HCl plays the following role in our stomach:
(a) Make the medium acidic for action of enzyme pepsin. It activates pepsinogen to pepsin.
(b) Kills the harmful bacteria that enter the body along with food.
 

Ultrafiltration is a process in the kidney by which urea, salt, water and glucose etc. is extracted from the blood. When blood passes through the top of the nephron, it enters a structure called the glomerulus which is a network of tiny capillaries. This causes the pressure to increase and fluid is forced through the "sieve-like" walls of the vessels into the Bowman's capsule. This fluid is called the filtrate. Blood cells and larger proteins do not pass through the capillaries as they are too large and so are not found in the filtrate. A large proportion what enters the filtrate is valuable to the body and so needs to be reabsorbed into the blood. This is a process called selective reabsorption.

The heart is roughly triangular in shape. It has four compartments called chambers inside it. The upper two chambers of the heart are called atria and the lower two chambers are called ventricles. The atria receive blood from the two main veins. The left atrium is connected to the left ventricle through a valve V1 and the right atrium is connected to the right ventricle through another valve V2. These valves prevent the backflow of blood into atria when the ventricles contract to pump blood out of the heart to the rest of the body because when the ventricles contract, the valves V1 and V2 close automatically so that the blood may not go back into the atria. The job of heart is to pump blood around our body. All the atria and ventricles of the heart contract and relax at appropriate times and make the heart behave like a pump. A sheath of tissue called pericardium protects the muscular heart.

Working of Human blood circulatory system takes place in the steps below:

(i) When the muscles of all the four chambers are relaxed, the pulmonary vein brings the oxygenated blood from the lungs in the left atrium of the heart.

(ii) When the left atrium contracts, the oxygenated blood is pushed into the left ventricle through valve V1.

(iii) When the left ventricle contracts, the oxygenated blood enters the main artery called aorta from which it goes to the different body organs through small branches called arterioles and capillaries.

(iv) The main artery carries the blood to all the organs of the body head, arms etc except the lungs. The oxygenated blood gives off oxygen, digested food and dissolved materials to the body cells. The carbon dioxide produced in the cells enters the blood. The deoxygenated blood enters main vein called vena cava which carried it to the right atrium of the heart.

(v) When the right atrium contracts, the deoxygenated blood enters right ventricle through valve V2.

(vi) When the right ventricle contracts, the deoxygenated blood enters the lungs through pulmonary artery and releases carbon dioxide and absorbs fresh oxygen from air. The blood becomes oxygenated again and is sent to the left atrium of heart by pulmonary vein for circulation in the body. This whole process is repeated continuously.

The fruiting bodies of fungi contain spores, which are dispersed for reproduction .Mushroom are a familiar example of a fruiting body .Their are formed from hyphae, the tiny threads that make up the bulk of most fungi. They can be:

Boletes & polypores

Boletes are mushroom-like, but with pores under the cap. Polypores vary from flat to mushroom-like and also have pores.

Stinkhorns, puffballs & birds nest fungi

Stinkhorns are smelly, puffballs are powdery and birds nest fungi are cup-like with "eggs" inside.

Cup fungi

The fruiting bodies are typically shaped like shallow cups or saucers

Coral & jelly fungi

Coral fungi are coral-like in shape, while jelly fungi are jelly-like to the touch.

Stereoid & paint (or skin) fungi

Stereoid fungi are mushroom-like to bracket-like, with a smooth underside. A paint (or skin) fungus looks like an extra skin growing on the surface of some wood.

Flask fungi

These produce their fruiting bodies in small chambers

Truffle-like fungi

Truffle-like fruiting bodies are generally out of sight, underground.

Inspired gas is the gas we take in during breathing which mostly contains oxygen in adequate amount whereas expired gas is the gas we breathe out which is mostly carbon dioxide. Inspired gas is mainly used for the oxidation of food for energy whereas expired gas is the bi product of oxidation and is toxic in nature. The air that we inspire is a mixture of gases. The most important of these are nitrogen, oxygen, carbon dioxide and water vapour. Dead space gas is not identical to inspired gas. It is warmed and humidified. Mixed expired air is somewhere in composition between alveolar gas and room air, so its oxygen level is higher than alveolar gas and its CO₂ lower.

