxide are produced in the tissues during metabolic process. In addition, residue of undigested food, heavy metals, drugs, toxic substances and pathogenic organisms like bacteria are also present in the body. All these substances must be removed to keep the body in healthy condition. Various systems/organs in the body are involved in performing the excretory function, viz.

Woman Kidney

 

1. Digestive system excretes food residues in the form of feces. Some bacteria and toxic substances also are excreted through feces

2. Lungs remove carbon dioxide and water vapor

3. Skin excretes water, salts and some wastes. It also removes heat from the body

4. Liver excretes many substances like bile pigments, heavy metals, drugs, toxins, bacteria, etc. through bile.

Although various organs are involved in removal of wastes from the body, their excretory capacity is limited. But renal system or urinary system has maximum excretory capacity and so it plays a major role in homeostasis. Renal system includes:

1. A pair of kidneys

2. Ureters

3. Urinary bladder

4. Urethra.

Kidneys produce the urine. Ureters transport the urine to urinary bladder. Urinary bladder stores the urine until it is voided (emptied). Urine is voided from bladder through urethra.

FUNCTIONS OF KIDNEY

Kidneys perform several vital functions besides formation of urine. By excreting urine, kidneys play the principal role in homeostasis. Thus, the functions of kidney are:

ROLE IN HOMEOSTASIS

Primary function of kidneys is homeostasis. It is accomplished by the formation of urine. During the formation of urine, kidneys regulate various activities in the body, which are concerned with homeostasis such as:

Excretion of Waste Products

Kidneys excrete the unwanted waste products, which are formed during metabolic activities:

a. Urea (end product of amino acid metabolism)

b. Uric acid (end product of nucleic acid metabolism)

c. Creatinine (end product of metabolism in muscles)

d. Bilirubin (end product of hemoglobin degradation)

e. Products of metabolism of other substances.

Kidneys also excrete harmful foreign chemical

substances such as toxins, drugs, heavy metals pesticides, etc.

Maintenance of Water Balance

Kidneys maintain the water balance in the body by conserving water when it is decreased and excreting water when it is excess in the body. This is an important process for homeostasis.

Maintenance of Electrolyte Balance

Maintenance of electrolyte balance, especially sodium is in relation to water balance. Kidneys retain sodium if the osmolarity of body water decreases and eliminate sodium when osmolarity increases.

Maintenance of Acid–Base Balance

The pH of the blood and body fluids should be maintained within narrow range for healthy living. It is achieved by the function of kidneys. Body is under constant threat to develop acidosis, because of production of lot of acids during metabolic activities. However, it is prevented by kidneys, lungs and blood buffers, which eliminate these acids. Among these organs, kidneys play major role in preventing acidosis. In fact, kidneys are the only organs, which are capable of eliminating certain metabolic acids like sulfuric and phosphoric acids.

HEMOPOIETIC FUNCTION

Kidneys stimulate the production of erythrocytes by secreting erythropoietin. Erythropoietin is the important stimulating factor for erythropoiesis. Kidney also secretes another factor called thrombopoietin, which stimulates the production of thrombocytes.

ENDOCRINE FUNCTION

Kidneys secrete many hormonal substances in addition to erythropoietin and thrombopoietin.

Hormones secreted by kidneys

i. Erythropoietin

ii. Thrombopoietin

iii. Renin

iv. 1,25-dihydroxycholecalciferol (calcitriol)

v. Prostaglandins.

REGULATION OF BLOOD PRESSURE

Kidneys play an important role in the long-term regulation of arterial blood pressure by two ways:

i. By regulating the volume of extracellular fluid

ii. Through renin-angiotensin mechanism.

REGULATION OF BLOOD CALCIUM LEVEL

Kidneys play a role in the regulation of blood calcium level by activating 1,25-dihydroxycholecalciferol into vitamin D. Vitamin D is necessary for the absorption of calcium from intestine.

FUNCTIONAL ANATOMY OF KIDNEY

Kidney is a compound tubular gland covered by a connective tissue capsule. There is a depression on the medial border of kidney called hilum, through which renal artery, renal veins, nerves and ureter pass.

DIFFERENT LAYERS OF KIDNEY

Components of kidney are arranged in three layers :

Outer cortex

Inner medulla

Renal sinuses

1. Outer Cortex

Cortex is dark and granular in appearance. It contains renal corpuscles and convoluted tubules. At intervals, cortical tissue penetrates medulla in the form of columns, which are called renal columns or columns of Bertini.

2. Inner Medulla

Medulla contains tubular and vascular structures arranged in parallel radial lines. Medullary mass is divided into 8 to 18 medullary or Malpighian pyramids. Broad base of each pyramid is in contact with cortex and the apex projects into minor calyx.

3. Renal Sinus

Renal sinus consists of the following structures:

i. Upper expanded part of ureter called renal pelvis

ii. Subdivisions of pelvis: 2 or 3 major calyces and about 8 minor calyces

iii. Branches of nerves, arteries and tributaries of veins

iv. Loose connective tissues and fat.

TUBULAR STRUCTURES OF KIDNEY

Kidney is made up of closely arranged tubular structures called uriniferous tubules. Blood vessels and interstitial connective tissues are interposed between these tubules.

Uriniferous tubules include:

1. Terminal or secretary tubules called nephrons, which are concerned with formation of urine

2. Collecting ducts or tubules, which are concerned with transport of urine from nephrons to pelvis of ureter.

Collecting ducts unite to form ducts of Bellini, which open into minor calyces through papilla.

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