My Article no 2

                                                                HOMEOSTASIS 

Homeostasis may be defined as the maintenance of the internal conditions of body equilibrium, despite changes in the external  environment. For example, the core temperature of human body remains at about 37 Centigrade 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 examples 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 ar

e essential for the functioning of cells.

Thermoregulation: the maintenance of internal body temperature is called thermoregulation. The enzymes of body work best at particular temperatures are 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: Excretion is also process of homeostasis. In this process, the metabolic wastes are eliminated from body to maintain the internal conditions at equilibrium.

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 mesophyll 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.

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 lady finger) etc.

Osmotic Adjustments in Plants

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

·         Hydrophytes 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.

·         Xerophytes 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.

·         Halophytes 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. 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.

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.

·         The kidney filters excess water, salts, urea, uric acid etc. form the blood and forms urine.

Skin

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.

Lungs

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 human 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 form 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 27 grams. They are placed against the back wall of abdominal cavity just below diaphragm, on eon either side of vertebral column. They are protected by the last 2ribs. 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 duct. 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. Is 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.

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 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 Stones

When urine becomes concentrated, crystals of many salts e.g. calcium oxalate, calcium and ammonium phosphate, uric acid etc. are formed in it. Such large crystals cannot pass in urine and form hard deposits called kidney stones. Most stones start in kidney. Some may travel to ureter or urinary bladder.

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. 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.

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 stones are removed. Lithotripsy is another method for the removal of kidney stones. In this method, non-electrical shock waves 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.

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 and face and shortness of breath.

The kidney failure is treated with dialysis and kidney transplant.

a.       Dialysis

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

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.

2.       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. 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 three times per week.

b.      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-stag kidney failure is kidney transplantation. Is is the replacement of patient’s damaged kidney with a donor a healthy kidney.