CBSE CLASS 11 CHAPTER 18.Body Fluids and Circulation

Circulation is the transport of nutrients, oxygen, CO2, and excretory products to the concerned tissues or organs.

For circulation, simple organisms (sponges, coelenterates, etc.) use water from their surroundings. Complex organisms use body fluids (blood & lymph) for circulation.


Circulatory system is 2 types- Open and Closed.

  • Open circulatory system: Here, the blood pumped by the heart passes through large vessels into open spaces or cavities called sinuses. E.g. Arthropods and molluscs.

  • Closed circulatory system: Here, the blood pumped by the heart is always circulated through blood vessels. This is more advantageous as the flow of fluid can be more precisely regulated. E.g. Annelids and chordates.

All vertebrates have a muscular chambered heart.

Fishes: 2-chambered heart (an atrium + a ventricle).

Amphibians: 3-chambered heart (2 atria + a ventricle).

Reptiles (except crocodiles): 3-chambered heart (2 atria + a ventricle). Ventricle is incompletely partitioned.

Crocodiles, birds & mammals: 4-chambered heart.

Types of circulation

  • Single circulation: In fishes. In this, heart receives impure blood only (venous heart).

Deoxygenated blood → to heart → to gills → oxygenated blood → to body parts → deoxygenated blood → to heart.

Incomplete double circulation: In amphibians & reptiles. In this, left atrium receives oxygenated blood from gills/ lungs/skin and right atrium gets deoxygenated blood from other body parts. However, they get mixed up in the single ventricle. It pumps out mixed blood.

Double circulation: In birds & mammals. Right atrium gets deoxygenated blood and passes to right ventricle and left atrium gets oxygenated blood and passes to left ventricle. The ventricles pump it out separately without any mixing up.


It includes Heart, Blood & Blood vessels.

                            1. BLOOD                           

Formed of plasma (55%) & formed elements (45%).


Straw-coloured, slightly alkaline (pH 7.4) viscous fluid.

Constituents of plasma
  • Water (90-92%): It is a good solvent.
  • Plasma proteins (6-8 %): Include
    • Fibrinogen: For blood coagulation.
    • Globulins: Act as antibodies (for defense of the body).
    • Albumins: For osmotic balance. Regulate blood pressure.
  • Inorganic constituents: Na+, Ca2+, Mg2+, Cl-, HCO – etc.
  • Gases like CO2, O2, N2 etc.
  • Glucose, amino acids, lipids & cholesterol.

Plasma without clotting factors is known as Serum.


Red Blood Cells (RBC) or Erythrocytes:

  • Biconcave non-nucleated cells. No mitochondria, Golgi complex etc. Red colour is due to Haemoglobin (iron containing protein). Normal Hb level is 12-16 g/ 100 ml.
  • Count: 5 – 5.5 millions/ mm3.
  • Formed in: Red Bone marrow.
  • Average lifespan: 120 days. Worn-out RBCs are destroyed in spleen (graveyard of RBCs).
  • Function: CO2 and O2 transports.

White Blood Cells (WBC) or Leucocytes:

  • Colourless nucleated cells. Different types.

Count: 6000-8000 /mm3.

  • Formed in: Bone marrow, lymph glands, spleen.
  • Average lifespan: Generally short lived (1- 15 days).
  • Function: Part of immune system.
Types of WBC: Granulocytes & Agranulocytes


They are 3 types:

  1. Neutrophils (Heterophils): 60-65%. Soldier of the body. Function: Phagocytosis.
  2. Eosinophils (Acidophils): 2-3%. Resist infections. Cause allergic reactions.
  3. Basophils (Cyanophils): 0.5-1%. Secrete histamine, serotonin, heparin etc. Cause inflammatory reactions.

2.  Agranulocytes

They are 2 types:

  1. Lymphocytes (20-25%): Smallest WBC with largest nucleus. Includes B- lymphocytes & T- lymphocytes. Cause immune responses. Secrete antibodies.
  2. Monocytes (6-8%): Largest WBC. Function: Phagocytosis.

Platelets (Thrombocytes):

  • Colourless non-nucleated cell fragments.
  • Count: 1.5 – 3.5 lakhs /mm3.
  • Formed in: Megakaryocytes in Bone marrow.
  • Average lifespan: 7 days.
  • Function: Blood clotting.


