Digestion, Transportation, Respiration & Excretion

The activities by which living organisms take in food, derive energy, remove waste from their body and respond to changes in the environment are called life processes.

Digestion

The food that we eat cannot be used by the cells in the body in the form in which it is eaten. Conversion of complex food material into smaller substances so that it can be absorbed by the cells is called digestion. Taking in of food is termed ingestion.

Digestive System

Alimentary canal is a long continuous tube constituted made by mouth, pharynx, oesophagus, stomach, small intestine, large intestine, and rectum. The glandular organs, salivary glands, liver and pancreas and the alimentary canal form the digestive system.

Enzymes

The process of digestion requires enzymes present in the digestive juices secreted by the organs of digestive system. They convert complex substances into simpler ones. Enzymes are chemicals which speed up chemical reactions taking place in cells. Almost all enzymes are complex proteins and remain unchanged during the chemical reaction. They can, therefore, be used repeatedly.

Process Involved in Nutrition

The entire process of nutrition includes the following steps: ingestion, digestion, absorption, assimilation and egestion.

Ingestion and digestion

The process of taking in food through the mouth is called ingestion. The digestion of food starts from the mouth and ends in the small intestine.

Mouth: Carbohydrates, such as starch, are broken down or digested to form sugar. Saliva contains an enzyme salivary amylase that breaks down starch into sugar. It also helps in lubricating the food and making it easier for swallowing.

Oesophagus: There is no digestion in this part, also called gullet. The oesophagus or the food pipe by the contraction of muscles in its wall pushes the food into the stomach. Muscle movement is termed peristalsis and helped food travel down the alimentary canal.

Stomach: The stomach is a highly muscular organ. The gastric glands present in its walls secrete gastric juice containing hydrochloric acid (HCl) and enzymes like pepsinogen. HCl activates pepsinogen into pepsin and kills bacteria. Proteins are broken into smaller fragments called peptones by the enzyme pepsin.

Small intestine: The food moves from the stomach to duodenum, which is the upper part of the small intestine. Emulsification of fat (fat is broken into fat droplets) takes place with the help of the bile juice secreted by the live and stored in gall bladder. Bile does not have any digestive enzymes but it creates an alkaline medium which is essential for the action of pancreatic enzymes.

The pancreatic juice contains three enzymes:

  • Trypsin - converts peptones and proteoses to smaller peptides.
  • Amylase - converts starch into maltose.
  • Lipase - converts fats into fatty acids and glycerol.

The digestion of proteins into the end products amino acids, carbohydrates into glucose, and fats into fatty acids and glycerol is completed in the small intestine.

The inner surface of the small intestine contains thin finger-like projections called villi, which increase the surface area for absorption of digested food into the blood capillaries lining the villi. The blood then carries the absorbed food to different parts of the body and undigested food is pushed into the large intestine.

Large intestine: This part of the body absorbs water from the undigested food and solid waste is lubricated to form the faeces. The faeces pass on to the lower part of the large intestine, called the rectum, and are thrown out of the body through the anus.

Absorption

Blood capillaries in the villi pick up digested food and take it to all cells.

Assimilation

The absorbed food supplied to cells is used to release energy and also to build up the cell components. This is called assimilation.

Egestion

The process by which the undigested food material or waste is released from the body is called egestion.

Transportation

The distribution of food and oxygen to all parts of the body as well as removal of body wastes is performed by a transport system within the body of all living organisms. Our body also secretes many hormones, which have to be carried to their target organs. The flow of fluid (blood or lymph) within the body for transport purposes is termed circulation and the organs for circulation constitute circulatory system.

Transport of Materials in Plants

Transport of water

Roots of plants take up water and minerals from the soil. Tracheids and vessels, which are non-living cells of xylem, transport water picked up by root hairs from soil to the leaves. 

The upward movement of water and minerals from soil termed "ascent of sap" is against gravity and is due to transpiration pull. Transpiration is the process in which a lot of water evaporates (as water vapour) from stomata. This evaporation creates a vacuum and pulls up water through the xylem. This is transpiration pull.

Transport of food material

Sugars and other food molecules synthesized in the leaves are transported to other parts of the plant through phloem. Transport of food material from leaves to other parts of the plant is called translocation. This food may then be stored in fruits, stem or roots.

Transportation in Human Beings

Human circulatory system consists of centrally located muscular pump called heart, circulating fluid (blood, tissue fluid and lymph) and blood vessels, which are tube-like structures, connected to the heart. 

Blood Vessels

Blood vessels are of three kinds:

  1. Arteries: Carry blood from heart to various parts of body.
  2. Veins: Bring blood from various parts of body to the heart.
  3. Capillaries: Thin vessels between the artery and the vein. The capillaries allow the exchange of materials between blood and tissues.

Heart

Heart is a powerful muscular organ lying between lung. It is four-chambered - two (right and left) atria (also called auricles), and two (right and left) ventricles.

The heart is made of specialised muscle cells, also called cardiac muscle fibers, which contract and relax all the time without getting tired. The contraction and relaxation follows a rhythm called heartbeat. Heart pumps blood into the blood vessels.

