Hypothyroidism is a disorder in which thyroid gland is underactive and does not produce or release enough thyroid hormones or thyroxin.

As the metabolism continues to slow down, signs and symptoms develop. Here are a few signs and symptoms of hypothyroidism:

1. Fatigue.
2. Weight gain with high cholesterol.
3. Memory problems.
4. Coarse and pale skin.
5. Coarse and dry hair with hair loss.
6. Cold intolerance.
7. Muscle cramps with frequent muscle aches.
8. Frequent constipation.
9. Depression.
10. Irritability.

• The sequential event in the heart which is cyclically repeated is called the cardiac cycle and it consists of systole and diastole of both the atria and ventricles.
• Duration of a cardiac cycle is 0.8 seconds.
• During a cardiac cycle, each ventricle pumps out approximately 70 mL of blood which is called the stroke volume.
• The stroke volume multiplied by the heart rate gives the cardiac output.
• Cardiac output can be defined as the volume of blood pumped out by each ventricle per minute and averages 5000 mL or 5 litres in a healthy individual.
• As the tricuspid and bicuspid valves are open, blood from the pulmonary veins and vena cava flows into the left and the right ventricle respectively through the left and right atria and the semilunar valves are closed at this stage.
• The SAN now generates an action potential which stimulates both the atria to undergo a simultaneous contraction – the atrial systole.
• The action potential is conducted to the ventricular side by the AVN and AV bundle from where the bundle of HIS transmits it through the entire ventricular musculature, which causes the ventricular muscles to contract, (ventricular systole), the atria undergoes relaxation (diastole), coinciding with the ventricular systole.
• Ventricular systole increases the ventricular pressure causing the closure of tricuspid and bicuspid valves.
• As the ventricular pressure increases further, the semilunar valves guarding the pulmonary artery (right side) and the aorta (left side) are forced open, allowing the blood in the ventricles to flow through these vessels into the circulatory pathways.
• The ventricles now relax (ventricular diastole) and the ventricular pressure falls causing the closure of semilunar valves which prevents the backflow of blood into the ventricles.
• As the ventricular pressure declines further, the tricuspid and bicuspid valves are pushed open by the pressure in the atria exerted by the blood which was being emptied into them by the veins and the blood now once again moves freely to the ventricles, leading the ventricles and atria again in a relaxed (joint diastole) state.
• Again, the SAN generates a new action potential and the events are repeated.
• During each cardiac cycle two prominent sounds are produced which can be easily heard through a stethoscope.
• The first heart sound (lub) is associated with the closure of the tricuspid and bicuspid valves, whereas the second heart sound (dub) is associated with the closure of the semilunar valves.

4 Disorders Of The Circulatory System are as listed below:

• Coronary artery disease (CAD) – also referred to as atherosclerosis, it affects the vessels supplying blood to the heart muscles.
• Heart failure – in this state of the heart, blood is not being pumped effectively to fulfil the needs of the body. One of the main symptoms of this disease is the lung-congestion hence it is also referred to as congestive heart failure
• Angina – it is a symptom of acute chest pain surfaces when there is a dearth of oxygen supply to the muscles of the heart.
• High blood pressure (hypertension) – is a condition where the blood pressure is higher than the normal blood pressure (120/80).

Heart beat is the rhythmic contraction and relaxation of the heart. Each heart beat includes one systole (contraction phase) and one diastole (relaxation phase) of the heart to distribute and receive blood to and from the body. The heart of a healthy person beats 72 times per minute. Origin of heart beat. The mammalian heart is myogenic (myo = muscle, genic = originating from). It means the heart beat originates from a muscle, (however, it is regulated by the nerves). The heart beat originates from the sinoatrial node (SANode)—pace maker, which lies in the wall of the right atrium, near the opening of the superior vena cava. The SA node is a mass of neuromuscular tissue.

Conduction of heart beat. Another mass of neuromuscular tissue, the atrio-venticular node (AV node) is situated in the wall of the right atrium. The AV node picks up the wave of contraction propagated by SAnode. Amass of specialized fibres, the bundle of His, originates from the AV node. The bundle of His divides into two branches, one going to each ventricle. Within the myocardium of the ventricles the branches of bundle of His divide into a net work of fine fibres called the Purkinje fibres. The bundle of His and the Purkinje fibres convey impulse of contraction from the AV node of the myocardium of the ventricles.

1. The arteries receive oxygenated blood from the heart and supply it throughout the body. Whereas, the veins carry the deoxygenated blood from all the body parts to the heart for oxygenation.
2. The right atrium receives blood from the veins and pumps it to the right ventricle.
3. The right ventricle pumps the blood received from the right atrium to the lungs.
4. The left atrium receives oxygenated blood from the lungs and pumps it to the left ventricle.
5. The left ventricle pumps the oxygenated blood throughout the body.

