The Heart

 Diagram of Heartheart

The Structure of the Heart – Layers and Functions

The protective outer covering is called the Pericardium, which is two layers of connective tissue – one attaching to the major blood vessels above it and the other epicardium enveloping the heart itself.


In between the two layers is the pericardial space, full of lubricating serous fluid, which reduces friction.

The heart is a muscular organ comprised of three layers:

  1. Epicardium (inner layer of pericardium) provides an outer protective layer to the heart.
  2. Myocardium – middle, bulky layer of cardiac muscle.  Cardiac muscle cells are specialised so that they can continue to contract without becoming fatigued.  Many cardiac muscles are electrically coupled into a ‘syncytium’ therefore they can pass an electrical potential rapidly along the ehart wall and ‘in synch’.  The myocardium is responsible for the contractions of the heart.
  3. The Endocardium – is a lining of endothelium – a membranous tissue that lines all blood vessels including the heart. It’s made up of, mainly, epithelial cells and connective tissue and is also where the PERKINJE FIBERS are located.  It helps to regulate the contractions of the heart.

The heart separates its two pumps into two disctinct chambers.

The two upper chambers where blood is received from the veins are called the Atria (atrium).  The two lower chambers are called ventricles and depending on what side of the heart they are decides whether they are right or left atrium or ventricle.

The right and left chambers are separated by the septum, an extension of the heart wall.

A special set of valves separates the upper and lower chambers called cuspid valves or ‘lock gate’ valves.  The right atrioventricular valve is called the tricuspid valve as it has three flaps and the left atrioventricular valve is called the mitral or bicuspid valve as it has two.

Another set of valves – the semi lunar valves (half moon shaped) are located in the ventricles and regulate the flow to and from the aorta and the pulmonary artery.

Venous de-oxygenated blood enters from the right atrium into the right ventricle through the tricuspid valve.  At the same time oxygenated blood that has just returned from the lungs is re-entering from the left atrium into the left ventricle through the bicuspid valve.  The heart is at rest (no noise through a stethoscope).

Coronary arteries branch off from the ascending aorta here.  The heart itself receives the freshest, most oxygenated blood directly from the lungs.  It is squeezed out of the left ventricle up the ascending aorta.

It is the coronary arteries that can become clogged by fatty deposits or blood clots in coronary thrombosis or embolisms, causing the cardiac muscle cells to become ischemic, or deprived of oxygen as a result of decreased blood supply to the tissues.   This leads to the heart’s metabolic functioning becoming impaired and possible tissue death.  Myocardial infarction may be the result, with severe damage to the myocardial tissue and a heart attack is said to have occurred.

Cardiac veins somewhat mirror these coronary arteries and return the used up de-oxygenated blood directly back to the right atrium of the heart via the coronary sinus.


Blood Pressure

Maintaining appropriate blood pressure is so important to sustaining the life of every cell in the body.  This pressure is constantly being checked and measured by various systems of the body and several mechanisms are in place to control and regulate blood volume.

All of these mechanisms have an impact on water.  If blood volume drops then more water needs to move into the blood plasma to increase the blood volume and therefore the pressure.  Also vice versa.

The control of blood pressure takes place between the heart, kidneys, central nervous system and endocrine system.

The hypothalamus in the brain stimulates the posterior pituitary gland to secrete ADH; antidiuretic hormone – this has the effect of raising blood pressure by instructing the kidneys to reabsorb more water from the urine before it is excreted.

The kidneys release renin when they sense that blood pressure is too low; which leads to the secretion of Aldosterone (from the adrenals).  Aldosterone stimulates sodium to be retained and as water follows the sodium it causes water to be retained.  As water follows sodium water will also be retained, so having the effect of increasing the blood volume and raising blood pressure.

If blood pressure is too high and the heart carries too much blood; ANH (Anti Natriuretic Hormone) is triggered by the action of the heart walls being ‘overstretched‘.  This hormone promotes the loss of water from the blood plasma, decreases blood volume therefore lowering blood pressure.  This is achieved by telling the kidneys to excrete more sodium and therefore water.   It also suppresses thirst and dilates blood vessels.

There are more mechanisms to conserve water than to excrete it.

Cardiac Cycle

Atrial Systole: Atria contract, blood flows into ventricles, AV valves are open.

Isovolumetric Ventricular Contraction – pressure increases inside the ventricles because they are contracting.  !st sound of heartbeat.

Ejection SL valves open, when the pressure is greater in ventricles than arteries – not all blood is eliminated

Isolvolumetric ventricular relaxation – a very brief period between the SL valves closing and the opening of he AV valves.  The pressure is now much less in the ventricles and the AV will not reopen until the pressure in the atrial chambers is greater than in the relaxed ventricles.  2nd sound of heartbeat.