It is a mechanism for haemostasis (prevention of blood loss through injuries). At the site of injury, following events occur:



Blood groups were discovered by Carl Land Steiner.


ABO grouping

It is based on presence or absence of 2 surface antigens (chemicals that induce immune response) on RBCs namely A & B. Similarly, plasma contains 2 antibodies (proteins produced in response to antigens) namely anti-A & anti-B.

Blood group  Antigens  AntibodiesCan donate blood toCan receive blood from (Donor’s group)
AAAnti-BA & ABA, O
BBAnti-AB & ABB, O
ABA, BNilAB onlyA, B, AB & O
ONilAnti-A & Anti-BA, B, AB & OO only
  • Antigen A reacts with anti-A.  Antigen B reacts with anti-B.
  • If bloods with interactive antigens & antibodies are mixed together, it causes clumping (agglutination) of RBCs.
  • Persons with O Group are called Universal donors because they can donate blood to persons with any other blood group. Persons with AB group are called Universal recipients because they can accept blood from all groups.

2.    Rh grouping

  • Rhesus (Rh) factor is another antigen found on RBC.
  • Rh+ve means the presence of Rh factor and Rh-ve means absence of Rh factor. Nearly 80% of humans are Rh+ve.
  • Anti-Rh antibodies are not naturally found. So Rh-ve person can receive Rh+ve blood only once but it causes the development of anti-Rh antibodies in his blood. So, a second transfusion of Rh+ve blood causes agglutination. Therefore, Rh-group should be matched before transfusion.

Erythroblastosis foetalis


It is a Rh incompatibility between the Rh-ve blood of a pregnant mother and Rh+ve blood of the foetus.

Rh antigens do not get mixed with maternal blood in first pregnancy because placenta separates the two bloods.

But during first delivery, the maternal blood may be exposed to small amount of foetal blood (Rh+ve). This induces the formation of Rh antibodies in maternal blood.

In case of her subsequent pregnancies, the Rh antibodies from the mother leak into the foetal blood (Rh+ve) and destroy the foetal RBCs. This is fatal to the foetus or causes severe anaemia and jaundice to the baby. This condition is called Erythroblastosis foetalis.

It can be avoided by administering anti-Rh antibodies to the mother immediately after the first delivery.

                    2. BLOOD VESSELS                   

Blood vessels are 3 types: Arteries, Veins & Capillaries.



They carry blood from heart to other tissues. They contain oxygenated blood  (except  pulmonary  artery). Their smaller branches are called arterioles.

Arteries are 3- layered- inner tunica intima (squamous endothelium), middle tunica media (smooth muscles & elastic fibres) and outer tunica externa (fibrous connective tissue).


They carry blood towards the heart. They contain deoxygenated blood (except pulmonary vein). Their smaller branches are called venules. Veins are also 3-layered but tunica media is comparatively thin.


In tissues, arterioles divide into thin-walled and single-layered vessels. They are called capillaries. They unite into venules.


                            3. HEART                           

It is a mesodermally derived organ located in mediastinum.

It has the size of a clenched fist.

It is protected by double-layered pericardium.

The pericardial space (between pericardial membranes) is filled with pericardial fluid. It reduces the friction between the heart walls and surrounding tissues.


Heart has 4 chambers- two upper atria (auricles) and two lower ventricles.

  • The walls (cardiac muscles) of the ventricles are much thicker than that of the atria.

The atria are separated by an inter-atrial septum and the ventricles are separated by an inter-ventricular septum.

In between the atrium and ventricle, there is a thick fibrous atrio-ventricular septum with an opening.

A tricuspid valve (3 muscular flaps or cusps) guards the opening between right atrium and right ventricle. A bicuspid (mitral) valve guards the opening between left atrium and left ventricle.

These valves allow the flow of blood only in one direction, i.e. from atria to ventricles.

The right ventricle has an opening to the pulmonary artery and the left ventricle has an opening to the aorta. These openings have semi-lunar valves. They prevent the backward flow of blood.



It includes nodal tissues, bundles & fibres.

Nodal tissues are specialized cardiac musculature present in the heart wall. They are 2 types:

Sino-atrial node (SAN) in the right upper corner of the right atrium.

Atrioventricular node (AVN) in the lower-left corner of the right atrium close to the atrioventricular septum.