Characteristics of Living things

• Living things are made up of a cell or cells.
• They obtain and use energy to survive.
• A unique ability to reproduce, ability to grow, ability to metabolize, ability to respond to stimuli, ability to adapt to the environment, ability to move and last but not the least an ability to respire.

A large variety of plants, animals, and microbes found on earth and they differ in size, shape, colour, habitat, and many other characteristics. So, studying each of them is impossible. Therefore, scientists have devised mechanisms to classify all living organisms on certain rules and principles that allow identification, nomenclature, and finally classification of an organism.
For example, based on presence of three pairs of legs and 2 pairs of wings, an organism is identified as an insect, it will be given a scientific name and then grouped with other similar organisms. Thus, various groups or taxon include organisms based on their similarity and differences.
Therefore, the biological classification helps in revealing the relationship between various organisms. It also helps in making study of organisms easy and organized.

• Fragmentation is seen in multi-cellular organisms with relatively simple body organisation. Here the body of the organism simply breaks up into smaller pieces upon maturation. These pieces or fragments grow into new individuals. Eg- spirogyra.
• Fragmentation is a form of asexual reproduction where an organism is split into fragments. Each of these fragments develop into mature, fully grown individuals that are a clone of the original organism. Fragmentation is caused by mitosis.
• Fragmentation is seen in many organisms such as molds, some annelid worms, and starfish.
• Fragmentation is observed in nonvascular plants as well, liverworts and mosses.
• As this process is a form of asexual reproduction, it does not produce genetic diversity in the offspring.

Key is a taxonomical aid that helps in identification of plant and animal species. These keys are based on similarities and dissimilarities in characters, generally in a pair called couplet. Each statement in a taxonomic key is referred to as a lead. It is also useful in identification of unknown organisms. All living things are organized into groups by scientists as they are identified. Different scientists use various systems of classification to organize all living things into groups. Overall, the reason scientists classify living things is to understand the relationships between different organisms. Key is a device used by biologists for identifying unknown organisms. Keys are constructed so that the user is presented with a series of choices about the characteristics of the unknown organisms; by making the correct choice at each step of the key, the user is ultimately led to the identity of a specimen.

Hydronephrosis is a condition that typically occurs when a kidney swells due to urine failing to properly drain from the kidney to the bladder. 

Water molecules possess kinetic energy. In liquid and gaseous form they are in random motion that is both rapid and constant.

The greater the concentration of water in a system, the greater is its kinetic energy or 'water potential'.

Hence, it is obvious that pure water will have the greatest water potential. If two systems containing water are in contact, random movement of water molecules will result in net movement of water molecules from the system with higher energy to the one with lower energy.

Thus water will move from the system containing water at higher water potential to the one having low water potential. This process of movement of substances down a gradient of free energy is called diffusion.

Water potential is denoted by the Greek symbol Psi or y and is expressed in pressure units such as pascals (Pa), By convention, the water potential of pure water at standard temperatures, which is not under any pressure, is taken to be zero.

Economic importance of algae: Algae are useful to man in a variety of ways. They perform half of the total carbon dioxide-fixation on earth by photosynthesis, acting as the primary producers in aquatic habitats. Many species of marine algae such as Porphyra, Sargassum, and Laminaria are edible. Chlorella and Spirulina are rich in proteins. Thus, they are used as food supplements. Agar is used in the preparation of jellies and ice-cream. It is obtained from Gelidium and Gracilaria. Carrageenin is used as an emulsifier in chocolates, paints, and toothpastes. It is obtained from the red algae. Many red algae such as Corallina are used in treating worm infections.

Economic importance of gymnosperms: Gymnospermous plants are widely used as ornamentals.  Many conifers such as pine, cedar, etc., are sources of the soft wood used in construction and packing. Medicinal uses: An anticancer drug Taxol is obtained from Taxus. Many species of Ephedraproduce ephedrine, which can be used in the treatment of asthma and bronchitis. The seeds of Pinus gerardiana are edible. Resins are used commercially for manufacturing sealing waxes and water-proof paints. A type of resin known as turpentine is obtained from various species of Pinus.

Taxonomy is the practice of identifying different organisms, classifying them into categories, and naming them. All organisms, both living and extinct, are classified into distinct groups with other similar organisms and given a scientific name. The classification of organisms has various hierarchical categories. Taxonomy based on the four aspects, namely Characterisation, Identification, Nomenclature, Classification.