Rhythmic heart beat results in the proper transport of substances to the various organs by means of blood. In one minute, normal human heart beats about 72 times. Abnormalities in heartbeat can be seen by taking ECG or Electrocardiogram.

The oxygen laden blood from the left ventricle gets pumped into a large artery called aorta. It carries oxygenated blood to all parts of the body.

Blood Pressure

It is the force with which blood pushes against the walls of the arteries. It is generally measured in terms of how high it can push a column of mercury. When ventricles contract, pressure of blood inside the arteries is highest. In a healthy young human being, it is about 120 millimetres of mercury (120 mm Hg). When the ventricles relax, pressure of blood inside the arteries is comparatively less It is about 80 millimetres of Hg (80 mm Hg) in a healthy young man. Thus, a healthy young man has a normal blood pressure of 120 / 80 mm of Hg. The instrument used to measure blood pressure is called sphygmomanometer.

Pulse Rate

The systemic contraction of the heart can be felt as a jerk in certain arteries like the radial artery in the wrist and neck artery below the jaw which are superficial in position. This is called arterial pulse. Pulse rate is the same as the rate of heartbeat.

Circulatory Medium

Our body has three different types of fluids:

  1. Blood: found in heart and blood vessels (arteries, veins and capillaries)
  2. Tissue fluid: found in spaces between cells in organs
  3. Lymph: found in lymph vessels and lymphatic organs (e.g. spleen and tonsils)

Blood

Blood is a connective tissue that circulates throughout the body. It is made up of a fluid medium called plasma in which float two types of blood cells, called red blood cells, white blood cells and cell fragments called blood platelets. Blood cells are manufactured in the bone marrow.

Blood carries nutrients, oxygen, carbon dioxide, hormones and waste material to the relevant parts of the body. Some medicines when taken in the body are also distributed through blood.

Red blood cells (RBC or Erythrocytes)

  • These are circular in shape, and contain a red coloured pigment called haemoglobin
  • No nucleus is present in RBC
  • RBC carry oxygen to tissues and bring back carbon dioxide from tissues

White blood cells (WBC or Leucocytes)

  • Since they carry no pigments, they are colourless
  • They have irregular shape
  • They prevent body from infections by eating up germs or by producing antibodies to fight antigens

Blood platelets (Thrombocytes)

  • These are very small fragments of cells
  • They have no nuclei
  • They participate in clotting of blood

Lympthatic System

Lymph is also a circulatory fluid and flows in the lymph vessels.

  • It is light yellow in colour.
  • It always flows only in one direction from tissues to heart.
  • Cells called lymphocytes present in lymph eat up germs and prevent body from infections.
  • It returns proteins and fluids from circulation to tissues.

Disorders due to Circulatory System

Heart attack

Like all other organs, heart also needs nutrients and oxygen. When arteries supplying the heart become thick due to age or faulty diet consisting of excessive fatty food, muscle cells of the heart cannot beat in the proper rhythm. Heart attack occurs which can be detected in an abnormal ECG.

Anemia

When haemoglobin level falls below a certain point, the condition is called anemia. It makes the person weak and look pale and inactive. Iron in the diet helps remove anemia.

Leukemia

This is blood cancer. The bone narrow makes excessive WBCs and few RBCs.

Hypertension

It is another term for high blood pressure and leads to headache, dizziness and fatigue. Normal blood pressure is 120/80. Proper diet, exercise, medicines and tension free mind helps to cure high blood pressure.

Respiration

Breathing provides oxygen to the cells of our body for oxidation of food in order to generate energy for various activities.

Respiration in Plants

Plants do not have any special respiratory organs. Roots take up oxygen from air trapped in the soil by means of root hairs. Root hairs are embedded in the soil. Oxygen in the air surrounding them diffuses into the root hair and from there into the roots. Carbon dioxide given out, similarly, diffuses out through roots. Stomata in leaves opens to let in oxygen and release carbon dioxide.

In the older parts of roots or bark of woody plants, tiny openings called lenticels are present. It is through these lenticels that oxygen reaches the inner living tissues and carbon dioxide moves out.

Breathing and Respiration in Humans

Respiration may be divided into two steps.

  1. Breathing involves inhalation of air containing oxygen and exhalation of carbon dioxide.
  2. Cellular respiration is responsible for release of energy by oxidation of food (glucose), and its conversion into ATP (adenosine triphosphate) - The energy module.

Breathing is the physical process of respiratory gaseous exchange between the organism and the environment by diffusion. It takes place in the lungs. On the other hand, respiration involves oxidation of food and release of energy which takes place in the cells along with respiratory gaseous exchange.

Respiratory System

Respiratory system of human beings has the following parts:

  • External nares or nostrils.
  • Nasal cavities inside the nose.
  • Internal nostrils opening into pharynx.
  • Pharynx that leads into the wind pipe or trachea.
  • Trachea divides into two bronchi (one bronchus) which lead into the two lungs.

The opening of the pharynx into the trachea is called glottis. Trachea is thin walled but its walls do not collapse even when there is not enough air in it as it is supported by rings of cartilage. Trachea bifurcates into bronchi.