From the AVN, a bundle of the fibrous atrioventricular bundle (AV bundle) passes through atrioventricular septa and divides into right & left branches.

Each branch passes through the ventricular walls of its side. In the ventricular wall, it breaks up into minute fibres (Purkinje fibres). These fibres along with the bundles are known as the bundle of His.

Nodal tissues generate action potential without any external stimuli, i.e. it is autoexcitable. SAN initiates and maintains contraction of heart by generating action potentials (70-75/min). So, it is called the pacemaker.



Is the cyclic contraction and relaxation of heart for pumping blood. It involves 3 stages:

Joint diastole: It is the relaxed state of all chambers of heart. When the tricuspid and bicuspid valves open, blood from pulmonary vein and vena cava flows into left & right ventricles respectively through left and right atria. Semilunar valves are closed at this stage.


Atrial (Auricular) systole: SAN generates an action potential. As a result, both the atria contract. It is called atrial systole. This increases the flow of blood into the ventricles by about 30%.


Ventricular systole: The action potential is conducted to the ventricular side by AVN & AV bundle from where the bundle of His transmits it through the ventricular musculature. As a result, ventricles contract. It is called ventricular systole.


During this, the atria undergo diastole. Ventricular systole increases the ventricular pressure causing

Closure of tricuspid and bicuspid valves due to attempted backflow of blood into the atria.

Semilunar valves open. So deoxygenated blood enters the pulmonary artery from right ventricle and oxygenated blood enters the aorta from left ventricle.

The ventricles now relax (ventricular diastole) and the ventricular pressure falls causing

  • The closure of the semilunar valves which prevents the backflow of blood into the ventricles.
  • The tricuspid and bicuspid valves are opened by the pressure in the atria.

The ventricles and atria again undergo joint diastole and the above processes are repeated.

A cardiac cycle is completed in 0.8 seconds.

·   One heartbeat = a cardiac cycle.

So, normal heartbeat: 70-75 times/min (average: 72/min).

  • Stroke volume: It is the volume of blood pumped out by each ventricle during a cardiac cycle. It is about 70 ml.
  • Cardiac output: It is the volume of blood pumped out by each ventricle per minute, i.e. stroke volume x heart rate (70 x 72). It is about 5000 ml (5 litres).

Cardiac output of an athlete is very high.

  • Heart sounds: During each cardiac cycle, 2 sounds are produced. The first sound (lub) is due to the closure of tricuspid and bicuspid valves. The second sound (dub) is due to the closure of the semilunar valves.

One heartbeat = a lub + a dub.

           ELECTROCARDIOGRAPH (ECG)         

  • It is an instrument used to obtain an electrocardiogram.
  • An electrocardiogram is the graphical representation of the electrical activity of the heart during a cardiac cycle.
  • To get an ECG, a patient is connected to the machine with 3 electrical leads (one to each wrist and to left ankle) that monitor heart activity. For a detailed evaluation of heart’s function, multiple leads are attached to the chest region.
  • An ECG consists of the following waves:
    • P-wave: Represents the excitation (depolarization) of atria which causes atrial systole.
    • QRS-complex: Represents depolarization of ventricles

(Ventricular systole).

  • T-wave: Represents the repolarisation of ventricles.

Deviation in the ECG indicates the abnormality or disease. So, ECG has great clinical significance.

                 DOUBLE CIRCULATION                

It is the circulation in which blood flows through the heart twice for completing its circuit.

It includes:

  1. Pulmonary circulation: Circulation b/w lungs and heart. Deoxygenated blood from right ventricle → to pulmonary artery → to lungs oxygenated blood → to pulmonaryveins → left atrium.
  • Systemic circulation: Circulation b/w heart and various body parts.

Oxygenated blood from left ventricle → to aorta arteries arterioles capillaries tissues deoxygenated blood from tissues → venules veins vena cava → to right atrium.


Systemic circulation provides nutrients, O2 and other substances to the tissues and takes CO2 and other harmful substances away for elimination.

  • Hepatic portal system: It is a system which includes the hepatic portal vein that carries blood from intestine to the liver before it is delivered to the systemic circulation.
  • Coronary circulatory system: It is a system of coronary vessels that circulate blood to and from cardiac musculature.