Passive ventricular filling – The veins return blood back to the atria, this increases pressure to a point that AV valves are forced open and can flush down to relaxing ventricles.

Main Blood Vessels

The Artery

Has three layers:

  • tunica adventitiatough connective tissue
  • tunica mediathick muscular layer
  • tunica intimaendothelial lining that becomes thinner as the vessels get smaller.  there is a thin insulating layer of fatty tissue.

Arteries are distributors.    Arteries and arterioles can contract or dilate – sending blood flow inwards to the core and vital organs or vice versa.

Arteries are under pressure from the heart so need to resist the force of the blood from blowing them up like balloons.

Arteries have a large internal diameter and thus offer little resistance to the flow of blood.

Arteries also have a smooth muscular layer that functions to regulate the flow of blood through the artery. Contraction of the smooth muscle decreases the internal diameter of the vessel in a process called vasoconstriction. Relaxation of the smooth muscle increases the internal diameter in a process called vasodilation.

Arterioles  determine blood pressure and blood flow to the individual organs.  Arterioles have a considerably smaller diameter than arteries and so provide significant resistance to the flow of blood. This resistance creates pressure in the circulatory system. Pressure is required to provide adequate flow of blood to all parts of the body.  Vasodilation of an arteriole lowers resistance and results in an increase in flow through that particular arteriole. Vasoconstriction of an arteriole increases resistance and results in decreased flow through that particular arteriole.


These are the smallest and most numerous of blood vessels. Capillaries  offer the site of exchange of nutrients and wastes between blood and tissues. Capillary walls are composed of a single layer of epithelial cells surrounded by a basement layer of connective tissue. The thin nature of the walls facilitates efficient diffusion of oxygen and carbon dioxide. Most capillaries also have pores between cells that allow for bulk transport of fluid and dissolved substances from the blood into the tissues and visa versa.

Although capillaries are extremely numerous (40 billion in the body), collectively they hold only about 5% of the total blood volume at any one time. This is because most capillaries are closed most of the time. Precapillary sphincters, which are bands of smooth muscle that wrap around arterioles, control the amount of blood flowing in a particular capillary bed. Contraction of the sphincter shuts off blood flow to a capillary bed, while relaxation of the sphincter allows blood to flow.


Veins need to stretch as they pool the blood and can act as reservoirs, they are more elastic than arteriesThey are under much less pressure than arteries.

Some mechanisms are required to assist the pooling blood to travel against gravity back to the heart:

Skeletal Muscular Pump as physical activity increases major muscles contract, this exerts pressure on blood vessels and squeezes blood upwards, driving it back to the heart.

Thoracic Pressure ‘respiratory pump’ that pulls and pushes.

Series of ‘Lock Gate Valves‘ shunts blood onwards and as pressure lapses the gates close to prevent backflow.


Hepatic Circulation

In this system blood is filtered before it reaches the heart by means of a venous secondary capillary network.  Blood from the spleen, stomach, pancreas and gallbladder and intestines are filtered by the liver.

Blood sugar control and toxicity reduction are the main reasons for this detour.  There is a network of veins that surround the small intestine and colon, ever ready to absorb any nutrient that can freely pass through the intestinal mucosa.  They all lead ultimately to the hepatic portal vein which receives nutrients and waste from the many tributaries. The hepatic portal vein leads to the liver, the body’s chemical factory.  This whole network is encased within the protective mesentary.

The portal vein and its tributaries

This helps to explain why enemas and particularly coffee enemas work so effectively upon the liver.  Anything introduced to the rectum that is absorbable through the lumen will be transferred directly to the liver very very quickly where it can effect change.

Foetal Circulation

Before birth 6 important structures counteract the absence of pulmonary circulation and digestion in the foetus as well as providing an interface for the exchange of blood nutrients, gases and wastes between the foetus and mother.

The nutrients dissolved in the placental sac enter the body through the umbilical vein which forms, with the two umbilical arteries, the umbilical cord.

The mother’s blood and the foetus’s blood are kept separate as oxygen and nutrients are absorbed from the mother’s blood through the placenta.  The placenta acts as an interface.  Most of the blood from the foetus’s liver by means of the ductus venosus – which in turn empties into the inferior vena cava.

The foetus does not use its lungs and therefore does not need the right side of it’s heart to pump blood to it’s lungs.  As a result a temporary hole connecting the right and left atrium exists called the foramen ovale (the oval opening).  this allows the incoming blood to bypass the foetus’ absent pulmonary circulation and going straight to the left atrium allowing it to be pumped around the foetus.  Some blood does enter the pulmonary artery but another temporary vessel, the ductus arteriosus diverts most of this back to the aorta.

The returning systemic blood leaves the foetus from where it came that is through two umbilical arteries that leave through the umbilicus where they exchange, once more with the mother’s blood, through the placenta membrane.

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