CLASS 7TH

BIOLOGY

Introduction

The excretory system is a vital biological system that removes excess and waste products from the body to maintain homeostasis(stability or balance within the body). Most of these products are in fact used and broken down components of metabolism that leave the body in the form of urine, sweat, or faeces. While many organs are linked indirectly to the removal of metabolic waste, the term excretory system refers to those organs that are used strictly for the elimination and excretion of these broken-down components; this limits the focus of a discussion of the excretory system to mainly the urinary, or renal, system, which consists of the kidneys, ureters, a bladder, and a urethra.

Excretion It is the process of removal of metabolic waste materials from body of

organisms through excretory organs is called as excretion.

Excretory System in Humans

Excretion takes place by the means of excretory organs .Various life processes like nutrition, respiration, etc. produce many unwanted and toxic substances like CO2 , urea and uric acid in animals.

The organs that help the body to get rid of metabolic waste are called excretory organs.

In humans, excretion involves the removal of urea and sweat.

The urea is formed by the combination of ammonia and carbon dioxide. There are several ways in which excess substances can be excreted from the body.

Carbon dioxide is breathed out by the lungs.

Sweat glands present in the skin help in the elimination of

sweat. It mainly consists of water salts and urea.

The liver helps in the breakdown of excess of proteins in the body.

URINARY SYSTEM IN HUMANS

The urinary system's function is to filter blood and create urine as a

waste by-product. The organs of the urinary system include the kidneys, ureters, urinary bladder and urethra. Act of passing urine outside is known as urination or micturition.

The body takes nutrients from food and converts them to energy.

After the body has taken the food components that it needs, waste

products are left behind in the bowel and in the blood.

The kidney and urinary systems help the body to eliminate liquid

waste called urea, and to keep chemicals, such as potassium and

sodium, and water in balance. Urea is produced when foods

containing protein, such as meat, poultry, and certain vegetables, are

broken down in the body. Urea is carried in the bloodstream to the

kidneys, where it is removed along with water and other wastes in the

form of urine.

Other important functions of the kidneys include blood pressure

regulation and controls red blood cell production in the bone marrow.

Kidneys also regulate the acid-base balance and conserve fluids.

Kidney and urinary system parts and their functions

Two kidneys. This pair of reddish-brown, bean shaped organs is

located below the ribs toward the middle of the back. Their function is

to:

o Remove waste products and drugs from the body

o Balance the body's fluids

o Release hormones to regulate blood pressure

o Control production of red blood cells

o Helps in osmoregulation(maintains water contents constant in the

body)

The kidneys remove urea from the blood through tiny filtering units

called nephrons. Nephrons are the functional unit of kidneys. Each

nephron consists of a ball formed of small blood capillaries, called a

glomerulus, and a small tube called a renal tubule. Urea, together with

water and other waste substances, forms the urine as it passes through

the nephrons and down the renal tubules of the kidney.

Two ureters. These narrow tubes carry urine from the kidneys to the

bladder. Muscles in the ureter walls continually tighten and relax

forcing urine downward, away from the kidneys. If urine backs up, or

is allowed to stand still, a kidney infection can develop. About every

10 to 15 seconds, small amounts of urine are emptied into the bladder

from the ureters.

Bladder. This triangle-shaped, bag like organ is located in the lower

abdomen. It is held in place by ligaments that are attached to other

organs and the pelvic bones. The bladder's walls relax and expand it

acts as reservoir for urine before it leaves the body. It can hold about

0.5-1litre of urine.

Two sphincter muscles. These circular muscles help keep urine from

leaking by closing tightly like a rubber band around the opening of

the bladder.

Nerves in the bladder. The nerves alert a person when it is time to

urinate, or empty the bladder.

Urethra. This tube allows urine to pass outside the body. The brain

signals the bladder muscles to tighten, which squeezes urine out of the

bladder. At the same time, the brain signals the sphincter muscles to

relax to let urine exit the bladder through the urethra. When all the

signals occur in the correct order, normal urination occurs. In females

it is 3-4cm long, in males it is 18-20cm long.

Facts about urine

Normal, healthy urine is a pale straw or transparent yellow color.

Darker yellow or honey colored urine means you need more water.

A darker, brownish color may indicate a liver problem or severe

dehydration.

Pinkish or red urine may mean blood in the urine.

There are over a million nephrons in each kidney and each nephron is

about 3cm long.

The quantity of urine excreted by adult is 1500ml every 24 hours.

Know about the Nephron

One end of a nephron has a cup like shape called Bowman’s

capsule which encloses a tuft of capillaries called the

glomerulus. The other end leads into a large tube called collecting end.

Glomerulus helps in filtration of nitrogenous wastes from the

blood plasma.

Proximal convoluted tubule (PCT): It is primarily involved in

reabsorption of salts, water and nutrients. It is also important for regulating blood pH.

Loop of Henle: It functions to establish the osmotic

concentration gradient in the kidney which is necessary for concentration of urine.

Distal convoluted tubule (DCT): It is the major site of hormonal

regulation of sodium chloride and water reabsorption.