Structure of Human Heart

• The heart is a muscular organ covered by a double membrane called pericardium.
• The pericardial fluid of the pericardium lubricates the heart.
• The heart is divided into four chambers — two upper atria and two lower ventricles.
• The inter-atrial septum separates the two atria and the inter-ventricular septum separates the two ventricles.
• The atrium and the ventricle of each side are separated by the atrioventricular septum.
• Superior and inferior vena cavae open into the right atrium.
• The right ventricle opens into the pulmonary artery.
• Four pulmonary veins open into the left atrium.
• The left ventricle opens into the aorta.
• The opening between the right atrium and right ventricle is guarded by the tricuspid valve.
• The opening between the left atrium and left ventricle is guarded by the bicuspid (mitral) valve.
• The opening of the right ventricle into the pulmonary artery and the opening of the left ventricle into the aorta is guarded by the semilunar valves.

• Blood coagulation or clotting is the mechanism to prevent excessive loss of blood from the body.
• Reddish brown scum formed at the site of a cut is due to clot formed mainly of a network of threads called fibrins in which dead and damaged formed elements of blood are trapped.
• Fibrins are formed by the conversion of inactive fibrinogens in the plasma by the enzyme thrombin.
• Thrombins are formed from another inactive substance present in the plasma called prothrombin by an enzyme complex known as thrombokinase.
• Calcium ions play a very important role in clotting.

• Two blood groupings are done
• ABO and
• Rh

ABO grouping

• ABO grouping is based on the presence or absence of two surface antigen on the RBCs namely A and B.
• The plasma of different individuals contains two natural antibodies.
• The distribution of antigens and antibodies in the four groups of blood, A, B, AB and O.
• The blood of a donor has to be carefully matched with the blood of a recipient before any blood transfusion to avoid severe problems of clumping, which leads to destruction of RBC.
• Group ‘O’ blood can be donated to persons with any other blood group and hence ‘O’ group individuals are called ‘universal donors’.
• Persons with ‘AB’ group can accept blood from persons with AB as well as the other groups of blood, and such persons are called ‘universal recipients’.

Rh grouping

• The Rh antigen similar to one present in Rhesus monkeys is also observed on the surface of RBCs of majority of humans, hence the antigen is known as Rh antigen.
• The individuals having Rh antigen are called Rh positive (Rh+ve) and those in whom this antigen is absent are called Rh negative (Rh-ve).
• An Rh-ve person, if exposed to Rh+ve blood, will form specific antibodies against the Rh antigens, and hence Rh group should also be matched before transfusions.
• A special case of Rh incompatibility has been observed between the Rh-ve blood of a pregnant mother with Rh+ve blood of the foetus , which leads to a disease known as erythroblastosis foetalis.
• Rh antigens of the foetus do not get exposed to the Rh-ve blood of the mother in the first pregnancy as the two bloods are well separated by the placenta, during the delivery of the first child, maternal blood may get exposed to small amounts of the Rh+ve blood from the foetus and the mother starts preparing antibodies against Rh in her blood.
• In case of subsequent pregnancies, the Rh antibodies from the mother (Rh-ve) can leak into the blood of the foetus (Rh+ve) and destroy the foetal RBCs, which cause severe anaemia and jaundice to the baby leading to a condition known erythroblastosis foetalis.
• Erythroblastosis foetalis can be avoided by administering anti-Rh antibodies to the mother immediately after the delivery of the first child.

• Blood is a special connective tissue consisting of a fluid matrix, plasma, and formed elements.

There are many cellular structures in the composition of blood. When a sample of blood is spun in a centrifuge machine, they separate into the following constituents: Plasma, buffy coat and erythrocytes.

Plasma

The liquid state of blood can be contributed to plasma as it makes up  ~55% of blood. It is pale yellow in colour and when separated, it consists of salts, nutrients, water and enzymes. Blood plasma also contains important proteins and other components necessary for overall health. Hence, blood plasma transfusions are given to patients with liver failure and life-threatening injuries.

Red Blood Cells (RBC)

Red blood cells consist of Haemoglobin, a protein. They are produced by the bone marrow to primarily carry oxygen to the body and carbon dioxide away from it.

White Blood Cells (WBC)

White blood cells are responsible for fighting foreign pathogens (such as bacteria, viruses, fungi) that enter our body. They circulate throughout our body and originate from the bone marrow.

Platelets

Tiny disc-shaped cells that help regulate blood flow when any part of the body is damaged, thereby aiding in fast recovery through clotting of blood.

Spleen has a similar structure to a large lymph node, which primarily functions as a blood filter. The spleen plays an important role in the red blood cells also known as aserythrocytes and the digestive system. Old and damaged RBC’s are destroyed in the spleen and It is known as the RBCs Graveyard.

