IGCSE Biology Section B (Animal Physiology)

Section B IGCSE Biology notes :)

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Gas Exchange

The gas exchange system takes air into and out of the body to supply oxygen and remove carbon dioxide.]

The movement of air in and out of the lungs is brought about by breathing movements of the ribs and diaphragm. The air movement maintains a steep concentration gradient for diffusion of oxygen and carbon dioxide between blood and air in the lungs.

Gas exchange takes place in the lungs and depends on efficient diffusion of the gases. (Alveoli)

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Gas Exchange 2

The gas exchange system is also referred to as the respiratory system, but this can be confusing as respiration takes place in all cells, and is quite distinct from gas exchange. The actual gas exchange surface is on the alveoli inside the lungs. This surface meets the three requirements of Fick’s law:

A large surface area. Although each alveolus is tiny, an average adult has about 600 million alveoli, giving a total surface area of about 100m², so the area is huge.

A small distance between the source and the destination. The walls of the alveoli are composed of a single layer of flattened epithelial cells, as are the walls of the capillaries, so gases need to diffuse through just two thin cells.

A mechanism to maintain a high concentration gradient across the gas exchange surface. The steep concentration gradient across the gas exchange surface is maintained in two ways: by blood flow on one side and ventilation on the other side. This means oxygen can always diffuse down its concentration gradient from the air to the blood, while at the same time carbon dioxide can diffuse down its concentration gradient from the blood to the air.

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Ventilating the lungs

Air is moved in and out of the lungs by movement of ribs and diaphragm. The ventilation movement of the ribs and diaphragm bring about changes in the volume and therefore the pressure of the chest cavity.


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In the alveoli

The lungs contain alveoli. They are adapted for efficient gas exchange. Blood from heart to the lungs passes through the network of capillaries surrounding the alveoli. Carbon dioxide diffuses from the blood into the air in the alveoli. Oxygen diffuses into the blood from the alveoli.

Alveoli has:

  • a large surface area
  • a rich blood supply
  • short diffusion distances
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Nicotine is an addictive drug found in tobacco. It is addiction to nicotine which makes smoking hard to give up. Carbon MONOXIDE reduces the amount of oxygen carried in the blood. Many diseases are caused by smoking.

Tobacco also has the effects of:

  • Cilia are destroyed so dirt and bacteria cannot be removed
  • The walls of the alveoli are damaged and break down to form large irregular air spaces so gas exchange is no longer efficient
  • Carbon MONOXIDE binds haemoglobin
  • Increased risk of heart attacks
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Food and digestion

Food is broken down by enzymes.

  • Bile is released to neutralise stomach acids and emulsify fats
  • The villi of the small intestine increase SA for the absorption of digest food molecules

Carbohydrates are sugars and starches, made of carbon, hydrogen and oxygen. They provide energy for the cells. Broken down by carbohydrase.

Proteins are long chains of amino acids, they also have carbon, hydrogen and oxygen but as well as this they contain nitrogen or sulfur atoms. Broken down by protease.

Lipids are made up of fatty acids and glycerol joined together. They contain a lot of energy. They are broken down by lipase.

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Minerals and vitamins

A- makes chemicals in retina, protects surface of eye and connective tissue.

C- sticks together cells lining surfaces in the body

D- helps bones absorb calcium and phosphorus

Calcium- makes bones and teeth

Iron- makes haemoglobin in red blood cells

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The mouth: Food is partly broken down by the process of chewing and by the chemical action of salivary enzymes (these enzymes are produced by the salivary glands and break down starches into glucose). (Carbohydrase and amylase)

On the way to the stomach:  After being chewed and swallowed, the food enters the esophagus. The esophagus is a long tube that runs from the mouth to the stomach. It uses rhythmic, wave-like muscle movements (called peristalsis) to force food from the throat into the stomach.

In the stomach - The stomach is a large, sack-like organ that churns the food and bathes it in a very strong acid (gastric acid). Food in the stomach that is partly digested and mixed with stomach acids is called chyme.

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How? Part 2

In the small intestine - After being in the stomach, food enters the duodenum, the first part of the small intestine. It then enters the jejunum and then the ileum (the final part of the small intestine). In the small intestine, bile (produced in the liver and stored in the gall bladder), pancreatic enzymes, and other digestive enzymes produced by the inner wall of the small intestine help in the breakdown of food.

In the large intestine - After passing through the small intestine, food passes into the large intestine. In the large intestine, some of the water and electrolytes (chemicals like sodium) are removed from the food. Many microbes  in the large intestine help in the digestion process. The first part of the large intestine is called the cecum (the appendix is connected to the cecum). Food then travels upward in the ascending colon. The food travels across the abdomen in the transverse colon, goes back down the other side of the body in the descending colon, and then through the sigmoid colon.

The end of the process - Solid waste is then stored in the rectum until it is excreted via the ****.

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Carbohydrase, (amylase and maltase)- found in salivary glands, pancreas and small intestine to act on starch creating glucose.

Protease (pepsin) - found in stomach pancreas and small intestine to break down protein into amino acids.