Normal activities of heart are auto-regulated by nodal tissues. So, it is called myogenic heart.

Medulla oblongata regulates cardiac activity through ANS.

Sympathetic nerves of ANS increase the rate of heartbeat, the strength of ventricular contraction and cardiac output.

Parasympathetic nerves of ANS decrease the heartbeat, conduction of action potential and the cardiac output.

Adrenal medullary hormones increase the cardiac output.


Includes Lymph, Lymph vessels & Lymph nodes (glands).


As the blood passes through the capillaries in tissues, some water and soluble substances are filtered out from plasma to the intercellular spaces, to form tissue (interstitial) fluid. It has same mineral distribution as that in plasma.

Some tissue fluid enters lymphatic system and the tissue fluid in them is called lymph. It drains back to major veins.

Lymph is a colourless fluid containing lymphocytes.

Functions of lymph

  • It is the middleman between blood & tissues. Tissue fluid helps to exchange nutrients, gases, etc. b/w blood and cells.
  • It carries plasma proteins synthesized in the liver to the blood.
  • Transports digested fats (through lacteals in the intestinal villi), fat-soluble vitamins, hormones, etc.
  • Filtration of bacteria and foreign particles.
  • Lymph nodes produce WBC (lymphocytes) & antibodies.


Hypertension (High Blood Pressure): The pressure of circulating blood on the walls of blood vessels is called blood pressure. Normal BP is 120/80 mm Hg. It includes systolic (pumping) pressure (120 mm Hg) and diastolic (resting) pressure (80 mm Hg).

When the blood pressure is higher than normal BP, it is called hypertension. If an individual repeatedly has the BP of 140/90 or above, it shows hypertension. It leads to heart diseases and affects vital organs (brain, kidney etc).

·   Coronary  Artery  Disease  (CAD)  or Atherosclerosis:

Here, Ca, fat, cholesterol and fibrous tissue are deposited

in coronary arteries. It makes the lumen of arteries narrower and thereby affects the blood supply.

Angina (angina pectoris): An acute chest pain due to O2 deficiency to heart muscles. It occurs due to improper blood flow. It is common among middle-aged and elderly.

Heart Failure (congestive heart failure): It is the inability of heart to pump blood enough to meet the needs of the body. Congestion of the lungs is the main symptom.

Cardiac arrest: Heart stops beating.

Heart attack: Sudden damage of heart muscle due to inadequate blood supply.

Breathing and Exchange of Gases NCERT Class 11 Notes for NEET

Respiration is the oxidation of nutrients in the living cells to release energy for biological work.

Breathing is the exchange of O2 from the atmosphere with CO2 produced by the cells.


  •  General body surface: E.g. lower invertebrates (sponges, coelenterates, flatworms etc).
  • Skin or moist cuticle (cutaneous respiration): E.g.earthworms, leech, amphibians etc.
  • Tracheal tubes: E.g. insects, centipede, millipede, spider.
  • Gills (Branchial respiration): E.g. fishes, tadpoles, prawn.
  • Lungs (Pulmonary respiration): E.g. most vertebrates.



It consists of a pair of air passages (air tract) and lungs.

 1. Air passages

– Conducting part which transports the atmospheric air into the alveoli, clears it from foreign particles, humidifies and brings the air to body temperature.

External nostrils → nasal passage → nasal chamber (cavity) → pharynx → glottis → larynx → trachea → primary bronchi → secondary bronchi → tertiary bronchi → bronchioles → terminal bronchioles → respiratory bronchiole → alveolar duct.


– Each terminal bronchiole gives rise to many very thin and vascularised alveoli (in lungs).

– A cartilaginous Larynx (sound box or voice box) helps in sound production.

– During swallowing, epiglottis (a thin elastic cartilaginous flap) closes glottis to prevent entry of food into larynx.

– Trachea, all bronchi and initial bronchioles are supported by incomplete cartilaginous half rings.

2. Lungs

– Lungs situate in thoracic chamber and rest on diaphragm.


– Right lung has 3 lobes and left lung has 2 lobes. – Lungs are covered by double-layered pleura (outer parietal pleura and inner visceral pleura).

– The pleural fluid present in between these 2 layers lubricates the surface of the lungs and prevents friction between the membranes.

Lungs= Bronchi + bronchioles + alveoli.

