Homeostasis - Home of Knowledge




Question 1: What is homeostasis? What are the different types of homeostasis?


Homeostasis is the combination of two Greek words, Homios means similar and stasis means standing still. The maintenance of the internal conditions of body at equilibrium, despite changes in the external environment is called homeostasis. “OR” The ability of a living organism to maintain or nearly maintain constant internal environmental conditions is called homeostasis. “OR” The response of a living organism to changing environment is called homeostasis. Homeostasis helps an organism to survive and utilize its environment in best possible way.

For example, the core temperature of human body remains at about 37°C despite fluctuations in the surrounding air temperature. Similarly, the blood glucose level remains about 1g per litre despite eating a meal rich in carbohydrates. Body cells need the internal environment in which conditions do not change much. Stable internal conditions are important for the efficient functioning of enzymes.

Aspects of homeostasis: There are three main aspects of homeostasis.


Osmoregulation:  It is maintenance of the amounts of water and salts in body fluids (i.e. blood and tissue fluids). We know that the relative amounts of water and salts in body fluids and inside cells control the processes of diffusion and osmosis, which are essential for the functioning of cells.

Thermoregulation:  The maintenance of internal body temperature within a tolerable range is called thermoregulation. The enzymes of body work best at particular temperatures (optimum temperature). Any change in body temperature may affect the functioning of enzymes. For example: Optimum temperature for human enzymes is 37 (35.8 to 37.7 OC).


Question 2: What is excretion? How do the plants excrete extra gases, water, salts and other metabolic wastes from their bodies?

Answer: Excretion: The removal of metabolic wastes from the body is called excretion. By excretion toxic materials are removed from the body like nitrogenous materials (urea, ammonia, uric acid) and various gases. Metabolic waste means any material that is produced during body metabolism and that may harm the body.



Plants respond to environmental changes and keep their internal conditions constant i.e. homeostasis. They apply different mechanisms for the homeostasis of water and other chemicals (oxygen, carbon dioxide, nitrogenous materials etc). 


Excretion in plants: Plants do not have specialized organs for excretion. Plants get rid of various wastes by diffusion process.


1) Removal of Extra Carbon dioxide and Oxygen:

In daytime, the carbon dioxide produced during cellular respiration is utilized in photosynthesis and hence it is not a waste product. At night, it is surplus because there is no utilization of carbon dioxide. It is removed from the tissue cells by diffusion. In leaves and young stems, carbon dioxide escapes out through stomata.

In young roots, carbon dioxide diffuses through the general root surface, especially through root hairs. Oxygen is produced in mesophyll cells only during daytime, as a by-product of photosynthesis. After its utilization in cellular respiration, the mesophyll cells remove the extra amount of oxygen through stomata.


2) Removal of Extra Water:

Plants obtain water from soil and it is also produced in the body during cellular respiration. Plants store large amount of water in their cells for turgidity. The extra water can be removed by plants from their body by following processes.

Transpiration: Loss of water from aerial parts of plants is called transpiration. During day time Extra water is removed from plant body by transpiration through stomata or lenticles . At night, transpiration usually does not occur because most plants have their stomata closed.

Guttation: The oozing of water through hydathodes on leaf margins on the absence of transpiration is called guttation. If there is a high water content in soil, water enters the roots and is accumulated in xylem vessels. Small plants such as grasses force this water through special pores (hydathodes) , present at leaf tips or edges, and form drops.



Transpiration is the loss of water from plant surface in the form of vapors.


Guttation is not to be confused with dew, which condenses from the atmosphere onto the plant surface.


Removal of Metabolic Wastes: Plants deposit many metabolic wastes in their bodies as harmless insoluble materials. For example, calcium oxalate is deposited in the form of crystals in the leaves and stems of many plants e.g. in tomato.


In trees which shed their leaves yearly, the excretory products are removed from body during leaf fall.

Besides these processes some plants also have special secretory glandular tissues that secrete products like gums (by acacia or keekar), resin (by coniferous trees or pinus), mucilage (by carnivorous plants and ladyfinger) and latex (by rubber plant). These products are found in special canals in plants and ooze out when some part of plant is damaged.