Lungs enclose within them branches of bronchi called bronchioles which branch further and end in very thin walled sac-like structures called air sacs or alveoli.

The voice box or larynx is present on the trachea.

Mechanism of Breathing or Ventilation of Lungs

Lungs are located in the chest cavity or the thoracic cavity. Below the chest cavity is the abdominal cavity. These two cavities are separated from each other by a dome-shaped (upwardly arched) muscular sheet called diaphragm. The movement of diaphragm helps in breathing. Breathing, also called ventilation involves two processes - inhalation and exhalation.

Inhalation

Inhalation (drawing the air inwards) is the result of increase in the volume of the thoracic cavity. This increase is caused by the changes that take place in the position of diaphragm and ribs.

  • Diaphragm straightens out due to contraction of its muscles.
  • Ribs are raised upward and outward and volume of chest cavity enlarges by contraction of rib muscles.. As volume of chest increases pressure of air in it decreases.
  • Atmospheric air rushes in and reaches the alveoli. It brings in oxygen which diffuses into the capillaries from the alveoli.

Exhalation

Exhalation is the result of decrease in the volume of the thoracic cavity. This decrease in the volume is caused when:

  • Diaphragm relaxes and resumes its dome shape, arching upwards.
  • Ribs are lowered downwards and inwards.
  • Thoracic cavity is compressed and the pressure inside the lungs is increased.
  • The alveolar carbon dioxide diffuses out and is pushed out through the trachea and nose.
  • This breathing out of carbon dioxide laden air is called exhalation.

Breathing Rate

When at rest, an adult human breathes about 16 to 18 times per minute. Breathing rate increases during physical exercise, disease, fever, pain and under stress.

Exchange of gases between blood and tissues

Inhalation fills in the alveoli of lungs with oxygenated air. This oxygen has to reach the various tissues of the body. Thus as the first step, blood capillaries on alveoli pick up oxygen from alveoli and carbon dioxide brought by the capillaries from the tissues is exchanged for oxygen. Oxygen diffuses into alveoli.

In the tissues, oxygen gets used up and carbon dioxide is accumulated which is now exchanged for oxygen in blood. The carbon dioxide picked up by blood from tissues is carried to the heart by veins.

Cellular Respiration

Once inside the tissues, oxygen acts upon the digested food (glucose) which has reached the cells of the tissues. As a result energy and carbon dioxide are released. This occurs in the mitochondria of the cells and is called cellular respiration.

Excretory System

Many chemical reactions take place inside body cells. Some products of these chemical reactions are not needed by the body. They may even be harmful if they accumulate in the body. Their removal from the body is called excretion.

In human beings, excretion is carried out by an organ system known as the urinary system or the excretory system:

  • Two bean shaped kidneys, located below the diaphragm in the abdomen and towards the back.
  • Two excretory tubes or ureters, one from each kidney.
  • One urinary bladder, ureters open into it.
  • A muscular tube called urethra arises from the bladder. The urinary opening is at the end of urethra.

Nephron

Each kidney is made of tube like structures called nephrons (renal tubules). A nephron is the structural and functional unit of the kidney. The cup-shaped upper end of nephron is called Bowman’s capsule, has a network of capillaries within it called glomerulus. Glomerulus is a knot of capillaries formed from the artery which brings blood containing wastes and excess of water to the kidney.

Bowman’s capusle leads into a tubular structure. The tubular part of the nephron or renal tubule has three sub-parts, the proximal convoluted tubule (PCT), a thinner tube called loop of Henle and the distal convoluted tubule (DCT). Blood capillaries surround these tubules.

Mechanism of Excretion

Filtration and reabsorption are two important processes of excretion.

Blood entering the glomerulus gets filtered in the Bowman’s capsule and is called the nephric filtrate. The red blood corpuscles and proteins do not filter out. They remain in the blood stream.

The filtrate entering the renal tubule not only contains waste but also useful substances. The useful substances get reabsorbed from the tubule into the blood capillaries surrounding the tubule. Excess water and salts like sodium and chloride also get reabsorbed into the blood from the renal tubule. Thus, waste alone which is primarily in the form of urea enters into collecting tubules from various renal tubules. It is the urine.

From the kidneys, the urine enters the ureters to reach the urinary bladder where it is temporarily stored. Urine is thrown out periodically through the urinary opening.

Kidney Failure, Dialysis and Kidney Transplant

Since the number of nephrons is as large as almost one million in each kidney, a person can survive even with one kidney. However, in case both the kidneys are damaged, it is difficult to remain alive. Modern technology can now save such patients with the help of new techniques like dialysis and kidney transplant. 

A tube is inserted in an artery in the patient’s arm or leg. The tube is connected to the kidney machine. This plastic tube has two membranes so as to form one tube within the other. In the inner tube flows blood from patient’s artery. This blood is surrounded by fluid (dialysis fluid) in the outer tube, separated from it by the membrane of the inner tube. Wastes move out of blood into the fluid. The blood cleaned of its waste goes back from the kidney machine into the vein in the arm or leg and back into the body. The dialysis fluid carrying waste is removed from the machine. This technique is termed dialysis.