Function of spleen :

• The spleen is the largest organ of the lymphatic system. It keeps all the body fluids balanced.
• It is made up of a red pulp tissue that filters the old and damaged red blood cells.
• The important function of the spleen is to filter the blood. The spleen recycles the old and damaged red blood cells and the white blood cells are stored.
• Spleen also helps to fight against bacteria that cause diseases such as meningitis and pneumonia.

Based on the position of the ovary flowers are categorized to 3 types they are
: Hypogynous, Epigynous, and Perigynous flowers.
Hypogynous: Flowers in which the sepals, petals, and stamens are attached below the ovary are called hypogynous, and the ovaries of such flowers are said to be superior. 
Eg: tomato, tulip, and snapdragon.
Epigynous: Flowers in which the sepals, petals, and stamens appear to be attached to the upper part of the ovary due to the fusion of the hypanthium are called epigynous, and the ovaries of such flowers are said to be inferior.  Eg: daffodil
Perigynous: Flowers types in which the hypanthium forms a cuplike or tubular structure that partly surrounds the ovary are called perigynous. In such flowers, the sepals, petals, and stamens are attached to the rim of the hypanthium, and the ovaries of such flowers are superior.
 Eg: cherry, Prunus

• The mitochondria is a double-membraned cell organelle, known as the powerhouse of the cell which is present in all eukaryotic cells.
• It was first discovered by Albert von Kolliker in the year 1857.
• It was named as bioblast by Richard Altman in the year 1886.
• The term mitochondria was coined by Carl Benda in the year 1898. 

The discovery of mitochondria is accredited to the several scientists who contributed to the discovery of mitochondrion and identification of its structure and functions. The earliest accounts of description of mitochondria go back to 1840. However, Richard Altmann was the first one to recognize the occurrence of these organelles and called them bioblasts. The name mitochondria was coined by Carl Benda in 1898. Christian de Duve was a Belgian researcher who discovered lysosomes in 1949.

Macromolecules are basically polymers, long chains of molecular sub-units called monomers. Carbohydrates, proteins and nucleic acids are found as long polymers. Due to their polymeric nature and large size, they are known as macromolecules.

The large complex molecules having molecular weights more than one thousand Dalton which occur in colloidal state in the intercellular fluid are called macromolecules. They are formed by the polymerization of low molecular weight micromolecules. For example: Polysaccharides, proteins, lipids, etc.

Key is a taxonomical aid that helps in the identification of plant and animal species. These keys are based on similarities and dissimilarities in characters, generally in a pair called a couplet. Each statement in a taxonomic key is referred to as a lead. It is also useful in the identification of unknown organisms.

The full form of PPLO is Pleuron pneumonia-like organisms and the term used to describe the mycoplasmas. PPLO is the smallest cell or organism with the size of between 0.1 and 0.3 mm. Mycoplasma has included species discovered from pleural fluid of cattle suffering from pleuropneumonia.

The study of tissue is called as histology study of forms and shapes of any living thing is called morphology study of the internal cellular organs or parts of any living thing is called anatomy. Pathophysiology or physiopathology is a convergence of pathology with physiology.

• Tissues are organized in specific proportion and pattern to form an organ. Examples- stomach, lung, heart and kidney.
• When two or more organs perform a common function by their physical and/or chemical interaction, they together form organ system. Examples- digestive system, respiratory system, etc.
• Cells, tissues, organs and organ systems exhibit division of labor for the survival of the whole body.

The gram-positive bacteria retain the crystal violet colour and stains purple whereas the gram-negative bacteria lose crystal violet and stain red. Thus, the two types of bacteria are distinguished by gram staining.

The most abundant protein present in the animal world is collagen.

Collagen: - It is a protein made from amino acids, specifically glycine, proline, hydroxyproline, and arginine.

 In plants, the most abundant protein is Rubisco.

Glycolysis is the first stage of the breakdown of glucose in the cell. During glycolysis 2 ATP molecules are used up and four ATP molecules are generated. In the entire process of glycolysis, two NADH₂ molecules are also generated. When these molecules undergo ETS they will form 3 ATP per NADH₂  which means 6 ATP. Therefore the total ATP that are forming are 10 and as 2 ATP is used up the net gain will be 8.

In glycolysis, two molecules of ATP are consumed initially in converting glucose to fructose 1, 6-bisphosphate. Two triose phosphate molecules are formed from one glucose molecules. Four molecules of ATP are produced at substrate level phosphorylation. Therefore, net gain of ATP is 2ATP×2−2ATP=2.

Neurons or nerve cells can be up to 3 feet long. A typical neuron has a cell morphology called soma, hair-like structures called dendrites and an axon. Neurons are specialized in conveying knowledge throughout the body. The sensory neurons, motor neurons, and interneurons are three types of neurons. Neurons have a membrane built to forward information to other cells.