The removal of excretory products is a secondary function of leaf fall. If the leaves are not shed, the calcium oxalate just remains as harmless crystals in the leaves


Osmosis is the movement of water from hypotonic solutions (less solute concentration) to hypertonic solutions (higher solute concentration), through

semipermeable membrane.


Question 3: What are the different types of plants based on osmotic adjustments? “OR” Types of plants based on osmoregulation?


Osmoregulation:  Osmoregulation is the maintenance of water and dissolved salts between an organism and its environment.

Types of plants based on osmoregulation: On the basis of the available amount of water and salts, plants are divided into three groups.





1) Hydrophytes: Hydro means water and phyton means plants. Hydrophytes are the plants which grow in areas with abundance of water. These are the plants which live completely or partially submerged in freshwater. Such plants do not face the problem of water shortage. They have developed mechanisms for the removal of extra water from their cells. Hydrophytes have broad leaves with a large number of stomata on their upper surfaces. This characteristic helps them to remove the extra amount of water. The most common example of such plants is water lily.

Characters of Hydrophytes: a) They have short roots with less or no root hairs.

b) Leaves have large surface area and large number of stomata.



2) Xerophytes: Xeric means desert (dry) and phyton means plant. Xerophytes are those plants that are found in habitats with scarcity or limited supply (dry environment) of water. They possess the following main properties that help to reduce loss of water.

They have strong or deep roots to absorb maximum water from soil.

Stem with water storage tissues.

Leaves are very small or modified into spines to reduce transpiration.

 They possess thick, waxy cuticle over their epidermis to reduce water loss from internal tissues.

They have less number of stomata to reduce the rate of transpiration.

They have special parenchyma cells in stems or roots in which they store large quantities of water. This makes their stems or roots wet and juicy, called succulent organs. The parenchyma cells store water in rainy season which is then used in dry season.

They have sunken stomata.

Hairs on aerial leaves.

Some xerophytes have no leaves where green stem perform process of photosynthesis.

Cacti (Singular Cactus) and Opuntia are the common examples of such plants.


3) Halophytes: Halo means salt and phyton means plant. plants that live in sea waters and are adapted to salty environments are called halophytes. Salts enter in the bodies of such plants due to their higher concentration in sea water. On the other hand, water tends to move out of their cells into the hypertonic sea water. When salts enter into cells, plants carry out active transport to move and hold large amount of salts in vacuoles. Salts are not allowed to move out through the semi- permeable membranes of vacuoles. So the sap of vacuoles remains even more hypertonic than sea water. In this way, water does not move out of cells.

 Many sea grasses are included in this group of plants.



* Important: Another type of plants is Mesophyte (Meso means moderate and phyton means plant) which live in areas with normal or moderate supply of water. Ordinary land plants are examples of mesophytes, such as rose, brassica, etc.


Question 4: Write a note on homeostasis in humans? What are the main organs of homeostasis in humans?


Like other complex animals, humans have highly developed systems for homeostasis.

The following are the main organs which work for homeostasis:

  • Lungs remove excess carbon dioxide and keep it in balance.
  • Skin performs role in the maintenance of body temperature and also removes excess water and salts.
  • The kidney filters excess water, salts, urea, uric acid etc. from the blood and forms urine.


1) Skin as homeostatic organ:

Skin consists of two layers. Epidermis is the outer protective layer without blood vessels while dermis is the inner layer containing blood vessels, sensory nerve endings, sweat and oil glands, hairs and fat cells.

Skin performs important role in the regulation of body temperature. The thin layer of fat cells in the dermis insulates the body. Contraction of small muscles attached to hairs forms ‘Goosebumps’. It creates an insulating blanket of warm air. Similarly, skin helps in providing cooling effect when sweat is produced by sweat glands and excess body heat escapes through evaporation. Metabolic wastes such as excess water, salts, urea and uric acid are also removed in sweat.


