Physiology
Lec: 1 د. زيــد الاطرقجي Cardiovascular system[Introduction]
Heart
Within about three weeks after conception the heart of a developing embryo starts to function. It is the first organ to become functioning at this time, even the human embryo is only a few millimeters long.
Throughout an average life span the heart contract and never stops except for a fraction of a second to be filled with blood between beats
The circulatory system
It has three basic components:
1.The heart : which serves as the pump that impart pressure to the blood to establish the pressure gradient which is needed to flow the blood to the tissues .Blood like all liquids flows down a pressure gradient from an area of higher pressure to an area of a lower pressure.
2. The blood vessels which serve as passage ways through which blood is directed and distributed from the heart to all parts of the body and subsequently returned back to the heart.
3. Blood is the transport medium within which materials being transported through the body such as O2, CO2, nutrient, waste, electrolytes, and hormones.
The heart is a dual [two] pump:
Even though anatomically the heart is a single organ but the right and left sides of the heart function as two separate pumps.
Both sides of the heart simultaneously pump equal amounts of blood,
the volume of O2 poor blood being pumped to the lungs by the right side of the heart soon becomes the same volume of O2 rich blood being delivered by the left side of the heart to the body
In pulmonary circulation all the blood flows through the lungs, but in the systemic circulation it may be viewed as a series of parallel pathways.
Part of the blood which is pumped out by the left ventricle goes to the muscles, part to the kidneys, part to the brain and so on . Thus the out put of the left ventricle is distributed so that each part of the body receives a fresh blood supply.
Note that: The same arterial blood does not pass from organ to organ.
The pulmonary circulation is a low pressure, low resistance system. Whereas the systemic circulation is high pressure, high resistance system
Pressure: is the force exerted on the vessel walls by the blood which is pumped into the vessels by the heart.
Resistance: is the opposition to blood flow, it is largely caused by friction between the flowing blood and the vessel wall.
Blood flows through the heart in one direction from veins to atria to ventricles to arteries.
The presence of four [4] one way valves ensure this unidirectional flow of blood.
The valves are positioned so that they open and close passively because of pressure gradient similar to one way door.
Forward pressure gradient [that is a greater pressure behind the valves] forces the valves open much as open a door by pushing on one side of it. Whereas a backward pressure gradient [ that is a greater pressure in front of the valves ]forces the valve closed just as you apply pressure to the opposite of the door to close it.
Fibrous-skeleton of the valves:
Four interconnecting rings of dense connective tissue provide a firm base for attachment of the four heart valves. The function of fibrous skeleton is to separate the atria from the ventricles, it also provides a fairly rigid structure for attachment of the cardiac muscle.
The atrial muscle mass is anchored above the rings and the ventricular muscle mass is attached to the bottom of the rings.
The inlet valve to the ventricles [atrio-ventricular valves] and the outlet valves from the ventricles [the semi lunar valves] all lie on the same plane through the heart.
Heart muscle[myocardium]
The myocardium consists of interlacing bundles of cardiac muscle fibers arranged spirally around the circumference of the heart. The spiral arrangement is due to the hearts complex twisting during development. As a result of this arrangement when the ventricular muscle contracts and shorten, the diameter of the ventricular chambers is reduced while the apex is simultaneously pulled upward toward the top of the heart in rotating manner, this exerts a wringing effect efficiently exerting pressure on the blood within the enclosed chambers and directing it upward toward the openings of the major arteries that exit at the base of the ventricles.
The individual cardiac muscle cells are interconnected to form branching fibers with adjacent cells joined end to end at a specialized structures known as intercalated disc.
Within an intercalated disc there are two types of membrane junctions.
A. Desmosomes . B. Gap junctions
Desmosomes is type of a mechanical junction that hold cells together.
Gap junction is formed when the opposing membranes approach each other very closely to form gap junctions which are areas of low electrical resistance that allow action potential to spread from one cardiac cell to adjacent cells. Some cardiac muscle cells can generate action potential without any nervous stimulation. When one of the cardiac cells spontaneously undergoes an action potential the electrical impulse spreads to all the other
Cells that are joined by gap junctions in the surrounding muscle mass so that they become excited and contract as a single functional syncytium.
The atria and the ventricles each form a functional syncytium and contract as separate units. The synchronous contraction of the muscle cells that make up the walls of each of these chambers [atria and ventricles] produce the force needed to eject the enclosed blood.
No junction joins the atria and ventricular contractile cells and furthermore the atria and ventricles are separated by the electrically nonconductive fibrous skeleton that surrounds and support the valves. However an important specialized conductive system facilitates and coordinates transmissions of electrical excitation from the atria to the ventricles to ensure synchronization between atrial and ventricular pumping
Because of both the syncytial nature of cardiac muscle and the conduction system between the atria and ventricles an impulse spontaneously generated in one part of the heart spreads throughout the entire heart. Therefore unlike skeletal muscle when graded contractions can be produced by varying the number of muscle cells contracting within the muscle [recruitment of motor units]. So either all the cardiac muscle fibers contract or none do. A halfhearted contraction is not possible.
Cardiac muscle contraction is graded by varying the strength of contraction of all the cardiac muscle cells.
Electrical activity of the heart
Contraction of cardiac muscle cells to eject blood is triggered by action potentials sweeping across the muscle membranes.
The heart contracts or beats rhythmically as result of action potentials that is generate by itself which is called auto-rhythmicity.
There are two specialized types of cardiac muscle cells:
1. Contractile cells which are 90% of the cardiac muscle cells that do the mechanical work of pumping. These working cells normally do not initiate their own action-potentials.
2. auto-rhythmic cells do not contract but instead they are specialized for initiating and conducting the action potentials responsible for contraction of the working cells.
Auto-rhythmic cells do not have resting membrane potential instead they display pace maker activity that is their membrane potential slowly depolarizes or drifts between action potentials and threshold is reached at which time the membrane fires or has an action potential.
An auto-rhythmic cell membranes slow drift to threshold is called the pace-maker potential.
Through repeated cycles of drift and fire these auto-rhythmic cells cyclically initiate action potentials which then spread throughout the heart to trigger rhythmic beating without any nervous stimulation.
Complex interaction of several different ionic mechanism are responsible for the pace-maker potential.
The most important changes in ionic movement that give rise to the pace-maker potential are:
1. A decreased outward K current coupled with a constant inward Na current.
2. An increased inward Ca current.
The sinoatrial node is the normal pace maker of the heart .it discharge 70-80 action potential per minute.