Removal of waste material from the body is for proper functioning and for maintaining the chemical balance that is known as excretion and the system of the body which perform the excretion is called an excretion system. The excretion system consists of kidney, ureter, urethra and urinary bladder. In vertebrates, excretion is carried out by lungs, kidney and skin. In single-celled organisms, wastes are eliminated through its surface. Excretion plays an important role in the control of osmotic pressure and balance between ions. Water molecules also maintain acid-base balance. There are main four organs that involve in Excretion :
(i)    Kidney: Kidney is a primary organ that removes toxic waste products in the form of urine.
(ii)    Lungs: Involve in the removal of water vapour and CO2 by exhalation and diffusion.
(iii)    Skin: Skin excretes sweat through sweat glands which is sound throughout the body and control temperature.
(iv)    Liver: Liver produces bile and excretes drugs, cholesterol and steroid hormones and toxic substances.
4.1.    Physiological Anatomy of Kidney
In Human body, many biological processes such as respiration, circulation, digestion etc. take place simultaneously. By this process, many waste products are formed in different forms. It includes urea, ammonia, CO2 and other toxic compounds.

The kidney is two bean-shaped organs located below the ribs on the posterior wall of the abdomen. Its weight is around 150 grams and size is about the drench of fist. The right kidney is slightly lower than the left kidney due to the presence of liver which displaces it downwards.
Renal artery and vein, nerves, lymphatic and ureter pass through kidney which carries the urine and finally pours into the urethra via urinary bladder for its excretion. The kidney is coated with a fibrous capsule which protects the delicate inner structure. If a kidney is bisected laterally then we can observe cortex which is present on the outer side of the kidney.
The function of the kidney is to remove toxins and certain acids such as phosphoric acid, sulphuric acid which are generated by the procedure of metabolism. It also helps to maintain the blood level of a person. The kidney is also responsible for some Hormones, which maintain the balance of the body. These hormones are:-
(i)    Erythropoietin: Erythropoietin is synthesized 85% by the kidney and 15% by the liver. This erythropoietin triggers erythrogensis in the bone marrow. A person who suffers from kidney failure also suffers from severe anaemia as the patient lives on Dialysis.
(ii)    Renin: Renin is secreted by Juxtaglomerular cells that regulate blood pressure.
(iii)    Calcitriol: This is the active form of vitamin D. Calcitriol is also synthesized by kidney during prolonged fasting. Kidney triggers gluconeogenesis. Calcitriol helps to maintain a calcium level for bones.
Kidney has a structural and functional unit where blood is filtered. This unit is called as Nephron.
4.2.    Nephron
The main functional unit of the excretion system is Nephron which is present in the kidney. There are nearly one million nephrons in each kidney. The number of nephrons can degrade due to an injury or with ageing. Nephron has two components:-
(i)    Glomerulus, which takes part in filtration.
(ii)    A long tube where urine formation takes place and leads for excretion.

Glomerulus and Bowman's capsule is the beginning of the nephron, where filtration occurs. The glomerulus is a capillary network that receives blood from afferent arteriole. The glomerulus is surrounded by Bowman's capsule. Bowman's capsule is composed of two layers, outer layer and inner layer. The outer layer is composed of simple squamous epithelium and the inner layer is composed of podocyte cells. Blood is filtrated through these podocyte cells so it is called glomerular filtrate and this glomerular filtrate moves towards the renal tube. 

4.3.    The parts of renal tubules are:-
4.3.1.    Proximal convoluted tube (PCT):- Proximal convoluted tube is a part of nephron of kidney located between Bowman capsule and Henle loop. The proximal convoluted tube has a brush border (microvilli) simple cuboidal epithelium.
The most important transport mechanism in the Proximal convoluted tube is reabsorption. Proximal convoluted tube involves 80% water, 80% ions, 100% glucose and amino acids to be reabsorbed. These components reabsorbed into the peritubular capillaries. Sodium reabsorption is mainly driven by P-Type ATPase and potassium is reabsorbed by the paracellular mechanism. Glucose and amino acids and other solutes are reabsorbed by secondary active transport.

4.3.2.    Henle Loop: It is a U-shaped tube that begins from the proximal tube. The filtered fluid travels from the proximal convoluted tube to Henle Loop. The Henle Loop can be divided into two parts:-
(1)    Descending Arm: It is a thin part or tube of the loop that is highly permeable for water molecules but low permeable for ions. It has simple cuboidal epithelium.