2) Lungs as homeostatic organs: During cellular respiration CO2 and water vapors are produced as waste products. These are required to be eliminated from the body as its accumulation is very dangerous. CO2 is an acidic gas which changes PH of the blood. Lungs perform basic role in removal of these wastes products through expiration.


Question 5: What is the functional unit of the kidney? Describe its structure and draw labeled diagram? What steps are involved in the formation of urine in the kidneys?


Answer: THE URINARY SYSTEM OF HUMANS: The excretory system of humans is also called the urinary system. It is formed of one pair of kidneys, a pair of ureters, a urinary bladder and a urethra. Kidneys filter blood to produce urine and the ureters carry urine from kidneys to urinary bladder. The bladder temporarily stores urine until it is released from body. Urethra is the tube that carries urine from urinary bladder to the outside of body.


External structure of Kidney: Human beings possess a pair of kidney. Kidneys are dark-red, bean shaped organs. Each kidney is 10 cm long, 5 cm wide and 4 cm thick and weighs about 120 grams. They are placed against the back wall of abdominal cavity just below diaphragm, one on either side of vertebral column. They are protected by the last 2 ribs and a layer of adipose tissues known as peri-renal fat each kidney is held in place by connective tissues called renal fascia . The left kidney is a little higher and anterior than the right due to the presence of liver on the right side which displaces the right kidney downward.

Kidney has two sides, the laterally convex side and the medial concave side. The concave side of kidney faces vertebral column. There is a depression, called hilus, near the centre of the concave area of kidney. This is the area of kidney through which ureter leaves kidney and other structures including blood vessels, lymphatic vessels and nerves enter and leave kidney. From the ureter urine run downward to the urinary bladder, where urine is stored and then discharges through urethra (penis or vagina).

Internal structure of kidney: The longitudinal section of the kidney shows two regions. Renal cortex is the outer darker part of kidney and it is dark red in colour. Renal medulla is the inner lighter part of kidney and is pale red in colour. Renal medulla consists of several cone shaped areas called renal pyramids. Renal pyramids project into a funnel-shaped cavity called renal pelvis, which is the base of ureter.


Nephron: Nephron is the basic structural and functional unit of kidney. There are over one million nephrons in each kidney. There are two parts of a nephron i.e. renal corpuscle and renal tubule.


1) Renal corpuscle “OR” Renal Carpuscle “OR” Malphigian capsule: The renal corpuscle is not tubular and has two parts i.e. glomerulus and Bowman’s capsule. Glomerulus is a network of capillaries while Bowman’s capsule is a cup-shaped structure that encloses glomerulus. The artery which inter into capsule is called afferent artery. Afferent artery divides and re-divides to form glomerulus. The artery that drained out blood from glomerulus is called efferent artery. The efferent artery form a network upon renal tubule and finally form renal vein. In the renal capsule blood filtration occur.

2) Renal Tubule: Renal tubule is the tubular part of nephron and is commonly called filtrate refinery. which starts after Bowman’s capsule. Its first coiled portion where reabsorption occur is called the proximal convoluted tubule. Next portion is U-shaped and is called the Loop of Henle. The last highly coiled portion of renal tubule is the distal convoluted tubule. It plays a role in re- this process is called tubular secretion. The distal convoluted tubules of many nephrons open in a single collecting duct. Many collecting ducts join together to form several hundred papillary ducts which drain into renal pelvis.



Figure : The structure of a nephron

(The capillaries surrounding the renal tubule are not shown for simplicity)

Functioning of Kidney OR Process of urine formation: The main function of kidney is urine formation, which takes place in three steps .

1) Pressure filtration or glomerular filtration: When blood enters the kidney via the renal artery, it goes to many arterioles, and then to the glomerulus. The pressure of blood is very high and so most of the water, salts, glucose and urea of blood is forced out of glomerular capillaries. This material passes into the Bowman’s capsule and is now called glomerular filtrate. Due to filtration dissolved salts, glucose, drugs, nitrogenous wastes are filtered out of the glomerulus into bowman capsule. Due to the small pores in glomerulus only small particles are filtered. Blood cells, proteins and plasma being larger in size are not filtered.

