By trust mambemba

Sorry if this is too complex, but it give you the idea.
Pacemaker cells in the sinoatrial node, located over the right atrium, initiate the cardiac cycle. Systole begins when excitation spreads over both atria, activating atrial contraction. Excitation spreads to the atrioventricular node, near the atrioventricular border, from which excitation is conducted by the bundle of His and the Purkinje system (groups of specialized muscle cells) to the bottom of the ventricles. Excitation spreads upward in the ventricles, contracting them from the bottom up, like squeezing a toothpaste tube from the bottom. When the action potentials end, diastole (relaxation) begins, until excitation is again initiated by the pacemaker cells of the sinoatrial node.

Tabitha Farrant

You can find the following on the Biology A level Revision Summary pages
The plasma makes up most of the blood. It is mainly water but carries lots of other essential ingredients.
The following substances are carried in the plasma:
Dissolved carbon dioxide: This is the waste gas produced by respiration in cells
Dissolved glucose and amino acids: Food molecules for respiration, building and repairing cells
Urea: Waste product of digestion, this is lost from the kidney.
Antibodies and antitoxins: Chemicals that protect us from disease and poisons
Hormones: Chemicals that control some of our body functions
Plasma has a yellowish appearance. It sometimes oozes out of blisters. Nice!
BLOOD is a fluid tissue which provides a link between all the other tissues and organs of the body.
It has the capacity to carry essential items around the body.
These are:
Oxygen from lungs to body cells
Carbon dioxide from body to lungs
Waste products and water from cells to kidneys
White blood cells to sites of infection
Platelets to damaged capillary areas
Glucose and nutrients from digestive system to cells
Hormones from glands to where they are needed
Heat from warmer to cooler parts of the body

Tabitha Farrant
When do you normally start revising for exams?
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I revise all year
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