Human Excretory System

The generation of energy from food leads to the generation of waste too in the human body. The bi-products of the various chemical and physiological processes which are no longer needed for the human body need to be eliminated from the body. For simple organisms such as unicellular organisms, excretion is also simple and takes place by the elimination of gases and water or by diffusion. Inside the complex bodies of human beings, excretion is also a complex process that takes place through an organ system composed of various organs.

We all get our nutrients from various sources, which are then digested and processed in our bodies. The process of digestion and metabolism helps us absorb nutrients and generate energy in our bodies. These chemical reactions and processes lead to the production of toxic substances. Following metabolic responses, the body begins to separate helpful and hazardous molecules in an individual. Toxin buildup, as we all know, may be dangerous, and the body eliminates all metabolic wastes through a process known as excretion.

These toxic substances, if kept inside the body for a long time, will start affecting it. Hence, they need to be thrown out from the body regularly. The process of excretion makes sure that the body gets rid of the toxic substances from the body time to time.

There are various modes of excretion. Depending upon the morphology of the organism, the habitat where it lives, or the kind of waste products generated, excretion takes place in various ways by various means. 

Excretion can be carried out by lungs, skin, kidneys, etc.

Aquatic creatures, for example, fishes, have abundant availability of water, hence the waste generated in fishes is ammonia, which is highly soluble in water. Humans generate urea as a waste product that is fairly soluble in water. That is also why we are suggested to drink plenty of water, so that elimination of waste from our body takes place smoothly. In the case of birds, they hardly have the availability of water, and the waste generated in their bodies is uric acid, which requires less water to keep the process of excretion going.

Let us take a closer look at the excretory system notes to learn more about the excretory system in humans, distinct organs of the excretory system, and the excretory mechanism in humans.

Human Excretory System

A dedicated system of organs that removes waste products from the human body is called the human excretory system. The major waste product generated in the human body is urea. Along with which some other toxins are also generated. Urea is eliminated by kidneys in the form of liquid by the process of urination and solid wastes are removed from the body by the intestines.

Organs of the Excretory System

Organs of the human excretory system include:

• A set of kidneys

• A ureter and a uretero-uretero-uretero

• A bladder that stores urine

• The urethra

Excretory System Organs

The main organs of the human excretory system are-

• Kidneys

• Ureters

• bladder

• urethra

These organs work together to remove nitrogenous waste- urea from our bodies.

The kidney's structure is described below: 

Kidneys

The kidneys are bean-shaped organs that are positioned on either side of the backbone and are protected by the ribs and back muscles. They are primarily responsible for the production of urine. Kidneys collect waste from our bloodstream and turn that waste into a simpler form (urine) which is then eliminated from the body. Kidneys are present in the abdomen, just below the diaphragm. Each adult human kidney measures 10-12 cm in length, 5-7 cm in breadth, and weighs 120-170g.

The kidneys have a concave inner structure. The blood vessels, ureter and nerves enter the kidneys through the hilum, which is a notch at the inner concave surface of the kidney. The renal pelvis, a large funnel-shaped space that is present inner to the hilum, has many projections known as calyces.

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

Capsule

The capsule refers to the outer layer. The kidney is divided into two zones: the cortex on the outside and the medulla on the inside. As renal columns are known as columns of Bertin, the cortex spreads between the medullary pyramids.

1. 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 and 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. 

2. Inner renal medulla

The inner smooth part of the kidney which consists of the Loop of Henle and renal pyramids forms the Renal Medulla.

Nephrons

The functioning units of the kidney are known as nephrons. Each of our kidneys has as many as 1 million nephrons in each human kidney. The basic functions of nephrons are to carry out secretion as well as the process of excretion. Each nephron is made up of two parts: the glomerulus and the renal tubule. 

• Glomerulus - Glomerulus is a mass of capillaries Glomerulus is a collection of capillaries created by afferent arterioles which absorb protein from the blood that travels through the Malpighian Body. Different arterioles transport blood out from the glomerulus.