2) Selective reabsorption OR tubular reabsorption: The second step is the selective re-absorption. In this step about 99% of the glomerular filtrate is reabsorbed into the blood capillaries surrounding renal tubule. It occurs through osmosis, diffusion and active transport. Some water and most of the glucose is reabsorbed from the proximal convoluted tubule. Here, salts are reabsorbed by active transport and then water follows by osmosis. The descending limb of loop of Henle allows the reabsorption of water while the ascending limb of Loop of Henle allows the reabsorption of salts. The distal convoluted tubule again allows the reabsorption of water into the blood.

 3) Tubular secretion: The third step is the tubular secretion. Different ions, creatinine, urea, Hydrogen ions, etc. are secreted from blood into the filtrate in renal tubule. After the above mentioned steps, the filtrate present in renal tubules is known as urine. This is done to maintain blood at a normal pH (7.35 to 7.45).

 It moves into collecting ducts and then into pelvis. At the final stage urine is only 1% of the originally filtered volume. The typical volume of urine produced by an average adult is around 1.4 litres per day.


Question 6: What chemicals are found in urine?


Chemical composition of urine:

Table: Normal chemical composition of urine (Source: NASA Contractor Report)




9.3 g/I

Chloride ions

1.87 g/I

Sodium ions

1.17 g/I

Potassium ions

0.750 g/I

Other ions and compounds

Variable amounts



Question 7: Along with excretion, kidneys also play role in Osmoregulation.” Comment on this statement.

Answer: Osmoregulatory Function of Kidney:

Osmoregulation is defined as the regulation of the concentration of water and salts in blood and other body fluids. Kidneys play important role in osmoregulation by regulating the water contents of blood. It is an important process as excessive loss of water concentrates the body fluids whereas excess intake of water dilutes them.

When there is excess water in body fluids, kidneys form dilute (hypotonic) urine. For this purpose, kidneys filter more water from glomerular capillaries into Bowman’s capsule. Similarly less water is reabsorbed and abundant dilute urine is produced. It brings down the volume of body fluids to normal.

When there is shortage of water in body fluids, kidneys filter less water from glomerular capillaries and the rate of reabsorption of water is increased. Less filtration and more reabsorption produce small amount of concentrated (hypertonic) urine. It increases the volume of body fluids to normal. This whole process is under hormonal control. ADH is a hormone of the posterior pituitary gland that increase permeability of the walls of renal tubule. When there is shortage of water the secretion of ADH increase and when there is excess of water the secretion of ADH decreases.


Question 8: What are different disorders of kidney? Write in short words about the treatment of these disorders? Also write a note on work of famous Muslim scientists in the treatment of kidney disorders?

DISORDERS OF KIDNEY: Human kidneys start working during fetal life right after four month of gestation and keep working throughout the life. Sometime due to some internal or external factors the normal functioning of the kidney might be disturbed. This can lead to various disorders. Some common disorders of kidney are as follows.

 Kidney Stones: The hard stone like materials formed in the kidney due to accumulation of calcium and nitrogenous salts are called kidney stones.

Formation of kidney stones:  When urine becomes concentrated, crystals of many salts e.g. calcium oxalate, calcium and ammonium phosphate, uric acid etc. are formed in it (in bowman capsule). Such large crystals cannot pass in urine and form hard deposits called kidney stones.

Location of kidney stones: Most stones start in kidney.Some may travel to ureter or urinary bladder.

 Causes of kidney stones: The major causes of kidney stones are age, diet (containing more green vegetables, salts, vitamins C and D), recurring urinary tract infections, less intake of water, and alcohol consumption.

Symptoms of kidney stones: The symptoms of kidney stones include severe pain in kidney or in lower abdomen, vomiting, frequent urination and foul-smelling urine with blood and pus.

