The human heart is the most important part of our body and its continuous functioning is what keeps our body alive and active, a minor injury caused internally or from outside of the body to this piece could stop its functioning and cause immediate death. It is a scientific fact that all the blood in the entire body consistently circulates through this one muscle, it keeps the entire body functioning, it is as we can say if the entire body was a state then the capital of the state is the heart. If the capital was suddenly taken over or destroyed, then all organs of the state would cease to function and result in collapse of the entire state.
Pulmonary Circulation in Islamic Medicine
This most important function of the heart that keeps all organs alive is the Pulmonary Circulation, which brings deoxygenated blood from the entire body into the right side of the heart and from there into the lung. The purified and oxygenated blood from the lung flows to left side of the heart which pushes blood out to the rest of the body. In the world of medical science the discovery of this function is largely attributed to William Harvey, a 17th century physician and his European predecessors after their investigation of Greek physician Galen’s observations of the hearts and blood circulation. This common notion eclipses the 400 year earlier finding of the circulation system by a 13th century Islamic Scholar, philosopher and physician named Ibn Nafis (Ala ad-Din Abu al Hasan Ali Ibn Abi-Hazm al Qarsh). Ibn Nafis had already studied Galen’s theory and concluded that movement of blood initiates from the right side of the heart then flows to the lungs, then circulates back to the left side. For the first time in the history of medicine, in addition to describing blood circulation, Ibn Nafis stated that blood is refined in the lungs. He also suggested that the spirit that gives life to the heart resides in it.
The Islamic scholars and scientists began their medical career by first deriving information from Greek, Persians, Indian, Byzantine sources. They then added to this body of knowledge and accomplished great achievements by their own observation, research and experience. This is how Ibn Nafis’s pulmonary circulation came to be. It was a result of correcting and improving shortcomings of Galen’s research. But what is most important to note that Islamic Scientists assimilated the Islamic medicine, primarily based on guidance from the Quran and Prophetic Sunnah (practices), with their science-based medicine. For instance, 9th century Islamic physician, also a heart specialist, Al Razi (Abu Bakr Muhammad ibn Zakariyya al-Razi – Rhazes) made historical contribution in the medicine field. He listed certain principles for the preservation of health which included moderation and balance in one dietary lifestyle, adopting an active physical lifestyle, avoiding sins and evils acts, and balancing one’s will power with one’s ability. These principles are all part of Islamic medicine. 
Importance of Healthy Heart
The amount of work that the heart does is incredible. At rest, a normal heart performs 75 beats/minute, pumping around 5.25 liters of blood, thus circulating the entire blood supply throughout the body once every minute. To appreciate this incredible role, it is enough to note that the brain cells could be irreversibly destroyed if they were deprived of the supply of oxygenated blood only for five minutes. So we can see – as Prophet Muhammad ﷺ told us – on this small muscle depends the life of the entire cells in the body: “Beware! There is a piece of flesh in the body if it becomes good (reformed) the whole body becomes good but if it gets spoilt the whole body gets spoilt and that is the heart.”
Islam makes important recommendations to keep our heart and circulatory system healthy, the first of which is physical activity. Ibn Al Qayyim, one of the most important Islamic thinkers in history and major proponents of prophetic medicine in 14th century, said that any organ used regularly in a type of physical activity grow stronger and healthier. According to American Heart Association, one of the most important recommendations to enjoy healthy heart is to keep physically active. Even 30 minutes of daily physical activity/aerobic exercise will benefit the heart: reduces risk of heart disease by improving blood circulation, keeping weight under control, boosting energy and releasing tension. Physical exercise also reduces blood cholesterol levels and controls hypertension, strengthening body muscles including the heart. The salaah (5 times prayers) also allows stretching of muscles which enhances blood circulation through gently massaging blood vessels involved. During prostration alleviates pain of varicose veins and reduce the risk of thrombosis (formation of blood clots) in the legs. The bowing and prostrating help the pumping of blood by the heart. The movements of neck further vitalize the blood vessels that deliver blood to the brain. So, prayer not only revives spirit, but also energizes body and mind. In addition to exercise, Islamic medicine makes certain dietary recommendations proven healthy for heart such as tender meat fish (Al Quran 16:14), pomegranates (Al Quran 55:68), grapes and many others Just as the heart is beneficial for the entire body, the Prophet ﷺ also emphasized a healthy believer was good for the environment: “A believer who is strong (and healthy) is better and dearer to Allah than the weak believer, but there is goodness in both of them.”