– Alveoli and their ducts form the respiratory or exchange part of the respiratory system.

– Alveoli are the structural and functional units of lungs.

Steps of respiration


1. Pulmonary ventilation (breathing).

2. Gas exchange between lung alveoli & blood.

3. Gas transport (O2 transport & CO2 transport).

4. Gas exchange between blood & tissues.

5. Cellular or tissue respiration.


a. Inspiration


– Active intake of air from atmosphere into lungs. – During this, the diaphragm contracts (flattens) causing an increase in vertical thoracic volume (antero-posterior axis).

– Contraction of external intercostal muscles (muscles found between ribs) lifts up the ribs and sternum causing an increase in thoracic volume in the dorso-ventral axis.

– Increase in thoracic volume reduces thoracic pressure. So, lungs expand. Thus, pulmonary volume increases resulting in decrease of intra-pulmonary pressure to less than the atmospheric pressure. So, air moves into lungs. 

b. Expiration


– Passive expelling of air from the lungs.

– During this, intercostal muscles & diaphragm relax causing a decrease in thoracic volume and thereby pulmonary volume. So, air moves out.

– During forceful expiration, abdominal muscles and internal inter-costal muscles contract.

Respiratory volumes and capacities

• Tidal volume (TV): Volume of air inspired or expired during a normal respiration. It is about 500 ml. i.e., 6000-8000 ml per minute. 

• Inspiratory reserve volume (IRV) or complemental air: Additional volume of air that can inspire by forceful inspiration. It is 2500-3000 ml.

• Expiratory reserve volume (ERV) or supplemental air: Additional volume of air that can expire by a forceful expiration. It is 1000-1100 ml.

• Residual volume (RV): Volume of air remaining in lungs after a forcible expiration. It is 1100-1200 ml.

• Inspiratory capacity (IC): Total volume of air inspired after a normal expiration (TV + IRV). It is 3000-3500 ml.

• Expiratory capacity (EC): Total volume of air expired after a normal inspiration (TV + ERV). It is 1500-1600 ml.

• Functional residual capacity (FRC): Volume of air remaining in the lungs after a normal expiration (ERV + RV). It is 2100-2300 ml.

• Vital capacity (VC): Volume of air that can breathe in after a forced expiration or Volume of air that can breathe out after a forced inspiration (ERV + TV + IRV). It is 3500-4500 ml.

• Total lung capacity (TLC): Total volume of air in the lungs after a maximum inspiration. (RV + ERV + TV + IRV or VC + RV). It is 5000-6000 ml.


• Part of respiratory tract (from nostrils to terminal bronchi) not involved in gaseous exchange is called dead space. Dead air volume is about 150 ml.


– Respiratory cycle= an inspiration + an expiration

– Normal respiratory (breathing) rate: 12-16 times/min

– Spirometer (respirometer): To measure respiratory rate.


Gas exchange occurs between  1. Alveoli and blood and 2. Blood and tissues


Alveoli are the primary sites of gas exchange. O2 & CO2 are exchanged by simple diffusion. It depends upon the following factors:

• Pressure/ concentration gradient: The Partial pressures (individual pressure of a gas in a gas mixture) of O2 and CO2 (pO2 and pCO2) are given below.


pO2 in alveoli is more (104 mm Hg) than that in blood capillaries (40 mm Hg). So O2 diffuses into capillary blood.

pCO2 in deoxygenated blood is more (45 mm Hg) than that in alveoli (40 mm Hg). So, CO2 diffuses to alveoli.

• Solubility of gases: Solubility of CO2 is 20-25 times higher than that of O2.


So, the amount of CO2 that can diffuse through the diffusion membrane per unit difference in partial pressure is higher than that of O2.

Thickness of membranes: The diffusion membrane is made up of 3 layers:


  a) Squamous epithelium of alveoli.

 b) Endothelium of alveolar capillaries.

 c) Basement substance between them.

 Its total thickness is only 0.5 micrometer. It enables easy gas exchange.

• Surface area: Presence of alveoli increases the surface area of lungs. It increases the gas exchange.



It is the transport of respiratory gases (O2 & CO2) from alveoli to the systemic tissues and vice versa.


It is the transport of O2 from lungs to various tissues. It occurs in 2 ways:


a. In physical solution (blood plasma): About 3% of O2 is carried in a dissolved state through plasma.

b. As oxyhaemoglobin: About 97% of O2 is transported by RBC.