• Renal Tubule - Bowman's capsule, which encloses the glomerulus, forms the beginning of the renal tubule. The malpighian body is made up of the glomerulus and the Bowman's capsule. The proximal convoluted tubule is the heavily coiled structure in the tubule adjacent to Bowman's capsule. Renal Tubule is composed of a Proximal convoluted tubule that lies in the cortex and absorbs water, sodium, and glucose back into the blood, a distal convoluted tubule that reabsorbs sodium into the blood and absorbs potassium and acid from blood and Loop of Henle. 

Henle's loop

Henle's loop is the long ‘U tubule’ which has an ascending and descending limb and is the next section of the tubule. As a distal convoluted tubule, the ascending loop continues. Many nephrons have distal convoluted tubules that open into the collecting duct. It is primarily involved in the recovery of water and sodium chloride from urine is Loop of Henle. The Loop of Henle produces very concentrated urine. Therefore, desert animals have a highly efficient Loop of Henle to carry out excretion with a very less amount water. The Loop of Henle lies in the medulla region.       

Nephrons are of two types, namely

1. Cortical Nephrons - The nephrons which are at a higher position in the cortex with a Short Loop of Henle which does not penetrate into the medulla are called Cortical Nephrons.

2. Juxtamedullary Nephrons - Juxtamedullary nephrons have a longer Loop of Henle which runs deeper into the Renal Medulla.

Ureter

Each kidney is connected to the renal pelvis by a thin and muscular tube. This tube is called the Ureter. The function of the ureter is to propel urine from the kidneys to the urinary bladder from time to time.

Urinary Bladder

Urinary bladder is the organ that controls the passage of urine. It is a muscular sac-like structure present above the pubic bone that stores urine. The average capacity of a human bladder is 400-600 ml. It holds urine until its capacity and expels it by the process of micturition when it is full. The bladder is lined by muscular tissue, which squeezes during micturition allowing the urine to flow out.

Urethra

The urethra is a tube made up of fibro-muscular tissue and emerges from the lower opening of the urinary bladder and extends through urogenital and pelvic diaphragms and opens out of the body through an external urethral orifice. It aids in the removal of urine from the body. In males, it carries the sperms as well. The Urethra is guarded by the sphincter which relaxes during urination.

The function of the Human Excretory System

The main function of the human excretory system is to eliminate wastes from the body which are generated as the bi-products of metabolism. The excretory system works in order to maintain homeostasis. Homeostasis is maintaining an equilibrium in the body by keeping the temperature and fluid balance stable. Apart from the metabolic wastes, there are other chemical and hormonal wastes too which are generated in the body and are excreted by various processes. The urinary system, however, filters the blood of nitrogenous waste- urea, salts, and excess water and excretes them out of the body in the form of urination. Kidneys also help in the reabsorption of useful substances.

The filtration of blood occurs in the nephrons in kidneys. Millions of nephrons work together to absorb minerals like glucose, amino acid, salts and let the pure blood flow to other parts of the body. Urea and excess water are converted into urine which is passed to the bladder through the ureters and expelled from the body through the urethra. The Urinary bladder holds the urine for some time, till it's full and receives signals from the brain. Upon receiving the signal, the sphincters at the opening of the bladder relax and urine is expelled. 

Excretion Mechanism in Humans : Process of Urine formation

In humans, Urine is generated in the nephrons and the elimination process is divided into the following steps:

• Glomerular Filtration

• Tubular Reabsorption

• Secretion

Glomerular Filtration

The first step of urine formation occurs in the glomerulus of the kidneys. The blood enters the glomerular capillaries by afferent arteriole and leaves by an efferent arteriole. In this process excess water, ions, glucose, and waste products are eliminated from the blood into the urine collection which is then eliminated from the body.

The glomerular filtrate consists of water, glucose, ions along urea. Therefore, these necessary substances need to be reabsorbed into the body. It happens by tubular reabsorption.

The rate at which the glomerulus filters blood and produces a filtrate is called the glomerular filtration rate.

Tubular Reabsorption 

It is the second step of urine formation and is a very important step, as it makes sure that essential substances and ions are excreted from our bodies. In this process, ions and substances such as sodium ions, glucose, amino acids, water, etc are absorbed. It also ensures less water loss. There are two types of transport taking place during tubular reabsorption. 