How Emotions & Actions Impact Hearts
No one can deny the emotional, spiritual aspects attributed to the heart. Stress, depression, worry, anger and many other negative feelings burden the heart and weaken its performance. Ibn al Qayyim, in his famous work on prophetic medicine Al-Tibb al-Nabawi, recommended practicing love, joy, kindness, compassion, patient, perseverance and courage as best type of sports for the heart. He also recommended ethical and moral behaviors and righteous acts as great healers. Such morals and ethics are remedies for the spiritual diseases of the heart and spiritually strong hearts help defeat physical illnesses. This is precisely one of the main purposes for which the Quran was revealed and the Prophet ﷺ was given to us as an example to follow. One of the Divine Names of Allah ﷻ is Al Shafi (The Healer) and He says in the Quran: (Al Quran 10:57) “O mankind! There has come to you a good advice from your Lord (i.e. the Quran, enjoining all that is good and forbidding all that is evil), and a healing for that (disease of ignorance, doubt, hypocrisy and differences) which is in your chests (i.e. hearts), - a guidance and a mercy (explaining lawful and unlawful things) for the believers.”
Modern science considered the heart nothing more than a piece of flesh that pumps blood throughout the body and that learning involves primarily the nervous system, and secondarily, the immune system (that recognizes diseases it has defeated). But that is being updated especially with the consideration being given by doctors to the emotional wellbeing of their patients. A very interesting research done on patients that underwent heart transplant surgery revealed that after their surgery they exhibited personality changes of their donors! The evidence was gained during open-ended interviews with volunteer transplant recipients, recipient families or friends, and donor families or friends. Following surgery, the recipients reflected changes in personality and behavior of the donors including food, music, personal career preferences etc. A 17yr old African male donor’s heart was transplanted to an elderly white male. During interviews the recipient’s wife revealed how the recipient now loves classical music, something he never liked before. The donor’s family revealed that the black teenager did not like rap music, something that the white couple thought. In fact, he loved classical music and died in a shooting holding a violin while heading to his music class. This reveals for us a lot about the heart that what the Quran and prophetic hadith teach that our lifestyles can leave their positive or negative imprints on our hearts. This is also a wakeup call for anyone planning on getting heart transplant surgery. The recipient may unknowingly adopt the donor’s lifestyle!
