Thinking between sentences A significant amount of the time we spend writing we actually spend re-reading what we have already written and thinking about what to write next…

(I want to directly answer the question. The first sentence will be my topic sentence.) Carbon dioxide affects the physiology of organisms; in humans carbon dioxide can affect pulmonary ventilation and the mechanism of breathing. (20)

Carbon dioxide affects the physiology of organisms; in humans carbon dioxide can affect pulmonary ventilation and the mechanism of breathing. (I need to be more detailed with a specific description of what carbon dioxide does to human respiration.) Carbon dioxide is a waste product of respiration; it enters the blood when a respiratory organism is exercising or is anxious. (20)

Carbon dioxide is a waste product of respiration; it enters the blood when a respiratory organism is exercising or is anxious. (Now I need to develop the point, linking this to chemoreceptors in the blood.) A build up of carbon dioxide in the blood will be detected by chemoreceptors which will trigger the brain to make the heart beat faster. (25)

A build up of carbon dioxide in the blood will be detected by chemoreceptors which will trigger the brain to make the heart beat faster. (I need to finish this paragraph with a bit about the effect of the pulse rate increase.) The quickening pulse rate increases the rate of oxygen in-take and carbon dioxide expiration, ensuring the blood remains oxygenated. (20)

The quickening pulse rate increases the rate of oxygen in-take and carbon dioxide expiration, ensuring the blood remains oxygenated. (Next paragraph – I need to look back to the last paragraph and forward to the next.) Carbon dioxide warns the brain that the respiratory organism is exerting itself and requires more oxygen; in plants it is a key ingredient for photosynthesis. (25)

Carbon dioxide affects the physiology of organisms; in humans carbon dioxide can affect pulmonary ventilation and the mechanism of breathing. Carbon dioxide is a waste product of respiration; it enters the blood when a respiratory organism is exercising or is anxious. A build up of carbon dioxide in the blood will be detected by chemoreceptors which will trigger the brain to make the heart rate beat faster. The quickening pulse rate increases the rate of oxygen in-take and carbon dioxide expiration, ensuring the blood remains oxygenated. (85) Carbon dioxide warns the brain that the respiratory organism is exerting itself and requires more oxygen; in plants it is a key ingredient for photosynthesis. (25)

Other thoughts… We think about the next sentence whilst we are writing, especially under time pressure; Write in singles not doubles; Write in positives; Use the right word, not a high-sounding one; Write in shorter rather than longer sentences; Twentyish words per sentence maximum; Four sentences per paragraph; Eight to ten paragraphs (or thirty-forty sentences) make an essay.