Human Biology: Respiration, Microorganisms, and Nutrition

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Gas Exchange

Blood is circulated to the lungs to obtain oxygen. This happens in the alveoli. Air entering the lungs has a higher concentration of oxygen and a lower concentration of carbon dioxide than the air that the lungs release. The blood exiting the lungs has a higher oxygen concentration and a lower carbon dioxide concentration than when it enters. The amount of nitrogen remains constant. Oxygen passes through the alveolar wall. These gases traverse the membrane of the cells by diffusion.

What Happens Inside the Alveoli?

The total surface area of the lungs' alveoli is enormous. Each alveolus is surrounded by many tiny blood vessels called capillaries, and the wall is extremely thin.

Microorganisms in the Air

Upper respiratory illnesses like the common cold and influenza, and lower respiratory infections like bronchitis and pneumonia can be caused by microorganisms in the air.

The Dangers of Tobacco Smoke

Chronic bronchitis, the secretion of excess mucus that can obstruct the respiratory passageway, and lung cancer are all dangers of tobacco smoke.

The Composition of Food

The human diet consists of an immense variety of possible foods from a diverse array of sources. The chemical composition of foods, like milk and bread, are composed of the same basic molecules.

Inorganic Substances: Mineral Salts

  • Carbonate and phosphate forms, found in our bones and teeth. They provide structural strength.
  • Found dissolved in all our body fluids.
  • Are quite small.


  • The most abundant chemical compound.
  • We are constantly losing water and must replenish what is lost.
  • We obtain water through the water we drink and in the food we eat.

Organic Substances

Food contains many organic molecules, which are those found in, or produced by, living organisms, but never in abiotic substances.


  • Simple carbohydrates taste sweet and are known as sugars. Glucose is a simple sugar or monosaccharide.
  • Complex carbohydrates, like starch, glycogen, and cellulose, do not taste sweet. They are considered macromolecules, a product of assembling or connecting smaller molecules.


  • Includes fats and cholesterol.
  • Can be broken down into simpler molecules: glycerol and fatty acids.
  • Unsaturated fats: Usually of plant origin, liquid at room temperature, referred to as oil.
  • Saturated fats: Usually of animal origin, solid at room temperature, like butter and lard.


  • Macromolecules formed by hundreds or even thousands of smaller molecules called amino acids.
  • There are twenty different types of amino acids that fit and healthy humans need.
  • Proteins differ in the number, type, and sequence of amino acids.


  • Our bodies need very small quantities of vitamins, but they are essential.
  • As most vitamins cannot be synthesized by our bodies, we have to obtain them from our diet.

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