It is also involved in regulation of blood pH.

It is also the target site for ADH (Anti diuretic Hormone), the

hormone which regulates blood volume.

Diagram of Nephron

URINE FORMATION THROUGH NEPHRONS

The kidneys filter unwanted substances from the blood and produce urine to

excrete them. There are three main steps of urine formation: glomerular

filtration, reabsorption, and secretion. These processes ensure that only waste

and excess water are removed from the body.

1. Filtration: The Glomerulus Filters Water and Other

Substances from the Bloodstream

Each kidney contains over 1 million tiny structures called nephrons. Each

nephron has a glomerulus, the site of blood filtration. The glomerulus is a

network of capillaries surrounded by a cuplike structure, the glomerular capsule

(or Bowman’s capsule). As blood flows through the glomerulus, blood pressure

pushes water and solutes from the capillaries into the capsule through a

filtration membrane.

Inside the glomerulus, blood pressure pushes fluid from capillaries into the

glomerular capsule through a specialized layer of cells. This layer, the filtration

membrane, allows water and small solutes to pass but blocks blood cells and

large proteins. The filtrate flows from the glomerular capsule further into the

nephron.

Diagram of glomerulus

2. Reabsorption: Moves Nutrients and Water Back into

the Bloodstream

The glomerulus filters water and small solutes out of the bloodstream. The

resulting filtrate contains waste, but also other substances the body needs:

essential ions, glucose, amino acids, and smaller proteins. When the filtrate

exits the glomerulus, it flows into a duct in the nephron called the renal tubule.

As it moves, the needed substances and some water are reabsorbed through the

tube wall into adjacent capillaries. This reabsorption of vital nutrients from the

filtrate is the second step in urine creation.

3. Secretion: Waste Ions and Hydrogen Ions Secreted

from the Blood Complete the Formation of Urine

The filtrate absorbed in the glomerulus flows through the renal tubule, where

nutrients and water are reabsorbed into capillaries. At the same time, waste ions

and hydrogen ions pass from the capillaries into the renal tubule. This process is

called secretion. The secreted ions combine with the remaining filtrate and

become urine. The urine flows out of the nephron tubule into a collecting duct.

It passes out of the kidney through the renal pelvis, into the ureter, and down to

the bladder.

Composition of Urine

The nephrons of the kidneys process blood and create urine through a process of filtration,

reabsorption, and secretion. Urine is about 95% water and 5% waste products. Nitrogenous

wastes excreted in urine include urea, creatinine, ammonia, and uric acid. Ions such as

sodium, potassium, hydrogen, and calcium are also excreted.

Physical characteristics: Urine is the waste product that is eliminated by the kidneys. Urine

contains waste products like urea, salts, excess ions, water, and metabolized products of

drugs.

Urine is often light or pale yellow in color and fresh urine has a slight ammoniac smell. It is

often clear in turbidity with a pH of around 4-8. These characteristics vary depending upon

the nature of the disease in the body. Often a urine sample analysis helps to detect diseases

like diabetes, kidney failures etc.

Chemical composition: Chemically, urine is composed mainly of urea, sodium chloride,

potassium ions, creatinine, ammonia products, and some amount of protein, and other

metabolites.

Osmoregulation

Osmoregulation is the process of regulating body fluids and its compositions. It

maintains osmotic pressure of the blood and helps in the homeostasis. This is

why it is recommended to consume more water about 2-3 liters which help in

the proper functioning of our kidneys. For example, we consume lots of water

during summers but still, we urinate fewer times in summers than winters and

the concentration of the urine is also more. The reason is that we lose lots of

water from our body in summers through sweating. Thus, to maintain the fluid

balance in the body our kidneys reabsorb more water.

Common disorders of the urinary system

Hematuria- When blood is found in the urine, the condition is called as

hematuria. The usual cause of this disease is the toxin produced in certain

types of fever and infection in urinary tract, kidney stone or tumor.

Glycosuria- This is a condition characterized by the presence of glucose

in the urine. This is indicative of diabetes that is most likely uncontrolled.

Kidney stones: A small, hard deposit that forms in the kidneys and is often painful when passed. Kidney stones are hard deposits of minerals and acid salts that stick together in concentrated urine. They can be painful when passing through the urinary tract, but usually don't cause permanent damage. The most common symptom is severe pain, usually in the side of the abdomen, that's often associated with nausea. Treatment includes pain relievers and drinking lots of water to help pass the stone. Medical procedures may be required to remove or break up larger stones.

Uraemia: a raised level in the blood of urea and other nitrogenous waste compounds that are normally eliminated by the kidneys.

Uraemia is a dangerous condition that occurs when the kidneys no longer filter properly. It's likely to occur when a person is in the final stage of chronic kidney disease.

Kidney Failure

When the kidneys do not work properly, they are not able to remove the waste product urea from the blood. Complete failure of kidneys leads to

the accumulation of too much urea in the blood of a person.

Urea is a toxic substance (or poisonous substance), so the accumulation

of too much urea in the blood of a person can ultimately kill the person.