O2 binds with haemoglobin (red coloured iron containing pigment present in the RBCs) to form oxyhaemoglobin. This is called oxygenation.

Hb has 4 haem units. So, each Hb molecule can carry 4 oxygen molecules. Binding of O2 depends upon pO2, pCO2, H+ ion concentration (pH) and temperature.

 – In the alveoli, high pO2, low pCO2, lesser H+ ion concentration and lower temperature exist. These factors are favourable for the formation of oxyhaemoglobin. –

In tissues, low pO2, high pCO2, high H+ ions and high temperature exist. So Hb4O8 dissociates to release O2. –

Every 100 ml of oxygenated blood can deliver around 5 ml of O2 to the tissues under normal physiological conditions.

Oxygen-haemoglobin dissociation curve


It is a sigmoid curve obtained when percentage saturation of Hb with O2 is plotted against the pO2. It is used to study the effect of factors like pCO2, H+ concentration etc., on binding of O2 with Hb.


It is the transport of CO2 from tissues to lungs. In tissues, pCO2 is high and pO2 is low. In lungs, pCO2 is low and pO2 is high.


This favours CO2 transport from tissues to lungs. It occurs in 3 ways:

a. As carbonic acid: In tissues, 7% of CO2 is dissolved in plasma water to form carbonic acid and carried to lungs.

b. As carbamino-haemoglobin: In tissues, 20-25% of CO2 binds to Hb to form carbamino-haemoglobin. In alveoli, CO2 dissociates from carbamino-haemoglobin.

c. As bicarbonates: About 70% of CO2 is transported by this method. RBCs and plasma contain an enzyme, carbonic anhydrase. It facilitates the following reactions:

 In alveoli, the above reaction proceeds in opposite direction leading to the formation of CO2 and H2O.

Every 100 ml of deoxygenated blood delivers about 4 ml of CO2 to the alveoli.

In brain, there are the following Respiratory centres:


• Respiratory rhythm centre (Inspiratory & Expiratory centres): In In medulla oblongata, It regulates respiratory rhythms.


• Pneumotaxic centre: In Pons, It moderates functions of respiratory rhythm centre. The impulse from this centre reduces the duration of inspiration and thereby alter respiratory rate.


• Chemosensitive area: Seen adjacent to the rhythm centre.
Increase in the concentration of CO2 and H+ activates this centre, which in turn signals rhythm centre.

Receptors in the aortic arch & carotid artery also recognize changes in CO2 & H+ concentration and send signals to the rhythm center.

Role of oxygen in the regulation of respiratory rhythm is quite insignificant.


1.Asthma: Difficulty in breathing causing wheezing due to inflammation of bronchi and bronchioles.


2.Emphysema: Damage of alveolar walls. It decreases respiratory surface. Major cause is cigarette smoking.


3.Occupational respiratory disorders: Certain industries produce so much dust.


So, the defense mechanism of the body cannot cope with the situation.

Long exposure causes inflammation leading to fibrosis (proliferation of fibrous tissues). It results in lung damage.

Workers in such industries should wear protective masks.


NCERT UNIT 19 – Excretory Products and their Elimination CBSE Class 11 Biology

Excretion is the elimination of metabolic wastes like ammonia, urea, uric acid etc. from the tissues.

Types of excretion

1. Ammonotelism:

Process of excretion of NH3. Ammonotelic animals: Aquatic invertebrates, aquatic insects, bony fishes, aquatic amphibians, etc. NH3 is highly toxic.

So, excretion needs an excess of water.

NH3 is readily soluble in water and is excreted by diffusion through a body surface or gill surfaces (in fishes) as ammonium ions.

Kidneys do not play any significant role in its removal.

2. Ureotelism:

Process of excretion of urea.

Ureotelic animals: Cartilaginous fishes, terrestrial & semi-aquatic amphibians (frogs, toads, etc.), aquatic & semi-aquatic reptiles (alligators, turtles), mammals, etc.

In the liver, NH3 is converted into less toxic urea.

So, it needs only a moderate quantity of water for excretion. Some amount of urea may be retained in the kidney matrix of some animals to maintain the desired osmolarity.

