its not about sun but about Mars (who controls blood formation) and moon (who controls flow of all liquids in body)
If you are keen on learning Medical astrology, be prepared to also learn to be very familiar with the Human body.
You may learn the function of blood by doing some googling such as
Wiki
http://en.wikipedia.org/wiki/Blood
or
A PDF of an university teaching handout about the Functions of Blood.
http://faculty.rcc.edu/ivey/2b/ppt_pdf/Blood,%20Ch14.pdf
Mars is iron but it doesn't form the blood itself but be part of the main ingredient of blood in the colour of Red. Mars as iron, being the only mineral that can bind oxygen in the form of Hemoglobin.
The primary function of hemoglobin (Hb) is to transport oxygen. Since oxygen is not very soluble in water (the major constituent of blood), an oxygen transport protein must be used to allow oxygen to be 'soluble'. Hemoglobin (Hb) is the oxygen transport protein used in the blood of vertebrates. It is composed of 4 polypeptide chains (represented in this diagram by different colors. Each chain contains one heme group (colored orange), each of which contains one iron ion... The iron is the site of oxygen binding; each iron can bind one O2 molecule thus each hemoglobin molecule is capable of binding a total to four (4) O2 molecules.
It is important that you remember that the purpose of Hb is to pickup oxygen at the lungs and to deliver it to the tissues. So Hb must be able to both bind and release oxygen and must be able to do these at the right places!
Hemoglobin and Oxygen Binding
http://www.austincc.edu/~emeyerth/hemoglob.htm
By nature, Mars is Hot and Dry, so as we are warm blooded, it assists along with Jupiter, Sun the other 2 Fire Planets to give our body warmth and alive. As blood, Mars distributes warmth through the mechanical contractions/releases of the cardiac muscle to pump/distribute nutrients and warmth to our body and cells.
Edited: Maritian energy is in our blood as the active defense system by being the active line of defense through bleeding flashing away the potential infective agent trying to attack our body through entering the cuts, wounds after broken through the Capricorn barrier Skin and Fatty tissues Jupiter/Cancer/Moon, then to Bones Capricorn again. Mars causes inflammation by supplying/ammulating large amount of blood cells with various capabilities to destroy, to isolate, to remove, to decode the DNA of the infective bacterial cells.
Read further with the Dummies:
http://www.dummies.com/how-to/content/what-are-the-components-of-human-blood.html
Biologically, astrologically, Sun/Leo is the Planet/Sign of the actual initiator of the physical delivery of blood and the
Yang side of the circulation system which the orders obviously mainly coming from the central nervous system.
While Jupiter as Liver being the vital organs for metabolic and many more functions which can be better described on this site.
The Liver: Anatomy and Functions
http://medicalcenter.osu.edu/patien...sease/liver_anatomy_function/Pages/index.aspx
A large part of the blood formation and recycling occurring in the Liver. It works as a Factory of producing majority of the ingredients of blood and regulates glucose, fat, filtering toxic chemical (along with kidneys, bowels, sweat, lungs).
Blood is warm, tasted salty and metallic.
Moon is bodily fluid but
it doesn't rule the Active flow and by nature that is Wet, Cold and Passive but similar to the Yang side of the Circulatory system that is ruled by Sun and the fire planets. Moon along with modern Neptune or Ying side of Jupiter as Pisces, give the saltiness to the fluid. The way fluid of the Moon/Pisces transported is not through Active pumping actions of any kind but Passive in the mirroring of the Sun (Heart). Heart failure or poor cardiovascular circulation will also directly cause sluggish lymphatic system.
Along with the more Ying side of Jupiter, in traditional as Pisces in modern as ruled by Neptune; Pisces, Cancer, Moon, rules the more subtle way of bodily fluid distribution, such as lymphatic system which also rules Immunity like Mars, being part of the blood but does not form the blood.
Pisces and Moon/Cancer rules the Ying side of the circulatory system. Their natures are Wet, Cold and Passive.
The primary function of the lymphatic system is to transport lymph, a clear, colorless fluid containing white blood cells that helps rid the body of toxins, waste and other unwanted materials.
Lymphatic comes from the Latin word lymphaticus, meaning "connected to water," as lymph is clear.
The following described the very nature of the passive, Cancer/Moon and Pisces like behaviors of the lymphatic system.
When bacteria are recognized in the lymph fluid, the lymph nodes make more infection-fighting white blood cells, which can cause swelling. The swollen nodes can sometimes be felt in the neck, underarms and groin.
Unlike blood, which flows throughout the body in a continue loop, lymph flows in only one direction — upward toward the neck — within its own system. It flows into the venous blood stream through the subclavien veins, which are located on either sides of the neck near the collarbones.
http://www.livescience.com/26983-lymphatic-system.html
http://en.wikipedia.org/wiki/Lymphatic_system
I think I had done enough to clear some points.