Excretion:

Excretion is the removal of harmful and unwanted substances, metabolic wastes, especially nitrogenous wastes, from the body.

It is an essential life process in all forms of life.

Human Excretion:

We eat food and later it digested and metabolized in our body.
The body uses all the useful substances and sorts out all the toxic substances.
All these harmful toxic substances are removed out of the body by a process called excretion.
A human body a complex system where a lot of processes (respiration, circulation, digestion, etc.) take place simultaneously.
As a result, a lot of waste products produced in our body i.e. carbon dioxide, water, and nitrogenous products like urea, ammonia, and uric acid.
A simple diffusion or evaporation will not be sufficient to remove waste from our body.
Excretion is also one of the vital life processes.

In human beings, there is specialized organ for excretion which is consisted in excretory system of human beings.

A pair of kidneys
Kidneys are deep red, bean-shaped.
Kidneys separate useful substances by reabsorption and toxic substances from blood by producing urine.
Kidney has a structural filtration unit called nephron where the blood is filtered.
Pair of ureters:
Ureters are tube-like structures which arise from the notch, i.e. the hilum of each kidney.
The ureters connect with the urinary bladder.
The ureters carry the urine produced to the urinary bladder.
Urinary Bladder
It is a muscular sac-like structure.
Also, it stores urine temporarily.
Muscular sphincters guard its opening.
The urinary bladder is under the control of the Central Nervous System.
The sphincters open at the time of urination.
Urethra
It is a short muscular tube which expels urine out of the body.
The urethra is quite long in males while very short in females.
The brain signals the urinary bladder to contract and through the urinary opening called urethra, we excrete the urine.

Nephron is the structural and functional unit of the kidney.

Each kidney is composed of approximately 1 million Nephron or Uriniferous tubules.

They are also known as uriniferous tubules, renal tubules or kidney tubules.

Each nephron has a Malpighian body and body of tubules.

A malpighian tubule consists of a glomerulus which is surrounded by a glomerular capsule (Bowman’s capsule).

Bowman’s capsule, a cup-shaped depression, where a tuft of blood capillaries called glomerulus is situated.

The process by which urine forms takes place in two steps i.e ultrafiltration and reabsorption.

There are three renal processes involved in the formation of urine: Glomerular filtration, Tubular reabsorption of the filtrate, Tubular secretion.

There develops a hydrostatic pressure on the blood as efferent arteriole is narrower than the afferent arteriole.
Thus, the blood flows with high pressure through the glomerulus.
Due to the pressure, the liquid part of the blood filters out from the glomerulus and passes into the Bowman’s capsule.
The glomerular filtrate consists of water, urea, salts, glucose and other plasma solutes.
Blood corpuscles, proteins and other large molecules remain behind in the glomerulus.
Therefore, the blood carried away by the efferent arteriole is relatively thick.

The kidney selectively reabsorbs substances it has already secreted into the renal tubules, such as glucose, protein, and sodium. These reabsorbed substances are returned to the blood.
As the glomerular filtrate passes down the tubule, water and other substances required by the body are reabsorbed.
Potassium ions and certain substances such as penicillin are passed into the forming urine.

Dialysis performs the function of kidney (when kidney doesn’t work) i.e removes excess water, solutes, toxic wastes from blood.
Alone one kidney can perform the work of both the kidneys.
If one kidney got damaged, a person can still live.
When both the kidneys damages then toxic wastes get accumulated in the blood. Thus harms the body.
The patient whose both kidneys are damaged undergoes kidney dialysis.
Artificial kidney is used in kidney dialysis.
Dialysis is used as a temporary treatment in either kidney injury or patient who will have kidney transplant.
This kidney has tubes with semi-permeable membrane.
These tubes are passed through a small tank filled with dialysing solution (contains water and glucose having same conc. as blood).
Blood is taken from artery of the patient’s arm and it enters in the artificial kidney.
Here, excess salts and urea are removed.
Now, the pure blood is returned in the vein of same arm.
Both dialysis and kidney performs the same work, but reabsorption doesn’t take place in excretion.

The excretion in plants is not a complex process that of animals.
Cellular respiration, photosynthesis and other chemical reactions within plants produce a large number of excretory products.
The main excretory products of plants are CO₂ (during respiration), O₂(during photosynthesis) excess water, nitrogenous compounds (during protein metabolism).
Excretion of gaseous products and excess water take place through stomata and lenticels.
The loss of water from plants is called transpiration.
Some of the waste products, plants store in leaves which fall off.
Old xylem stores gums and resins as waste products.