3. Uricotelism:

Process of excretion of uric acid. It is water insoluble & less toxic. So, water is not needed for excretion.

Uricotelic animals: Insects, some land crustaceans, land snails, terrestrial reptiles & birds. Ureotelism & uricotelism are needed for water conservation.

Some excretory organs in animals

Protonephridia (flame cells): In Flatworms, rotifers, some annelids & cephalochordate. Protonephridia are primarily for osmoregulation.

Nephridia: In Annelids. Help in the removal of nitrogenous wastes and osmoregulation.

Malpighian tubules: In Insects. Help in the removal of nitrogenous wastes and osmoregulation.

Antennal or green glands: In Crustaceans (prawn etc.)

Kidneys: In higher animals.


NCERT - Excretory Products and their Elimination CBSE Class 11  Biology human excretory system

It includes kidneys, ureters, urinary bladder & urethra.

Structure of Kidney

– Reddish brown, bean-shaped structures situated between the levels of last thoracic & 3rd lumbar vertebra.

– Length: 10-12 cm, width: 5-7 cm, thickness: 2-3 cm.

Average weight: 120-170 gm. – It is enclosed in a tough, 3-layered fibrous renal capsule.

NCERT - Excretory Products and their Elimination CBSE Class 11  Biology kidney labelled

– On the concave side of the kidney, there is an opening (hilum or hilus) through which blood vessels, nerves, lymphatic ducts, and ureters enter the kidney.

– Hilum leads to funnel shaped cavity called renal pelvis with projections called calyces.

– A kidney has outer cortex & inner medulla.

– Medulla has few conical projections called medullary pyramids (renal pyramids) projecting into the calyces.

– Cortex extends in between the medullary pyramids as renal columns (Columns of Bertini).

– Each kidney has nearly one million tubular nephrons.


– Nephrons are the structural & functional units of kidney.

– Each nephron has 2 parts: Glomerulus & Renal tubule.

NCERT - Excretory Products and their Elimination CBSE Class 11  Biology nephron

o Glomerulus:

A tuft of capillaries formed by afferent arteriole (a fine branch of the renal artery). Blood from the glomerulus is carried away by an efferent arteriole.

o Renal tubule:

It begins with a double-walled cup-like Bowman’s capsule, which encloses the glomerulus.

Glomerulus + Bowman’s capsule = Malpighian body

NCERT - Excretory Products and their Elimination CBSE Class 11  Biology bowmans capsule

– The tubule continues with proximal convoluted tubule (PCT), Henle’s loop & distal convoluted tubule (DCT).

– Henle’s loop is hairpin-shaped. It has to descending and ascending limbs. – The DCTs of many nephrons open into a collecting duct.

The collecting duct extends from the cortex to the inner parts of the medulla. They converge and open into the renal pelvis through medullary pyramids in the calyces.

– Malpighian body (Renal corpuscle), PCT, and DCT are situated in the renal cortex. Loop of Henle dips into the medulla.

– The efferent arteriole forms a fine capillary network (peritubular capillaries) around the renal tubule.

A minute vessel of this network runs parallel to Henle’s loop forming a ‘U’ shaped vasa recta.

Types of nephrons

1. Cortical nephrons (85%): In this, the Henle’s loop is short and extends only very little into the medulla. Vasa recta is absent or highly reduced.

2. Juxtamedullary nephrons (15%): In this, Henle’s loop is long and runs deep into medulla. Vasa recta present.


– A cell is the fundamental, structural, hereditary, and functional unit of all living organisms

Robert Hooke: Discovered cell (dead cell, from cork plant)

Anton Von Leeuwenhoek: First observed and described a live cell.

– The invention of the compound & electron microscopes revealed all the structural details of the cell.

                               CELL THEORY

Matthias Schleiden (1838) observed that all plants are composed of different kinds of cells.

Theodore Schwann (1839) found that cells have a thin outer layer (plasma membrane). He also found that plant cells have cell wall.

-He proposed the hypothesis that animals and plants are composed of cells and products of cells.

Schleiden & Schwann formulated the cell theory.

Rudolf Virchow (1855) first explained that cells divide and new cells are formed from pre-existing cells (Omnis cellula-e cellular).

-He modified the cell theory. – Cell theory states that:

(i) All living organisms are composed of cells and products of cells.

