Human Body and Organ System Class 8 Science Notes Maharashtra Board

Human Body and Organ System Class 8 Science Notes Maharashtra State Board

In the last class, we studied some characteristics of living organisms. All the vital processes which are essential properties of living beings, are collectively called life processes. Different organs in our body work in groups to smoothly carry out various life processes. These life processes take place in various steps. Specific organs carry out specific steps. A group of organs working together to perform a specific function is called an organ system. Various organ systems like digestive, respiratory, circulatory, nervous, excretory, reproductive, skeletal, muscular, etc. are functioning in our body. Energy is essential to operate all the life processes in the human body. Energy production occurs within the cells. Cells need the supply of soluble nutrients and oxygen for this purpose. This supply takes place with the help of respiratory and circulatory systems. Respiration is carried out through the following three steps.

1. External Respiration:

  • Inspiration/Inhalation: Air is taken in through the nose and sent toward the lungs through the trachea (windpipe).
  • Expiration/Exhalation: Oxygen from the inspired air goes into the blood. Blood carries the CO2 from various parts of the body towards the lungs. This air is given out by exhalation.
  • Both of these processes occurring with the help of the lungs are collectively called external respiration.

2. Internal Respiration:
The exchange of gases between cells and tissue fluid is called internal respiration. Oxygen moves from blood into tissue fluid and carbon dioxide moves from tissue fluid into blood.

Human Body and Organ System Class 8 Science Notes Maharashtra Board

3. Cellular Respiration:
Dissolved nutrients like glucose are slowly burnt (oxidized) with the help of oxygen and energy is released in the form of ATP. Waste materials like CO2 and water vapors are produced during this process. Cellular respiration can be summarized as follows.
C6H12O6 + 6O2 → 6CO2 + 6H22O + Energy (38 ATP)

Respiratory System: Structure and Function

1. Nose:
The respiratory system and respiration begin with the nose. Air is filtered with the help of hair and mucus present in the nose.

2. Pharynx:
Food pipes and windpipes originate in the pharynx. A windpipe is present in front of the food pipe. There is a lid at the beginning of the windpipe. This lid closes the windpipe during the passing of food into the food pipe and thereby normally prevents the entry of food particles into the windpipe. Otherwise, the windpipe remains open. Hence air passes through the pharynx into the windpipe.

3. Wind Pipe:
The windpipe is swollen at the beginning due to the sound box. Wind pipe bifurcates in the thorax. One branch enters the right lung and the other into the left lung.

4. Lungs:
A lung is present on either side of the heart in the thoracic cavity. The maximum area of the thoracic cavity is occupied by the lungs and they cover the maximum part of the heart. Each lung has double double-layered covering. It is called a pleura. Lungs are elastic like a sponge. Lungs are made up of many small compartments, called alveoli. A rich network of capillaries is present around each alveolus.

Human Body and Organ System Class 8 Science Notes Maharashtra Board 1
The walls of the alveoli and capillaries are extremely thin. Gaseous exchange can easily take place across these thin walls. As a large number of alveoli is present in the lungs, a larger surface is available for gaseous exchange.

Exchange of Gases in Lungs:
Gaseous exchange occurs continuously while blood is circulating through the alveoli. An iron-containing protein-hemoglobin is present in the RBCs of blood. Hemoglobin absorbs the oxygen from the air within the alveoli. Simultaneously, CO2 and water vapors move from blood into the alveoli. Thus, oxygen is taken into the blood, and CO2 and water vapors are removed from the blood and given out by exhalation.

Human Body and Organ System Class 8 Science Notes Maharashtra Board

5. Diaphragm:
A muscular partition is present at the base of the thoracic cage. This partition is called as diaphragm. It is present between the thoracic cavity and the abdominal cavity. Simultaneous rising of ribs and lowering of the diaphragm causes a decrease in pressure on the lungs. Due to this, air moves into the lungs through the nose. When ribs return to their original position and the diaphragm rises, pressure on the lungs increases. Due to this, air moves out from it through the nose. Continuous upward and downward movement of the diaphragm is necessary to bring about the breathing.
Human Body and Organ System Class 8 Science Notes Maharashtra Board 2

Blood Circulatory System

The circulatory system performs the function of transporting various substances like water, hormones, oxygen, soluble nutrients, and waste materials through different organs. An independent system for blood circulation is present in humans and other animals. It consists of the heart, blood vessels, and capillaries.