A person having kidney failure cannot survive unless his blood is filtered

periodically through kidney machine to remove urea.

The procedure used for cleaning the blood of a person by separating the

waste product urea from it is called dialysis. In the procedure called dialysis, the blood of a person having kidney failure is filtered regularly by

using a kidney machine (or dialysis machine) to remove urea and other

waste salts from it.

Since a kidney machine can do the work of damaged kidneys,

therefore, kidney machine is sometimes also called artificial kidney. Thus, artificial kidney is a machine to remove urea and other waste salts

from the blood of a person by filtering it.

Dialysis

In kidney transplant, the damaged kidney is removed and a matching

kidney donated by a healthy person is transplanted in its place by

performing a surgical operation.

Accessory excretory organs

Skin

The skin is the largest organ in the body. Its primary function is to protect the different organs

of the body. In addition, the skin excretes sweat and sebum from sebaceous glands. In

particular, the skin eliminates compounds like NaCl, some amount of urea etc.

Lungs

Lungs are the primary respiratory organs. They help take in oxygen and expel carbon dioxide.

But, in this process, they also eliminate some amount of water in the form of vapour.

Liver

This organ has an important function in excreting waste from the body. It is the first line of

defense when it comes to hormones, fats, alcohol, and drugs. In fact, most drugs undergo

first-pass metabolism in this organ. The kidneys also eliminate a few drugs. Furthermore, the

liver eliminates excess fats and cholesterol. Breakdown of proteins which results in the

formation of ammonia the liver converts it into urea. It also helps in removing inactive steroid

hormones bile pigments (bilirubin and biliverdin) which are formed from the broken

fragments of RBC’s and cholesterol. This is essential to maintain the health of the body.

Excretion in Other Animals

Other animals also excrete (or urinate) waste products from their body.

The way in which waste products are excreted (or removed) from the

body of the animal depends on the availability of water to it.

Aquatic animals excrete cell waste in gaseous form (ammonia) which directly dissolves in water. The fish excretes cell waste in gaseous form

(ammonia) which directly dissolves in water.

The land animals usually have less water available to them. The land

animals such as birds, lizards, snakes and insects excrete a white colored

waste called uric acid in semi-solid form.

Worksheet:

Fill in the blanks:

1. Urine contains nitrogenous wastes in the form of ________.

2. In case of kidney failure ________ is used to remove the metabolic

wastes.

3. The ________ supplies impure blood to the kidney for filtration.

4. The knot of blood vessels inside the Bowman’s capsule is called

________.

5. The long tubes that connect the kidneys to the urinary bladder are

called ________.

6. ________ produces bile pigments from the hemoglobin of broken red

blood cells.

7. Kidneys are made up of number of filters called _______.

Choose the best answer

1. The structural and functional unit of kidney is called:

a. Neuron b. Nephron c. Nephridia

2. The opening of urinary bladder is called:

a. Ureters b. renal capsule c. Urethra

3. Filtration of blood occurs through the:

a. Glomerulus b. Loop of Henle c. Liver

4. The process of maintaining water balance in the body:

a. Excretion b. Osmoregulation c. thermoregulation

5. A bag like organ meant for temporary storage of urine:

a. Renal artery b. Urinary bladder c. Kidney

Correct the false statements:

1. Bile pigments are the excretory products excreted by then lungs.

2. Carbon dioxide and urea is excreted by lungs.

3. The opening of urinary bladder is called ureter.

4. Liver helps the body to eliminate sweat.

5. Neurons are the excretory units of the kidney.

Textbook short question/answer of chapter excretory system:

1. Define excretion.

Ans: Excretion is the process of removal of metabolic wastes from the

body of organisms through excretory organs.

2. Name the main organs of renal excretory system of human beings in

correct sequence.

Ans: Kidneys, Ureters, Urinary bladder, Urethra.

3. Which tube connects the kidney to the urinary bladder?

Ans: Ureters

4. What are the metabolic waste produce in the body of human beings?

Ans: Skin: excess water, salts, lactic acid and urea

Kidneys: water, urea, excess salts

Lungs: Carbon dioxide, water vapour

Liver: Bile pigment, toxins, urea.

5. Name the excretory product excreted by the liver?

Ans: Bile Pigments (bilirubin, biliverdin), toxins, urea (produced in the

liver and transported to the kidneys for excretion).

6. What are the other excretory organs beside kidneys in human beings?

Ans: Skin, lungs, liver.

7. Name the process involved in the formation of urine.

Ans: Glomerular filtration, tubular reabsorption, tubular secretion and

excretion.

8. What are the constituents of normal urine?

Ans: Urine is about 95% water and 5% waste products. Nitrogenous wastes

excreted in urine include urea, creatinine, ammonia, and uric acid.

9. Name the organ concerned with maintaining the water balance in human

beings.

Ans: Kidney

10. What are the pigments produced by the breaking down of red blood cells in

the liver?

Ans: Bilirubin and Biliverdin

VIDEO LINK

https://www.youtube.com/watch?v=zAczdDFyglA&t=172s