Meaning Of Excretion System In Humans [Latest 2020]

Meaning Of Excretion System In Humans: Various types of chemical activities within the body not only produce the useful growth products and energy but also produce certain harmful waste products. These harmful chemicals must be eliminated our from the body. For this purpose, our body has a well developed and advanced system, which can eliminate out these toxic and harmful wastes out from the body. This system is called excretory system. The process of removing out harmful metabolic waste products from the body is called excretion. 

Besides this, the excretory system also maintenance the balance of water and ions within the body fluids such as blood and ECF.(extra cellular fluid). This process of regulating the amount of water and salts (ions) in the body is called osmoregulation. Both these processes (i.e., excretion and osmoregulation) are very closely associated phenomena and both are performed by the excretory system.

Excretion System In Humans

· Human beings are terrestrial animals. So, urea is the main excretory waste.

· The main excretory structures in human beings are one pair of bean shaped organs, called kidneys,

· These are located in the abdominal cavity on dorsal (back) side.

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Structure of Kidney

· Each kidney is a small compact structure.

· It measures about 10 cm x 5 cm x 3 cm and weighs about 125 to 170 gm.

· The concave surface of kidney bears a large opening, called hilum.

· Through this opening, renal artery and nerves enter the kidney and renal vein and ureter leaves the kidney. Internally, the kidney contains a large funnel shaped space, called pelvis.

Each kidney contains a large number of (about 1.2 million) small filtering units. These units are called nephrons (or uriniferous tubules).

· These units are responsible for the filtration of blood and formation of urine.

· Urine, which is formed by nephrons, goes to pelvis and then to ureter. Finally, it is poured into the urinary bladder.

· Urinary bladder is a pear shaped, muscular, sac like structure, present in lower abdomen (pelvic cavity). It serves to store the urine temporarily.

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Nephron (Uriniferous Tubules)

Nephrons are the functional units of kidney. These are very small (microscopic) structures. Each kidney contains about 1.2 million nephrons. Nephrons serve to filter the blood and form urine.

Structure of Nephron

A nephron is in the form of a long coiled tube. The diameter of the tube is variable in different portions. A nephron shows following parts:

(1) Bowman's Capsule

· One end of the nephronic tube becomes a cup Bowman's capsule like structure. This is called Bowman's capsule.

· It contains a bunch of blood capillaries, called glomerulus.

(2) Convoluted Tubule

· The tubular part of nephron is coiled and it is called convoluted tubule. It has two regions: (a) proximal convoluted tubule (PCT) and (b) distal convoluted tubule (DCT).

· The main function of convoluted tubules is the Selective reabsorption of substances and secretion of unfiltered waste.

(3) Henle's Loop: Nephronic tubule has a long turn. This is called Henle's loop. It has two limbs.

1. Descending limb

2. Ascending limb.

(4) Collecting Duct

• It is a large duct that receives the tubular filtrate (urine) from convoluted tubules (DCT) of many nephrons, associated with it.

• Many collecting ducts finally drain the urine into the pelvis. 

Blood Supply to the Nephron

The main function of the nephron is to filter the blood. The blood comes to the nephron through afferent arteriole that forms glomerulus. The glomerular capillaries unite to form efferent arteriole. 

The efferent arteriole again divides to form many fine capillaries. These capillaries form an extensive network over all parts of the nephron. This network of capillaries is called PTCN (peritubular capillary network). The capillaries of peritubular capillary network again unite to form renal venule that leaves the nephron.

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Mechanism Of Urine Formation

The basic mechanism of urine formation includes three processes-(a) ultrafiltration, (b) selective reabsorption and (c) secretion.

1. Ultrafiltration

· Uttfaliltration occurs in Bowman's capsule

· The blood that comes to nephron is filtered through the walls of glomerulus.

· The filtrate (which is called initial filtrate) comes into the cavity of convoluted tubule.

· The filtered blood goes into the PTCN (peritubular capillary network).

2. Selective Reabsorption

· It is the second step during urine formation.

· The initial filtrate contains many useful substances like glucose, amino acids and various ions.

· These are essential substances, which must not be eliminated to outside from the body with urine.

· Therefore, the capillaries of the PTCN (peri tubular capillary network) selectively reabsorb the essential substances from the tubular filtrate. This is called selective reabsorption.

· Every day, about 180 L of initial filtrate is formed. But the actual amount of urine that is passed to outside is about 1 to 1.5 L/day. So, a large amount of water is reabsorbed. The initial filtrate is now called tubular filtrate.

3. Secretion

· The blood, which is filtered in glomerulus, still contains some harmful materials like creatine. creatinine, remains of drugs etc., which must be removed out from the blood.

· So, these substances are selectively secreted from blood, into the tubular filtrate. This process is called secretion.

· After these three steps, the tubular filtrate becomes the urine, which is collected by collecting duct. From collecting ducts, urine goes to pelvis and then to ureter. It is finally collected in the urinary bladder. Time to time, it is passed to outside through urethra. The act of passing out of urine is called micturition.

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Renal Failure and Hemodialysis (Artificial Kidney)

In some persons, due to some diseases or infection, kidneys do not function properly. In more severe cases, kidneys may completely stop their function. This condition is called renal failure.

In persons, suffering from renal failure, the blood urea level rises abnormally. Excessive urea in the blood may be dangerous. Thus, to remove the urea from the body of such persons, an artificial kidney is used. This artificial kidney is called hemodialyser and the mechanism of removing urea by using machine is called hemodialysis (or simply dialysis).

Mechanism and Concept of Dialysis

·Blood is taken out from an artery of the patient, and it is cooled to 0°C. It is mixed with an anticoagulant (heparin) and pumped into artificial kidney (hemodialyser).

· This machine contains channels, which are provided with a dialysing membrane, made of cellophane.

· A dialysing fluid is also used in the machine, which contains some small solutes and mineral ions, but does not contain nitrogenous waste products, such as urea, uric acid and creatinine.

· The blood of the patient, moving inside the machine, is kept separated from dialysing fluid through cellophane membrane.

· Now, due to concentration difference across the two sides of the membrane, the wastes (urea, uric acid and creatinine etc.) diffuse from the blood to the dialysing fluid across the cellophane membrane.

· In this way, the blood is considerably cleared of nitrogenous waste products without losing plasma proteins.

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