Cardiovascular System

The heart and the circulatory system together form the cardiovascular system.
The heart is situated in the chest cavity and surrounded by a fluid filled sac called pericardium.
The heart is the organ that helps supply blood and oxygen to all parts of the body. It is separated by a septum into 2 halves, which are in turn divided into atria and ventricles. Therefore, the heart is mainly formed of 4 chambers: 2 atria (upper 2 chambers) and 2 ventricles (lower 2 chambers).
The heart wall is formed of:
- epicardium - the outer layer
- myocardium - the middle layer
                                                                                                        - endocardium - the inner layer
One way flow through the heart is ensured by 2 sets of valves (4 valves):

- aortic valve: impedes blood flowing back into the ventricle, as it is pumped from the left ventricle into the aorta
- pulmonary semilunar valve:  prevents back flow of blood, as it is pumped from the right ventricle to the pulmonary artery
- tricuspid valve(3 cusps): prevents back flow of blood into the right atrium, as it flows from the right atrium to the right ventricle.
- mitral valve (bicuspid valve): prevents back flow of blood into the left atrium, as it is pumped from the left atrium to the left ventricle.

Cardiac output (CO) is the amount of blood pumped by the heart per unit time, measured in liters per minute. In a normal adult heart, the cardiac output is 4.9 L/min. The cardiac output is a function of heart rate and stroke volume.
Stroke volume (SV) is the amount of blood pumped by the left ventricle of the heart in one contraction. Stroke volume is about 70 mls in a normal adult.   
Heart rate (HR) is the number of heartbeats per unit of time and is 70 bpm in a normal adult.
CO = HR x SV

Reflex control of heart rate: control center in medulla oblongata.
- sympathetic neurons (like norepinephrine) and parasympathetic neurons (like acetylcholine)
Sympathetic neurons increase the heart rate (sympathetic system - "fight or flight").
Parasympathetic neurons decrease the heart rate (parasympathetic system - "rest and digest").

Blood pressure (BP) is the pressure exerted by circulating blood upon the walls of blood vessels. During each heart beat, BP varies between a maximum (systolic) and a minimum (diastolic) pressure.
BP = CO x Total Peripheral Resistance.
Total Peripheral Resistance is the sum of the resistance of all peripheral vasculature in the systemic circulation.























http://biology.about.com/od/humananatomybiology/ss/heart_anatomy.htm
http://www.medicinenet.com/stroke/article.htm

Myocardial Infarction (Heart Attack)

Heart and circulatory diseases are the leading causes of death for men and women in the UK. Heart disease includes conditions such as coronary heart disease, heart attack and congestive heart failure.

Myocardial Infarction (MI) is commonly known as a heart attack. It means interruption of blood supply to a portion of the heart, causing heart cells to die.

An acute myocardial infarction begins with the occlusion of a coronary artery. The region of myocardium that is vascularized by that coronary artery is deprived of oxygen and, unless blood flow resumes shortly, it will die. 

Complete, but transient blockade of blood flow to the myocardium, even though it does not lead to cell death, may lead to a patern of ECG changes similar to that seen during the acute phase of myocardium infarction. 

The areas of infarction are regional, because blood flow is regional. 

The heart muscle requires a constant supply of oxygen-rich blood to nourish it. The coronary arteries provide the heart with this critical blood supply. That is the reason why in coronary artery disease, those arteries become narrow and blood cannot flow as well as they should, which can finally cause a heart attack. Fatty matter, calcium, proteins, and inflammatory cells build up within the arteries to form plaques of different sizes. The plaque deposits are hard on the outside and soft inside.                                                                   

When the plaque is hard, the outer shell cracks (plaque rupture), platelets (disc-shaped particles in the blood that aid clotting) come to the area, and blood clots form around the plaque. If a blood clot totally blocks the artery, the heart muscle becomes ‘starved’ for oxygen. Within a short time, death of heart muscle cells occurs, causing permanent damage. This is a heart attack.

While it is unusual, a heart attack can also be caused by a spasm of a coronary artery. During a coronary spasm, the coronary arteries restrict or spasm on and off, reducing blood supply to the heart muscle (ischemia). It may occur at rest and can even occur in people without significant coronary artery disease. Ischemia without cell death due to a "fixed" degree of occlusion is often associated with changes in the ECG, typically ST segment and T-wave changes.

Each coronary artery supplies blood to a region of heart muscle. The amount of damage to the heart muscle depends on the size of the area supplied by the blocked artery and the time between injury and treatment.

Heart Attack 

Healing of the heart muscle begins soon after a heart attack and takes about eight weeks. Just like a skin wound, the heart's wound heals and a scar forms in the damaged area. But, the new scar tissue does not contract or pump as well as healthy heart muscle tissue. So, the heart's pumping ability is lessened after a heart attack. The amount of lost pumping ability depends on the size and location of the scar.

References: "Medical Physiology. A cellular and Molecular Approach", Walter F. Boron, Emile L. Boulpaep
                   http://www.webmd.boots.com/heart-disease/default.htm
                   http://www.webmd.boots.com/heart-disease/guide/heart-disease-heart-attacks