Removal of kidney stones: About 90% of all kidney stones can pass through the urinary system by drinking plenty of water. In surgical treatment, the affected area is opened and stone(s) are removed. Lithotripsy is another method for the removal of kidney stones. In this method non-electrical shock waves (above 20,000 HZ) from outside are bombarded on the stones in the urinary system. Waves hit the dense stones and break them. Stones become sand-like and are passed through urine. The machine used in this process is called lithotripter. Approximately half to an hour is used in this process.

Kidney (Renal) failure: When the glomerulus part of the nephron becomes destroyed and stops functioning, the condition is called renal failure. “OR “Kidney failure means a complete or partial failure of kidneys to function.

Causes of renal failure: Diabetes mellitus and hypertension are the leading causes of kidney failure. In certain cases, sudden interruption in the blood supply to kidney and drug overdoses may also result in kidney failure. Less water consumption, overdoses of antibiotics like gentamycin and streptomycin, pain killers like ibuprofen, kidney stones and tumor are also main causes.

Symptoms of kidney failure: The main symptom of kidney failure is the high level of urea and other wastes in blood, which can result in vomiting, nausea, weight loss, frequent urination and blood in urine. Excess fluids in body may also cause swelling of legs, feet and face and shortness of breath.

Treatment of kidney transplant: The kidney failure is treated with dialysis and kidney transplant.

  1. A) Dialysis: (Greek word means separation). Dialysis means the cleaning of blood by artificial ways. Dialysis is thE separation of particles in a liquid on the basis of differences in their ability to pass through a membrane by diffusion.


  1. Peritoneal Dialysis:In this type of dialysis, the dialysis fluid is pumped for a time into the peritoneal cavity which is the space around gut. This cavity is lined by peritoneum. Peritoneum contains blood vessels. When we place dialysis fluid in peritoneal cavity, waste materials from peritoneal blood vessels diffuse into the dialysis fluid, which is then drained out. This type of dialysis can be performed at home, but must be done every day.


  1. Hemodialysis:In hemodialysis, patient’s blood is pumped through an apparatus called dialyzer. The dialyzer contains long tubes, the walls of which act as semi-permeable membranes. Blood flows through the tubes while the dialysis fluid flows around the tubes.

Extra water and wastes move from blood into the dialysis fluid. The cleansed blood is then returned back to body. The hemodialysis treatments are typically given in dialysis centers three times per week.


B) Kidney Transplant:Dialysis needs to be repeated after every few days and is unpleasant for patients and attendants. Another treatment for the end-stage kidney failure is kidney transplantation. It is the replacement of patient’s damaged kidney with a donor healthy kidney.

Kidney may be donated by a deceased-donor or living-donor. The donor may or may not be a relative of the patient. Before transplant, the tissue proteins of donor and patient are matched. The donor’s kidney is transplanted in patient’s body and is connected to the patient’s blood and urinary system. The average lifetime for a donated kidney is ten to fifteen years. When a transplant fails, the patient may be given a second kidney transplant. In this situation, the patient is treated through dialysis for some intermediary time. Problems after a transplant may include transplant rejection, infections, imbalances in body salts which can lead to bone problems and ulcers.


Works of famous Muslim scientists in the cure of kidney disorders: Abu Nasr al-Farabi (872-951) was a prominent scientist who wrote many books that contained information about kidney diseases. The genius Abu al-Qasim Al-Zahrawi (known as Albucasis: 936-1013), is considered to be Islam’s greatest surgeon who invented many surgical procedures including the surgical removal of stones from the urinary bladder. His encyclopedia, Al-Tasrif (“The Method”), contained over 200 surgical medical instruments he personally designed.


Analyzing and interpreting:

• Rationalize why dialysis machine is considered as artificial kidney.

Answer: Dialysis is a process for removing waste and excess water from the blood. It works like kidney and remove wastes, so it is called an artificial kidney.


For your information:

Gestation: gestation is the carrying of an embryo or fetus inside female viviparous animals.



  1. Describe the process of selective re-absorption in the kidneys.

Answer: Please see answer of question no. 5.