Lipase- found in pancreas and small intestine to break down fats and creating fatty acids and glycerol.

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Blood and circulation

All organisms need to transport substance around the body for important life processes. Small organisms transport substances around the body by diffusion.
Diffusion would take too long in large organisms, therefore, a heart and circulation is needed.
There are 4 different types of circulatory systems-Open, Closed, Single and Double.
In an open circulatory system blood travels in large open spaces. A simple heart pumps blood to the cavities surrounding the organs. Diffusion takes place. The heart relaxes. Blood is drawn back to the heart.
In a closed circulatory system, blood travels in narrow channels=High blood pressure. However, this is more efficient and fast.
Fishes have single circulatory systems as the heart has 2 chambers.
Birds and mammals have a double circulatory system as the heart has 4 chambers

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Blood vessels

Carry blood AWAY from the heart, Have a thick wall to withstand pressure,Have thick elastic fibres to stretch and recoil, Have a small lumen, No valves,Carry oxygenated blood except the pulmonary artery!

Carry blood to the heart, Have thin walls, Are less elastic than arteries, Large lumen=low blood pressure, Carry deoxygenated blood except the pulmonary vein,Have valves to prevent backflow!.

From arteries to veins through tissues.They have close contact with cells which allows gas exchange. Thin wall-single layer of cells,No valves

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The human heart has four chambers: two ventricles, each of which is a muscular chamber that squeezes blood out of the heart and into the blood vessels, and two atria, each of which is a muscular chamber that drains and then squeezes blood into the ventricles. The two atria reside at the top of the heart; the two ventricles are at the bottom. And, the heart is divided into left and right halves, so there is a left atrium and left ventricle, as well as a right atrium and right ventricle.

The reason that the heart is divided into halves is because of the two-circuit circulatory system. The right side of the heart can pump blood to the lungs, while the left side of the heart pumps blood to the rest of the body. Blood goes in both directions on each and every pump.

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How blood flows

Every minute of your life, your heart pumps about 70 times. Every minute of your life, your heart pumps the entire amount of blood that is in the body — 5 liters, which is equivalent to 2-1/2 big bottles of soda. The heart never stops working from the time that it starts to beat when humans are nothing but wee little embryos in their mother’s wombs until the moment they die.

The 8/10th of a second that a heart beats is called the cardiac cycle. During that 0.8-second period, the heart forces blood into the blood vessels plus it takes a quick nap. Here’s what happens in those 0.8 seconds:

  • The left and right atria contract.

  • The left and right ventricles contract.

  • The atria and ventricles rest.

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Blood pressure

Blood pressure is measured in millimetres of mercury, mmHg. There are two measurements:

  • systolic pressure - the higher measurement when the heart beats, pushing blood through the arteries, and
  • diastolic pressure - the lower measurement when the heart rests between beats

A young, fit person may have a blood pressure of about 120 over 70, which means their systolic pressure is 120 mmHg, and their diastolic pressure 70 mmHg.

Blood pressure varies with age. It also varies with lifestyle factors such as:

  • diet
  • stress
  • exercise
  • body mass
  • alcohol consumption
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The eye



Front part of the tough outer coat, the sclera. It is convex and transparent.

refracts light - bends it as it enters the eye


Pigmented - decides the colour of your eyes - so light cannot pass through. Its muscles contract and relax to alter the size of its central hole or pupil.

controls how much light enters the pupil Lens

Transparent, bi-convex, flexible disc behind the iris attached by the suspensory ligaments to the ciliary muscles.

focuses light onto the retina Retina The lining of the back of eye containing two types of photoreceptor cells - rods - sensitive to dim light and black and white, cones - sensitive to colour. A small area called the fovea in the middle of the retina has many more cones than rods. contains the light receptors Optic nerve Bundle of sensory neurones at back of eye. carries impulses from the eye to the brain

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Neurones are nerve cells. They carry information as tiny electrical signals. There are three different types of neurones, each with a slightly different function:

  • sensory neurones carry signals from receptors to the spinal cord and brain
  • relay neurones carry messages from one part of the CNS to another
  • motor neurones carry signals from the CNS to effectors.

Where two neurones meet, there is a tiny gap called a synapse. Signals cross this gap using chemicals. One neurone releases the chemical into the gap. The chemical diffuses across the gap and makes the next neurone transmit an electrical signal.


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In a simple reflex action:

stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector

An effector is any part of the body that produces the response. Here are some examples of effectors:

  • a muscle contracting
  • a gland releasing (secreting) a hormone or other chemical.

Reflex actions are rapid and happen without us thinking. For example, you would pull your hand away from a flame without thinking about it. The animation below allows you to step through each stage of the reflex arc.

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The kidneys


To remove waste and regulate water levels

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Regulating water levels


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Many non-living substances such as fluids, hormones, and pheromones are also important accessories to the reproductive system. Unlike most organ systems, the sexes of differientiated species often have significant differences. These differences allow for a combination of genetic material between two individuals, which allows for the possibility of greater genitic fitness of the offspring.

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So useful, thanks for sharing!


Really want to print this one but it keeps saying that the file is damaged and cannot be repaired.. ??

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