Getting in the Heart

We are now going to go over the heart. This is an extremely vital part of the body.

The heart is split into two halves by the cardiac septum. Within these two parts, right and left, they are split again into the right and left atrium (top) and the right and left ventricles (bottom). The right atrium and ventricle are connected by a valve called the tricuspid valve. It is made up of three flaps that open from pressure from blood filling up in the right atrium and also from an electrical charge. The left atrium and left ventricle are connected by a mitral valve which opens the same way as the tricuspid valve. On the right side of the heart, blood flows from the vena cava to the right atrium, through the tricuspid valve, to the right ventricle, and out to the lungs. On the left side, blood flows from the lungs, to the left atrium, to the mitral valve, to the left ventricle, and out to the body. The left side of the heart is bigger and has to work much harder because it has to distribute blood all over the entire body.

To detail the cardiac cycle, I’m going to follow the route of blood. Because this is a cycle, it would work to start at any point, but I will start with the right atrium. Blood enters the right atrium from two areas, inferior vena cava and superior vena cava. While the blood is in the atrium, there is an electrical charge that squeezes the atrium and forces blood through the tricuspid valve and into the right ventricle. The ventricle then contracts and applies pressure to the blood. This pressure closes the tricuspid valve. The blood is then pushed out of the ventricle, through the pulmonary artery trunk, which splits into right and left pulmonary and goes into the right and left lungs. Blood passes through capillaries and air sacs and dumps carbon dioxide and picks up oxygen in the lungs. Blood is now oxygenated and goes out through the pulmonary vein. Blood then comes through the pulmonary vein to the left atrium and enters the left ventricle in the same way as into the right ventricle, but this time, the blood is passing through the mitral valve. From the ventricle, the blood goes through an aortic valve to the ascending aorta to the aortic arch and then is distributed all over the body. In the body, the blood delivers oxygen and picks up Co2. The blood is now deoxygenated and reenters the right atrium from the vena cava. Then the cycle repeats. One heartbeat is comprised of the atria contraction and ventricle contraction.

Because the heat pumps based on an electrical impulse, it would be helpful to understand where this impulse comes from. At the top of right atrium, there is an SA Node which sets off a sequence of events by generating electrical current. This current is what contracts the atrium. In the heart muscle wall surrounding the atria, there is a barrier that keeps the signal only within the atria. On the cardiac septum, there is an AV Node. There are bundles of his (AV bundles), which are bundles of fibers coming out of the AV Node. Out of these, there are fibers, called purkinje fibers, which spread out along the ventricles. The AV Node stimulates the fibers to contract the ventricles. The AV Node gets the energy from the contraction of the atria which hits the AV Node. The fibers are conductive and slow the current to delay the signal so that the atria have enough time to complete their contraction before the ventricles begin their contractions.

To monitor the heat, there is a test called an Electrocardiogram. It consists of a P wave, a QRS complex, and a T wave. The P wave is a signal from the atrium, and the QRS complex and the T waves are signals from the ventricle. At first, we see polarization, which is a static (resting) while the heart is electrically charged. Then we see depolarization, a discharge of electrical energy. This is followed by a repolarization, a recharge to polarization. The P wave is the depolarization of the atrium. The QRS complex is ventricular depolarization corresponding to the ventricular contraction. The T wave is the repolarization of the ventricular system, diastole (relaxation).

When we think about what a heart sounds like, we know that there is a silence, followed by two bumps, then silence again. What we just went over corresponds with these sounds. The silence is when the atria cycle is occurring, the first sounds is from the tricuspid and mitral valves and the second bump is the semi-lunar valves from the blood exiting the heart. An example of an abnormal heart sound would be a murmur, which sounds like a gurgle or a hissing. This represents a leaky valve in the heart.

Blood pressure management is very important in regards to the heart and healthy blood flow. Blood pressure tests measure the systolic pressure and the diastolic pressure. The systolic pressure is the pressure in the artery. The number that is measured shows the maximum pressure in that artery during left ventricular systole. The diastolic pressure is the elastic recoil drop of pressure. The number that is measured here shows the minimum to which pressure drops in the same artery during left ventricular diastole.



(image from wikipedia.com)

The Autonomic Nervous System

The autonomic nervous system (ANS) operates below levels of consciousness. It monitors and controls all basic life processes. The ANS is a motor system with 3 target tissues; smooth muscles, glands (exocrine and endocrine), and cardiac muscles.

The autonomic nervous system is organized into pre-ganglionic and post-ganglionic neurons. Synapses between the pre and post-ganglionic neurons are made in the autonomic ganglia. Pre-ganglionic neurons of both divisions, sympathetic and parasympathetic, have their cell bodies in the central nervous system, while post-ganglionic neurons have their cell bodies in the autonomic ganglia and synapse on various organs. The sympathetic ganglia are located in the sympathetic trunk along the vertebral column, while the parasympathetic ganglia are located in or near the target organs.

The ANS contains higher autonomic centers, including the hypothalamus, which monitors blood and hormones, as well as coordinates signals to other centers. The hypothalamus contains the temperature regulation center, as well as the thirst and food intake regulation centers. The ANS is divided into the sympathetic and parasympathetic nervous systems. The sympathetic division is for immediate survival, instant reactions, and instant change. The parasympathetic works to keep a homeostatic flow within the body and focuses on long term survival. When the sympathetic system is activated, it increases heart rate, force of heart beat, blood pressure, adrenaline, respiration, and decreases renal and digestive functions. The parasympathetic system decreases heart rate, respiratory rate, blood pressure, and increases digestive and renal functions.

The sympathetic system is made up of two types of ganglia. There are sympathetic trunk (chain) ganglia and collateral ganglia (in the abdominal cavity). The signal comes from a pre-ganglionic neuron to the ganglion. From this ganglion, the signal can synapse with 20 or more post-ganglionic neurons. This is known as amplification. The post-ganglionic neurotransmitter output is nor-epinephrine. There is also adrenaline releases by a gland on the kidney known as the adrenal medulla.

The parasympathetic nervous system focuses on rest and regeneration. The main functions are salivation, lacrination, urination, defecation, and digestion. It tends to be slow acting; organ by organ. This system has much less amplification than the sympathetic system.

The main neurotransmitters of the autonomic nervous system are acetylcholine and norepinephrine. Acetylcholine is released by the pre-ganglionic neurons in both the sympathetic and parasympathetic divisions. Norepinephrine is released by the post-ganglionic neurons of the sympathetic division, while acetylcholine is released by the post ganglionic neurons of the parasympathetic division.