  1. How do the plants excrete extra water and salts from their bodies?

Answer: Please see answer of question no. 2.


  1. What is the functional unit of the kidney? Describe its structure and draw labelled diagram.

Answer: Please see answer of question no. 5.


  1. What steps are involved in the formation of urine in the kidneys?

Answer: Please see answer of question no. 5.


  1. “Along with excretion, kidneys also play role in Osmoregulation.” Comment on this statement.

Answer: Please see answer of question no. 7.



  1. What are the major organs involved in homeostasis in human body? State the roles of each of these organs.

Answer: Human beings have a developed system for homeostasis. Following organs are involved in homeostasis.

a) Lungs:Lungs remove excess of carbon dioxide from the body.

b) Skin:Excess maintains the body temperature so helps in thermoregulation. Skin also helps in removal of excess water and salts (Osmoregulation).

c) Kidney:Kidney filters excess water, salts, urea and uric acid etc. from the blood and form urine, so helps in osmoregulation.



Bowman’s capsule: The cup shaped part of the nephron enclosing the glomerulus is called bowman’s capsule.

Collecting duct: The tubes into which the renal tubules of nephrons open.

Dialysis: The cleaning of blood (removing nitrogenous wastes and extra water) by artificial ways.

Dialyzer: The apparatus used for haemodialysis or artificial cleaning of the blood is called dialyzer.

Distal convoluted tubule: The part of the nephron between the collecting duct and loop of henle is called Distal convoluted tubule.

Excretion: The process by which the metabolic wastes are removed from the body.

Glomerular filtrate: The material that passes from glomerulus into the Bowman’s capsule.

Glomerulus: The network of capillaries in the nephron of kidney is called glomerulus.

Guttation: : Appearance of drops of xylem sap on the tips or edges of leaves.

Hemodialysis: The dialysis in which patient’s blood is pumped through the apparatus called dialyzer for cleaning.

Hilus: A depression near the centre of the concave area of the kidney; the area through which the ureter, blood and lymphatic vessels and nerves enter/leave the kidney.

Homeostasis: The maintenance of a constant internal environment in response to environmental changes.

Lithotripsy: Treatment for removing kidney stones; non-electrical shock waves are bombarded on the stones to break them.

Loop of Henle: The U-shaped portion of the renal tubule of nephron

Nephron: The functional unit of kidney is called nephron.

Osmoregulation: The regulation of water and salt contents in body fluids is called osmoregulation.

Papillary ducts: The ducts formed by the joining of many collecting ducts; open into renal pelvis.

Peritoneal dialysis: The dialysis in which the dialysis fluid is pumped into the abdominal peritoneal cavity; the wastes from the blood vessels of the peritoneum diffuse into the dialysis fluid which is then drained out.

Pressure filtration: The first step in urine formation; the process in which most of the water, salts, glucose and urea of the blood is forced out of the glomerulus and passed into Bowman’s capsule.

convoluted tubule Proximal: The part of the nephron between Bowman’s capsule and the loop of Henle.

Renal corpuscle or renal capsule: The collective name for the glomerulus and Bowman’s capsule of the nephron.

Renal pelvis: The funnel-shaped cavity into which the renal pyramids of kidney project

Renal pyramids: Cone-shaped areas in the renal medulla.

Renal tubule: The part of the nephron after the Bowman’s capsule; consists of proximal convoluted tubule, loop of Henle and distal convoluted tubule.

Selective reabsorption: The second step in urine formation; in it about 99% of the glomerular filtrate is reabsorbed into the blood capillaries surrounding the renal tubule.

Tubular secretion: The third step in urine formation; different ions, creatinine, urea etc. are secreted from the blood into the filtrate in the renal tubule.

Ureter: A tube that carries urine from a kidney to the urinary bladder is called ureter.

Urethra: The tube that carries urine from urinary bladder to the outside of the body

Urinary bladder: A sac-like organ where urine is stored before being excreted is called urinary bladder.

Urinary system: The system responsible for the production and excretion of urine; includes kidneys, ureters, urinary bladder and urethra.

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