Heart: Structure and Functions
The heart is present almost at the center of the thoracic cage. It is present behind the ribs, between two lungs, and slightly inclined on the left side. The size of our heart is equal to one’s fist and its weight is about 360 grams. It is covered by a double-layered peritoneal membrane. A fluid is present between two membranes due to which the heart is protected from friction and mechanical shocks. The human heart is a muscular organ. It is made up of involuntary cardiac muscles. They contract and relax with a definite rhythm. This is called as beating of the heart.
Human Body and Organ System Class 8 Science Notes Maharashtra Board 3
Internally, the heart is divided into left and right compartments by a vertical partition. Each of those compartments is again divided into two chambers. Thus, in all, the heart consists of four chambers. Upper chambers are called as atria (singular-atrium) and lower chambers as ventricles.
Human Body and Organ System Class 8 Science Notes Maharashtra Board 4

Blood Vessels – Structure and Functions:
The heart is beating continuously. Due to this, blood continuously circulates through blood vessels. There are two main types of blood vessels.

Blood vessels that carry the blood away from the heart are called arteries. Except for the one carrying blood toward the lungs, all carry oxygenated blood. These are deeply located in the body and their walls are thick. These vessels do not have valves.

Vessels carrying blood toward the heart from various parts of the body are called veins. All veins except the one carrying blood from the lungs transport deoxygenated blood. Most of the veins are superficially located in the body. Their walls are thin and these are provided with valves.
Human Body and Organ System Class 8 Science Notes Maharashtra Board 5
In 1628, William Harvey described the mechanism of circulation in the body. He proposed a theory that our heart is a muscular pump by which blood is circulated in the body. He discovered the working mechanism of the valves of the heart.

Human Body and Organ System Class 8 Science Notes Maharashtra Board

Arteries gradually branch out with a decrease in their diameter as they spread in the body and finally form fine hairlike vessels called as capillaries. Walls of capillaries are extremely thin and made up of a single layer of cells. Due to this, the exchange of materials between capillaries and cells becomes easy. During the exchange, the oxygen, nutrients, hormones, vitamins, etc. are sent toward the cells and waste materials of the cells move into the blood. Capillaries unite together to form vessels of greater diameter, called veins. A capillary network is present in each organ.
Human Body and Organ System Class 8 Science Notes Maharashtra Board 6
In the case of a healthy person, there are 72 beats of heart per minute. The rate of heartbeat increases due to physical exercise and emotions. Similarly, it has been observed that it decreases during rest and sleep. The number of beats is higher in the case of infants. Two types of sounds are heard during the beat. One is described as ‘lubb’ and the other as ‘dub’. The heart pumps about 75 ml of blood during each beat.

Blood Circulation through Heart/Functioning of Heart
The process of pumping blood towards various parts of the body and bringing it back toward the heart is called blood circulation. To maintain the continuity in circulation, the heart alternately contracts and relaxes. Consecutive single contraction and relaxation of the heart constitute a single heartbeat.

Blood is a red-colored fluid material. It is fluid connective tissue. The oxygenated blood is deep red colored, salty and its pH is 7.4. Blood is composed of mainly two components.


  • Plasma is pale yellow, clear, and slightly alkaline fluid. It contains 90-92 % water, 6-8 % proteins, 1-2 % inorganic salts and other components.
  • Albumin – Distributes the water all over the body.
  • Globulins – Protection.
  • Fibrinogen & prothrombin help in the blood clotting process.
  • Inorganic ions – Ca, Na, and K control the function of muscles and nerves.

Blood Cells

1. Red Blood Corpuscles (RBCs):
Small, circular, enucleated cells. These cells appear red due to hemoglobin. Oxygen dissolves in the blood due to hemoglobin. 50-60 lakh RBCs are present in each cubic millimeter of blood. RBCs are produced in red bone marrow and live for about 100 – 127 days.

2. White Blood Cells (WBCs):
These are large, nucleated, and colorless cells. 5000-10000 WBCs are present per mm3 of blood.

  • 5 types of WBCs are present
  • basophils, eosinophils, neutrophils, monocytes & lymphocytes.
  • WBCs are produced in red bone marrow.
  • WBCs act as soldiers in our bodies.
  • Function: These cells attack the pathogens entering our body. They protect us from microbial diseases.

Human Body and Organ System Class 8 Science Notes Maharashtra Board

3. Platelets

  • These are extremely small and disc-shaped.
  • 2.5-4 lakh platelets are present per mm3 of blood.
  • Function: Platelets participate in the blood clotting process.