1. Active transport- Glucose and Ions are absorbed by active transport.

2. Passive transport- Water is absorbed by passive transport.

Various tubules that carry out the process of reabsorption are as follows. 

1. Proximal Convoluted Tubule (PCT) - The PCT reabsorbs ion, water, and nutrients from the filtrate. By secreting potassium, hydrogen, and ammonia to filtrate and reabsorbing bicarbonate ions from the filtrate. It also eliminates toxins from the filtrate aid in regulating the ionic balance and pH of bodily fluids. thus maintaining the pH of the filtrate.

2. Ascending loop of Henle - Allows the reabsorption of sodium and chloride ions from the filtrate.

3. Descending Loop of Henle is impervious to water but permeable to electrolytes. The passage of electrolytes from the filtrate to the medullary fluid dilutes the filtrate. It allows the reabsorption of water. 

4. Distal Tubule - Reabsorbs selective ions from the filtrate including sodium chloride ions. The Distal Convoluted Tubule (DCT) is responsible for the reabsorption of water and sodium ions. It also aids in the maintenance of pH and ionic equilibrium by the secretion and reabsorption of ions such as PCT.

5. Collecting duct - Connect the nephrons to the minor calyx or renal pelvis and reabsorbs a substantial amount of solutes and water from the filtrate.

Secretion

Along with the formation of urine, the waste ions like hydrogen and potassium, and ammonia are secreted out which are then mixed with the urine and expelled from the body. This is called secretion.

After a series of these processes, the bladder gets filled with urine. Upon signaling from the brain, the bladder relaxes and releases urine from the body by the process of micturition or urination.

Micturition

The bladder is stretched and filled with urine produced by the nephrons. The receptors on the urinary bladder walls convey signals to the Central Nervous System, allowing the sphincter muscles to relax and discharge urine. This is referred to as micturition.

Human Urine Excretion:

Excretion

Excretion is the process through which all metabolic wastes are eliminated from the organism. Excretion in humans occurs via a succession of procedures that include various bodily components and internal organs.

Diffusion: Diffusion is the most prevalent excretory mechanism in lower species. A human body is a remarkable mechanism in which several living activities (respiration, circulation, digestion, and so on) occur at the same time. As a result, many waste products created in our bodies take numerous forms, such as carbon dioxide, water, and nitrogenous compounds such as urea, ammonia, and uric acid. In addition, toxins and other hazardous components derived from pharmaceuticals and hormonal products are created. Simple diffusion will not suffice to remove these wastes from our bodies. Kidneys, which separate beneficial substances via reabsorption from poisonous substances via urine production.

The nephron is a structural filtration unit of the kidney where blood is filtered. A million nephrons are found in each kidney. Capillaries in the kidneys filter the blood, reabsorbing vital chemicals such as glucose, amino acids, salts, and the appropriate quantity of water before returning the blood to circulation.

In humans, urine is formed from excess water and nitrogenous waste. The ureters transport the generated urine to the urinary bladder. The urine bladder is controlled by the Central Nervous System. The brain causes the urinary bladder to contract, and urine is excreted through the urinary opening known as the urethra.

Dialysis

The kidneys fail to fulfil their tasks under specific conditions, such as insufficient blood supply to the kidneys, infections, trauma, and so on. In such cases, artificial kidneys are employed for blood filtering, a process known as dialysis.

Aside from the urinary system, the skin, lungs, and even the eyes aid in the excretion of waste items in various forms. Sweating is a method of excretion in which water, urea, and other salts are expelled via the skin. Lungs assist us in exhaling gaseous wastes such as carbon dioxide, nitrogen, and others.

Important Information About the Human Excretory System

• The human excretory system consists of organs that aid in the elimination of nitrogenous wastes from the body.

• The kidney, ureter, urinary bladder, and urethra are the primary excretory organs.

• The blood is filtered by the kidneys, and the filtrate is urine.

• Urine travels from the ureter to the urinary bladder before being evacuated from the body. This is referred to as micturition.

• Through excessive purification and filtration, kidneys also manage the osmotic pressure of a mammal's blood. This is referred to as osmoregulation.

• This was a thorough examination of the human excretory system.

Conclusion

As you can read, we have provided complete insight into the human excretory system which is a vital part of the human biological system.