Securing Our Hearts from Temptations (Fitnah)
The Quran and Prophetic hadith teach that hearts are influenced and shaped (become good or bad) by one’s actions and intentions. The Prophet ﷺ warned us that in the last age (i.e. the age before Judgment day) the hearts will be exposed to trials and temptations (fitnah) and become tainted by them: “Temptations will be presented to men's hearts as reed mat is woven stick by stick (الْحَصِيرِ عُودًا عُودًا) and any heart which is impregnated by them will have a black mark put into it, but any heart which rejects them will have a white mark put in it. The result is that there will become two types of hearts: one white like a white stone which will not be harmed by any temptation, so long as the heavens and the earth endure; and the other black and dust-coloured like a vessel which is upset, not recognizing what is good or rejecting what is abominable, but being impregnated with passion.” حَصِيرٌ generally translated in English as “a mat woven of reeds” also means “captivates hearts in a manner peculiar to it” Leading Scholar Sheikh Hamza Yusuf interprets the Arabic wording حَصِيرٌ to a TV set and claims that “reed mat is woven stick by stick” is a reference to the vertical/horizontal grid lines placed on each other like mats, part of manufactured TV screen. It is these screens that captivate the hearts by placing negative impressions through time waste entertainment, disinformative news, immoral music, airing of unethical divisive politics, violent movies, video game addiction, etc. Whoever consistently exposes their hearts to such temptations will fall under their influence. They will gradually lose their sense of right and wrong and become like animals following their passions (basic instincts), led towards harming themselves emotionally and physically. Allah ﷻ condemns such hearts that allow themselves to blindly fall for the temptations: (Al Quran 5:41-42).…for, if God wills anyone to be tempted to evil (Fitnah), you can in no wise prevail with God in his behalf. It is they whose hearts God is not willing to cleanse. Theirs shall be ignominy in this world, and awesome suffering in the life to come - those who eagerly listen to any falsehood, greedily swallowing all that is evil!” (Asad Translation)- The Quran teaches that a group of Jewish people were transformed into apes because they were tempted towards disobedience of Allah ﷻ and blindly followed their passion: (Al Quran 2:65-66) "Be apes despised and hated by all.” Some of the Prophet’s companions commented that this was an inner transformation of their hearts and not their bodies.
A very important Surah that Prophet ﷺ advised for the Muslims to recite is Surah Al Kahf (chapter 18), especially in the last age. It teaches about ashab e kahf (young men of the cave) who used their reason and made efforts to protect themselves from the environment of kufr (disobedience & disbelief) as a result of which Allah ﷻ increased their guidance and strengthened their hearts: (Al Quran 18:13-14) "They were young men who believed in their Lord, and We gave them more and more guidance. We gave strength to their hearts…”
So, our intentions, actions, our emotions, all play a part in shaping our hearts in one way or another and they all are important for the improvement of ourselves. The Islamic science of the heart is very important to understand which helps us to make use of the Islamic medicine, the key for the physical, emotional and spiritual well-being of the hearts. The heart that succeeds on all three levels will succeed in this world and the hereafter: (Al Quran 26:89)…[and when] only he [will be happy] who comes before God with a heart free of evil!"
 Correcting the circulation myth, Lateral Magazine, Dana Crow, August 9, 2017 Philosophy & History, Issue 24 - http://www.lateralmag.com/articles/issue-24/forgotten-discovery-of-circulation
 Changizi Ashtiyani, Saeed. (2013). The Discoverer of Pulmonary Blood Circulation: Ibn Nafis or William Harvey. Middle East Journal of Scientific Research. 18. 562-568.. 10.5829/idosi.mejsr.2013.18.5.75133 - https://www.researchgate.net/publication/277005637_The_Discoverer_of_Pulmonary_Blood_Circulation_Ibn_Nafis_or_William_Harvey
 Ayad, Amira, and Jamila Hakam. Healing Body & Soul: Your Guide to Holistic Wellbeing following Islamic Teachings. International Islamic Publishing House (Saudi Arabia) pgs. 24
 ibid pg 57
 Sahih al-Bukhari 52 – Sunnah.com
 Ayad, Amira. Healing Body & Soul pg. 58-59
 Sahih Muslim 2664 – Sunnah.com
 Changes in Heart Transplant Recipients That Parallel the Personalities of Their Donors, Paul Pearsall, Ph.D. University of Hawaii, Gary E. R. Schwartz, Ph.D., Linda G. S. Russek, Ph.D., University of Arizona – UNT Digital Library, https://digital.library.unt.edu/ark:/67531/metadc799207/m2/1/high_res_d/vol20-no3-191.pdf
 Sahih Muslim 144 a – Sunnah.com
 An Arabic-English Lexicon by Edward William Lane, pg. 0583: Root حصر
 Shaykh Hamza Yusuf Satellite Television Prophecy, Youtube.com, Jul 26, 2016 - https://www.youtube.com/watch?v=1rFCvOe4OEk