Functions of Blood

  • Transport of gases: Oxygen is carried via blood from the lungs to cells in various parts of the body and carbon dioxide from tissues to the lungs.
  • Transport of nutrients: Simple nutrients like glucose, amino acids, and fatty acids are taken up by blood from the wall of the alimentary canal and transported up to each cell in the body.
  • Transport of waste materials: Nitrogenous wastes like ammonia, urea, and creatinine are released by tissues into the blood which carries those to the kidney for excretion.
  • Protection: Antibodies are produced in the blood and they protect the body from microbes and other harmful particles.
  • Transport of enzymes and hormones: Blood transports the enzymes and hormones from the site of their production to the site of their action.
  • Thermoregulation: Body temperature is maintained constant at 37°C by vasodilation and vasoconstriction.
  • Maintaining the balance of minerals like Na, and K in the body.
  • If bleeding occurs at the injury, platelets and a protein called fibrinogen in the blood form a clot and seal the injury.

Human Blood Groups
Depending upon the proteins like antigens and antibodies, different blood groups are formed. There are four main groups of human blood A, B, AB, and O. Besides, there are two types of ‘Rh’ negative and ‘Rh’ positive in each of those four groups. Thus, in all eight blood groups are formed. (Eg. A Rh +ve & A Rh -ve).

Blood Donation:
If a person meets an accident, bleeding occurs through wounds. Many times, blood transfusion is necessary during the surgical operation. Similarly, blood is transfused in the case of patients with anemia, thalassemia, and cancer. Blood transfusion is carried out to compensate for the blood shortage in the body. This is called a blood transfusion. From where the blood is supplied for blood transfusion?

  • Blood Banks: Blood is collected in blood banks by specific methods from healthy persons and supplied to the needful persons. If the collected blood is not to be used immediately, it can be stored for some days in the refrigerator.
  • Blood Donor: The person who donates the blood is referred to as a blood donor.
  • Blood Recipient: The person who receives the blood is referred to as the recipient.

A person of the blood group ‘O’ can donate blood to a person having any other blood group where whereas the person with ‘AB’ blood group can receive blood from a person with any other blood group. Hence, a person of blood group ‘O’ is called a universal donor and the person with blood group ‘AB’ is called a universal recipient.

Human Body and Organ System Class 8 Science Notes Maharashtra Board

Blood groups are hereditary and depend upon the genes inherited from parents. Blood transfusion is performed only after the blood group matching. If it is done without matching, it may prove fatal for the patient. A person who donates the blood may be a recipient in the future. Blood donation without any expectation is always life-saving. Blood is required in various situations like accidents, bleeding, parturition, surgical operations, etc. Blood donated by a healthy person is used to save the life of a needful person. Hence blood donation is considered as the best donation.

Blood Pressure:
Blood is continuously kept flowing through blood vessels due to contraction relaxation of the heart. Due to contraction of the heart, pressure is exerted on the wall of arteries and it is called blood pressure. Proper blood pressure is necessary to supply blood to all parts of the body. The pressure recorded during the contraction of the heart is called ‘systolic pressure’ and that one recorded during relaxation is called ‘diastolic pressure’. The blood pressure of a healthy person is about 120/80 mm to 139/89 mm of Hg. It is measured with the help of a sphygmomanometer’.
Human Body and Organ System Class 8 Science Notes Maharashtra Board 7

  • Blood production occurs continuously in our body.
  • About 350 ml of blood is collected from a person during donation and our body restores the fluid part of it within 24 hrs.
  • Pregnant and breastfeeding women cannot donate blood.
  • There is no trouble during or after the blood donation.
  • 1st October is observed as National Voluntary Blood Donation Day.
  • A healthy person of age more than 18 years can donate the blood for 3-4 times a year.

Hypertension (High Blood Pressure):
High blood pressure than normal is referred to as hypertension. In the arteries of the person with hypertension, unnecessary tension develops. The heart needs to perform more function than the normal condition in the case of hypertension. Both, systolic and diastolic pressures are high in hypertension.

Type Systolic Pressure Diastolic Pressure
Normal Blood Pressure 90 – 119 mm 60 – 79 mm
Primary Hypertension 120 – 139 mm 80 – 89 mm
Hypertension Stage-I 140 – 159 mm 90 – 99 mm
Hypertension Stage-II > 160 mm > 100 mm

A, B, and O blood groups were discovered by Carl Landsteiner in 1900. He won the Nobel Prize of 1930 for this discovery. Blood group AB was discovered by Decastello and Sturli in 1902.

Human Body and Organ System Class 8 Science Notes Maharashtra Board

The branch of medical science that deals with the study of blood, hematopoietic organs, and blood diseases is called as hematology. Research of diagnosis and remedies of blood diseases is also performed in this branch.

Well-organized Maharashtra State Board Class 8 Science Notes Human Body and Organ System can aid in exam preparation and quick revision.

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