Homeostasis may be defined as the maintenance of the internal conditions of body at equilibrium, despite changes in the external environment. 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. The following are some process 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 by the processes of diffusion and osmosis, which are essential for the functioning of cells
(Recall “the concept of tonicity” from Grade IX Biology). Thermoregulation: The maintenance of internal body temperature 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. Excretion is also a process of homeostasis. In this process, the metabolic wastes are eliminated from body to maintain the internal conditions at equilibrium.

Metabolic waste means any material that is produced during body metabolism and that may harm the body

Homeostasis In Plants:

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)

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 leaf cells remove the extra amount of oxygen through stomata.

Removal of Extra Water:

We know that 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. Extra water is removed from plant body by transpiration. At night, transpiration usually does not occur because most plants have their stomata closed. If there is a high water content in soil, water enters the roots and is accumulated in xylem vessels. Some plants such as grasses force this water through special pores, present at leaf tips or edges, and form drops. The appearance of drops of water on the tips or edges of leaves is called guttation.

Removal of Other 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

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

In trees which shed their leaves yearly, the excretory products are removed from body during leaf fall. Other waste materials that are removed by some plants are resins (by coniferous trees), gums (by keekar), latex (by rubber plant) and mucilage (by carnivorous plants and ladyfinger) etc.

Osmotic Adjustments in Plants:

On the basis of the available amount of water and salts, plants are divided into three groups.


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.


Live in dry environments. 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. Such plants have deep roots to absorb maximum water from soil. Some xerophytes 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. Cacti (Singular
Cactus) are the common examples of such plants.


Live in sea waters and are adapted to salty environments. 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.

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

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.


We know that our 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.


In the previous chapter we have learned how lungs maintain the concentration of carbon dioxide
in the blood. Our cells produce carbon dioxide when they perform cellular respiration. From cells,
carbon dioxide diffuses into tissue fluid and from there into blood. Blood carries carbon dioxide to
lungs from where it is removed in air.

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.

Structure 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. The left kidney is a little higher than the right. 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.

The longitudinal section of the kidney shows two regions. Renal cortex is the outer part of kidney and it is dark red in color. Renal medulla is the inner part of kidney and is pale red in color. 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.

The functional unit of the kidneys is called nephron. There are over one million nephrons in each kidney. There are two parts of a nephron i.e. renal corpuscle and renal tubule . 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 renal tubule is the part of nephron which starts after Bowman’s capsule. Its first portion is called the proximal convoluted tubule. Next portion is U-shaped and is called the Loop of Henle. The last portion of renal tubule is the distal convoluted tubule. The distal convoluted tubules of many nephrons open in a single collecting ducts. Many collecting ducts join together to form several hundred papillary ducts which drain into renal pelvis.

Functioning of Kidney:

The main function of kidney is urine formation, which takes place in three steps . The first
step is pressure 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.

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. The third step is the tubular secretion. Different ions, creatinine, urea etc. are secreted from blood into the filtrate in renal tubule. This is done to maintain blood at a normal pH (7.35 to 7.45).

After the above mentioned steps, the filtrate present in renal tubules is known as urine. It moves into collecting ducts and then into pelvis.

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.

Disorders Of Kidney:

There are many different kidney disorders.

Kidney (Renal) failure:

Kidney failure means a complete or partial failure of kidneys to function. 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. 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 face and shortness of breath. The kidney failure is treated with dialysis and kidney transplant.

1 Dialysis:

Dialysis means the cleaning of blood by artificial ways. There are two methods of dialysis.

 A. 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.

B. Haemodialysis:

In haemodialysis, patient’s blood is pumped through an apparatus called dialyzer. The dialyzer contains long tubes, the walls of which act as semi-permeable membranes (Fig. 11.12). 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 haemodialysis treatments are typically given in dialysis centres.

2. Kidney Transplant:

We know that 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.

Previous Lectures:

Gaseous Exchange