Excretory system

Key Concepts

• Excretory system
• Excretion
• Nephron
• Urinary bladder

Introduction

Excretion is the process of removing wastes and excess water from the body. The major wastes produced by us are carbon dioxide and urea.  

Carbon dioxide is produced by the process of respiration, and urea is produced by the decomposition of unused proteins in the liver.  

It is necessary to remove these wastes from our bodies as their accumulation is poisonous and harmful. The organs which remove waste from our body are the skin, lungs, and kidneys. 

Functions of Excretory System

It is an essential process in all living things, and it is one of the major ways the human body maintains homeostasis.  

It eliminates waste products in the body, such as urea, uric acid ammonia, and other waste products. 

These products are excreted from the body through urine. 

The excretory system helps in maintaining the osmotic level of blood and plasma. 

It is involved in the process of osmoregulation, a process that maintains the proper amount of electrolytes in the body regardless of external factors like temperature, diet, and weather conditions. 

Excretory Organs 

The main organs of the human excretory system are –

• Kidneys
• Bladder
• Ureters
• Urethra 

These organs work together to remove nitrogenous waste, i.e., urea, from our bodies. 

Kidneys 

The kidneys are the main organ of the excretory system. Kidneys are bean-shaped organs that are reddish-brown in color. The kidneys lie on either side of the vertebral column in the abdominal cavity attached to the dorsal body wall. The right kidney is placed lower than the left kidney as the liver takes up much space on the right side. Each kidney is about 11 cm long, 5 cm wide, and 3 cm thick. 

Kidneys are made up of a number of sub-units called nephrons. The kidney is divided into two major structures

Outer renal cortex 

The outer part of the kidneys consists of the glomerulus and convoluted tubules from the outer cortex. The outer cortex is surrounded by the renal capsule. The renal cortex provides the space for the renal artery, veins, and glomerular capillaries. Apart from providing a protective layer, it also manufactures the hormone erythropoietin, which is necessary for the synthesis of new blood cells.  

Inner renal medulla 

The smooth inner part of the kidney, which consists of the loop of Henle and renal pyramids, forms the renal medulla. 

Both of these regions contain uriniferous tubules or nephrons.  

Nephrons 

Nephrons are found in the kidneys and are responsible for the filtration of blood and the regulation of chemicals. Nephrons or uriniferous tubules are structural and functional units of the kidneys. Each nephron consists of renal corpuscle or Malpighian corpuscle and renal tubule. 

Each kidney consists of more than one million nephrons. Nephrons or uriniferous tubules are structural and functional units of the kidneys. 

Structure of nephron: 

Each nephron consists of two parts that are glomerulus and renal tubule. 

Glomerulus along with Bowman’s capsule, is called the Malpighian body or renal corpuscles. 

Glomerulus is full of capillaries (bunch) formed by afferent arterioles (branch of renal artery). 

Bowman’s capsule is a double-walled cup-like structure that surrounds the glomerulus. 

Just below the glomerulus, the tubule has three distinct regions: 

Proximal convoluted tubule (PCT) 

Loop of Henle’s 

Distal convoluted tubule (DCT) 

Proximal convoluted tubule: Behind the neck, the tubule continues to form a highly coiled network and is restricted to the cortical region of the kidney. 

Henle’s loop: It is a quite narrow and U-shaped hairpin-like loop with a descending limb that ends into the medulla and an ascending limb that coarse back to the cortex. 

Distal convoluted tubule: The ascending limb entering the cortex become a highly coiled distal convoluted tubule. It then continues as a short straight collecting tubule that joins the collecting duct. Each collecting duct receives the collecting tubules of a number of nephrons. 

Ureters: Ureters are thin muscular tubes emerging out from the hilum. The ureter is connected to the renal pelvis. The sole function of the ureter is to carry urine from the kidneys to the bladder. 

Urinary bladder: Urinary bladder is a sac-like structure, which lies in the pelvic cavity of the abdomen. It stores urine temporarily. 

Urethra: Urethra is a membranous tube, which conducts urine to the exterior. The urethral sphincters keep the urethra closed and opens only at the time of micturition (urination). 

Functions of the kidney 

Filtration – fluid pressure forces water and dissolved substances out of the blood. 

Reabsorption – returns useful items as blood cells, plasma protein, glucose, amino acids, some salts, and some water to the blood. Some urea and other salts are also reabsorbed. 

Secretion – involves active transport – removes residues from toxins drugs, moves    urea and uric acid into urine, removes excess potassium ions, and regulates pH of the blood. 

Mechanism of urine formation 

The process of urine formation includes the following three stages. 

• Glomerular filtration 
• Tubular reabsorption
• Tubular secretion 

Glomerular filtration: Urine formation begins with the filtration of blood through epithelial walls of the glomerulus and Bowman’s capsule. The filtrate is called the glomerular filtrate. Both essential and non-essential substances present in the blood are filtered. 

Tubular reabsorption: The filtrate in the proximal tubule consists of essential substances such as glucose, amino acids, vitamins, sodium, potassium, bicarbonates, and water that are reabsorbed into the blood by the process of selective reabsorption. 

Tubular secretion: Substances such as H+ or K+ ions are secreted into the tubule. Certain substances like potassium and a large number of drugs like penicillin and aspirin are passed into the filtrate in the distal convoluted tubule. This tubular filtrate is finally known as urine. Finally, the urine passes into collecting ducts to the pelvis and through the ureter into the urinary bladder by urethral peristalsis (waves of constriction in the ureters).  

The relaxation of sphincter muscles located at the opening of the urinary bladder controls the release of urine.   When the urinary bladder is full, the urine is expelled out through the urethra. This process is called micturition. A healthy person excretes one to two litres of urine per day. 

Disorders of excretory system

Uraemia: Malfunctioning of kidneys can lead to accumulation of urea in blood, a condition called uraemia that is highly harmful and may lead to kidney failure.  

In such patients, urea can be removed by a process called haemodialysis. 

Kidney transplantation is the ultimate method in the correction of acute renal failure that is kidney failures.  

Renal calculi: Stone or insoluble mass of crystallized salts (oxalate, etc.) formed within the kidney. 

Glomerulonephritis: Inflammation of glomeruli of the kidney. 

Summary

• Removal of nitrogenous waste from the body due to its metabolic activities is called excretion.
• The kidneys lie on the posterior abdominal wall, one on each side of the vertebral column. These are bean-shaped organs.
• The kidney contains 1-2 million nephrons which are the functional units of kidneys.
• The kidney is divided in 3 sections- outer fibrous capsule, cortex, and medulla.
• Filtration, reabsorption, secretion are the three steps of urine formation.
• Urine is a by-product of the osmoregulatory function of kidneys, which filter blood, reabsorb water and nutrients, and secrete wastes.
• Disorders of excretory system: uraemia, renal calculi, and glomerulonephritis.