(ii) Cells arise from pre-existing cells.

                                                               AN OVERVIEW OF CELL

– All cells contains

o Cytoplasm: A semi-fluid matrix where cellular activities and chemical reactions occur. This keeps the cell in ‘living state’.

o Ribosomes: Non-membrane bound organelles seen in cytoplasm, chloroplasts, mitochondria & on rough ER.

# Cells differ in size, shape, and activities.

o Smallest cells: Mycoplasmas (0.3 µm in length).

o Largest isolated single cell: Egg of ostrich.

o Longest cells Eg Nerve cell.

o Size of bacteria: 3 to 5 µm.

o Human RBCs are about 7.0 µm in diameter.

– Based on the functions, the shape of cells may be disc-like, polygonal, columnar, cuboid, threadlike, or irregular.

source: ncert biology

Cells are 2 types: Prokaryotic & Eukaryotic cells.

                                                                         PROKARYOTIC CELLS

They have no membrane bound nucleus and organelles.

– They include bacteria, blue-green algae, mycoplasma & PPLO (Pleuro Pneumonia Like Organisms) and are generally smaller and multiply more rapidly than the eukaryotic cells.

– They vary in shape & size. Bacteria have 4 basic shapes :-Bacillus, Coccus, Vibrio, and Spirillum.      



      # Cell organelles in prokaryotic cells

1.  Cell Envelope – It is a chemically complex protective covering. – It is made of 3 tightly bound layers.


o Glycocalyx: Outer layer. Its composition and thickness vary in different bacteria. It may be a slime layer (loose sheath) or capsule (thick & tough)

o Cell wall: Middle layer. Seen in all prokaryotes except mycoplasma. It gives shape to the cell and provides structural support to prevent the bacterium from bursting or collapsing.

o Plasma membrane: Inner layer. It is semi-permeable in nature and interacts with the outside. This is structurally similar to that of the eukaryotes.

– Based on the types of cell envelopes and response to Gram staining (developed by Gram), bacteria are 2 types:

o Gram-positive: They take up and retain the gram stain.

o Gram-negative: They do not retain the gram stain.


2. Mesosomes & Chromatophores (Membranous structures)

– Mesosome is formed by the infolding of plasma membrane. It includes vesicles, tubules & lamellae.

Functions : Mesosomes helps in 

o In cell wall formation.

o In DNA (chromosome) replication.

o In distribution of chromosomes to daughter cells.

o In respiration and secretion processes.

o To increase the surface area of the plasma membrane and enzymatic content.

– Chromatophores are membranous infoldings in some prokaryotes (e.g. cyanobacteria). They contain pigments.

3. Nucleoid – It is formed of non-membranous (naked) circular genomic DNA (single chromosome/ Genetic material) & protein.

– Many bacteria have small circular DNA (plasmid) outside the genomic DNA. It gives some unique phenotypic characters (e.g. resistance to antibiotics) to bacteria.

4. Flagella – These are thin filamentous extensions from the cell wall of motile bacteria.

Their number and arrangement are varied in different bacteria. – The bacterial flagellum has 3 parts

filament, hook, and basal body. The filament is the longest portion and extends from the cell surface to the outside.

5. Pili and Fimbriae – These are surface structures that have no role in motility.

– Pili (sing. Pilus) are elongated tubular structures made of a special protein (pilin).

– Fimbriae are small bristle-like fibers sprouting out of the cell. In some bacteria, they help to attach the bacteria to rocks in streams and to the host tissue


6. Ribosomes – They are associated with the plasma membrane of prokaryotes.

– They are about 15 nm by 20 nm in size. – They are made of 2 subunits – 50S & 30S (Svedberg’s unit). – They together form 70S prokaryotic ribosomes. (S= sedimentation coefficient; a measure of density & size)

– Function: Ribosomes are the site of translation (protein synthesis). Several ribosomes may attach to a single mRNA to form a chain called polyribosomes (polysome).

Ribosomes translate the mRNA into proteins.

7. Inclusion Bodies – These are non-membranous, stored reserve material seen freely in the cytoplasm of prokaryotic cells. – E.g. phosphate granules, cyanophycean granules, and glycogen granules, gas vacuoles, etc.

Gas vacuoles are found in blue-green and purple and green photosynthetic bacteria.

To be continue….next page….