(2)    Ascending Arm: It is a thick part of the loop that is highly permeable for ions and low permeable of water molecules. So, no water reabsorption occurs in the ascending arm.

4.3.3.    Distal convoluted tube (DCT): Distal convoluted tube is located between the Henle loop and collecting duct. It is responsible for the regulation of potassium, sodium, calcium, phosphate and pH. The cells of the distal convoluted tube have thiazide-sensitive sodium chloride co-transporter and also permeable to calcium via TRPVS channel. DCT has cuboidal cells and large lumen and abundant in mitochondria. In DCT, there is the absence of Brush border epithelium. In DCT, ion transport is regulated by the endocrine system.
(→)    Parathyroid Hormone: It is responsible for reabsorption of more calcium and secretion of phosphate.
(→)    Aldosterone: It is responsible for sodium reabsorption and potassium secretion.
(→)    Atrial natriuretic peptide: Responsible for sodium secretion.

In DCT, the regulation of plasma sodium concentration by a hormones system is known as Renin-angiotensin system. It mediates extracellular volume and arterial Vasoconstriction. Thus, it regulates the Arterial Blood pressure. In this system, renin is secreted by the specialized juxtaglomerular cells in the response of low arterial blood pressure, loss of sodium ions and water in the form of urine.

This renin clears the Angiotensinogen. Angiotensinogen is an a2 – globulin which is released by the liver. In human, it is 453 amino acids long. Angiotensinogen level is increased by plasma corticosteroid, estrogen and thyroid hormone. This Angiotensinogen is converted by renin into 10 amino acids long angiotensin I. Now this Angiotensin I is converted into Angiotensin II in the presence of  Angiotensin-converting enzyme [ACE].
Angiotensin II is responsible for vasoconstriction and also stimulate pituitary gland for releasing Adrenocorticotropic hormones [ACTH].
This Adrenocorticotropic hormones ACTH stimulate the adrenal cortex for releasing aldosterone hormone that causes vasoconstriction and reabsorbs more sodium ions from urine. In exchange for the reabsorbing sodium into blood, potassium ion is secreted into the renal tube and excrete in Urine.

4.3.4.    Collecting Duct: Collecting duct is a part of the nephron that is located between the Henle loop and ureter. It begins in the renal cortex and extends deep into the medulla.
Collecting duct is impermeable to water. Antidiuretic Hormone (ADH) released in collecting duct to increase its permeability by increasing the number of Aquaporins, resulting in the reabsorption of water molecules. The aquaporins are the membrane protein that is permeable for water molecules but not for ions. Most of the water is reabsorbed as it leaves the collecting duct by osmosis.
4.4.    Anti-diuretic hormones (ADH) :
Antidiuretic hormones (ADH) is also known as vasopressin. It contains arginine. Antidiuretic hormones (ADH) determine the urine volume and whether urine will be concentrated or diluted. As Antidiuretic hormones (ADH) level increases, urine will be concentrated and urine volume less. Antidiuretic hormones (ADH) is also responsible for the constriction of blood vessels. In the absence of ADH or low level of ADH, water remains in the filtrate that leads to dialysis. At certain places in nephron where glomerulus capillaries lie at the intersecting region of cortex & medulla nephron with such arrangement is known as cortical nephron. In cortical nephrons, the proximal convoluted tube (PCT) & Distal convoluted tube (DCT) are short and less at the intersecting region. Whereas at certain places the arrangement of capillaries lies completely in the medulla region of the kidney. This arrangement of the nephron is known as Juxta medullae nephron. In Juxta medullae nephron, the arrangement of the Distal convoluted tube (DCT) & proximal convoluted tube (PCT) lies completely in the medullary region of the kidney. The size of the Distal convoluted tube (DCT) & proximal convoluted tube (PCT) is large & sometimes touches the renal papillae.

4.5.    Dialysis :
If the kidney does not work properly due to injection, injuries etc. This leads to poor filtration. In such a situation, Artificial kidneys are used to work properly for Blood filtration. This process is called dialysis.
4.6.    Urine formation :
Urine formation is essential for maintaining Homeostasis. In the kidney, three main processes are involved in Urine formation.
4.6.1.    Filtration: Infiltration, soluble water and waste products are transformed from the blood into the glomerulus. The ultrafiltration rate of the kidney is 180 litres of fluid per-day or approx 125 ml/min. In this process, blood enters the afferent arteriole and flow into the glomerulus.
Filtration Fraction: 20% of the total blood pumped by the heart each minute will enter the kidney to undergo filtration, called filtration fraction. Filtration occurs in Bowman's capsule and this filtrate called glomerular filtrate. Glomerular filtrate. Includes water, ions, glucose, fatty acids, amino acids, nitrogenous waste and some vitamins. But the absence of proteins calcium, iron, Hormone etc. This glomerular filtrate moves to the renal tubes [PCT, Henle loop DCT, collecting DUCT) where further process i.e. reabsorption, secretions occurs and form urine.

4.6.2.    Reabsorption: In this process, the necessary molecules, ions and water are absorbed from glomerular filtrate back into the blood. Reabsorption is most important to maintain body Homeostasis. These components pass through PCT, Henle loop, DCT and Collecting Duct.
4.6.3.    Secretion: In secretion, some unnecessary substance such as creatinine and drugs will be removed from the blood and pour into collecting Duct.
E    =    U    –    R    +    S
E    =    Excretion Rate
U    =    Ultra filtrate Rate
R    =    Reabsorption Rate
S    =    Secretion Rate
For Glucose - Glucose is transported by carriers called sodium-glucose transport. Normally all glucose reabsorbed. No glucose in Urine
Excretion Rate    =    0,        Secretion Rate    =    0        So,      
Ultra filtrate Rate    =    Reabsorption Rate
For Ions - Most of the ions are reabsorbed by proximal convoluted tube but not all. But there is no secretion.
Secretion Rate    =    O    So,     Excretion Rate    =    Ultra filtrate Rate  –  Reabsorption Rate
For Drugs - Reabsorption of Drugs so,
Excretion Rate    =    Ultra filtrate Rate  +  Secretion Rate
For Inulin - Inulin is a type of polysaccharide (fructose) which is not reabsorbed and not secreted.
Secretion Rate    =    0        Reabsorption Rate    =    0        
Ultra filtrate Rate    =    Excretion Rate

After filtration, Reabsorption and secretion, the fluid of end product which is not reabsorbed during filtration moves into the collecting Duct, called urine. Urine is mainly composed of water that has not been reabsorbed, ammonia, carbon dioxide, salts and other waste products. Normally, RBC and glucose are absent in Urine.
Excretory System
Removal of waste material from the body for the proper functioning and for maintaining the chemical balance is known as excretion and the system of the body which performs the excretion is called an excretion system.
Excretion system consists of Kidney, Ureter, urethra & urinary bladder.
The main functional unit of the excretion system is Nephron which is present in the kidney.
The function of the kidney is to remove toxins & certain acids such as phosphoric acid, sulphuric acid generated by the procedure of metabolism. Other than removing waste, they also help in maintaining the blood level of a person. Hormone erythropoietin is synthesized 80% by kidneys & 15% by the liver. This erythropoietin triggers erythrogenesis in the bone marrow. A person who suffers from kidney failure also suffers from severe anaemia as the patient lives on dialysis. The active form of vitamin D (calcitriol) is also synthesized by kidneys. During prolonged fasting kidneys trigger gluconeogenesis. A person suffering from chronic kidney disease failure has an abnormal concentration of potassium & other ions in the body.
4.7.    Blood supply to kidneys
Renal arteries supply blood to the kidney where upon entering the kidneys they become branched & from afferent arterioles. These afferent arterioles are connected to glomerular capillaries. The urine formation starts in these glomerular capillaries where the fluids the solute collected from all over the body concept plasma is filtered & urine formation initiates. The other end of these glomerular capillaries is known as efferent arterioles. Efferent arterioles are connected with a large network of peritubular capillaries. Which covers the renal tubules.
Glomerular capillaries & peritubular capillaries are separated by efferent arteriole & regulates the hydrostatic pressure. The hydrostatic pressure in glomerular capillaries is nearly about 60 mmHg which is very high & helps in fast filtration of the solute & fluid. Whereas the peritubular capillaries have hydrostatic pressure of nearly about 13 mmHg which is very low & helps in fast reabsorption of fluids.

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