- Cardiac Cycle refers to one complete heartbeat.
- The heart is actually 2 pumps situated side by side, so a complete cycle consists of contractions of both atria and both ventricles.
- The average resting heart rate is approximately 75 beats per minute
- Resting heart rate will vary with degree of physical conditioning as well as with disease states
- The familiar “lub-dub” associated with the function of the heart are known as heart sounds.
- The heart sounds are made by the heart valves snapping closed under contractile pressure.
- The “lub” is caused by the closure of the AV valves
- The “dub” is caused by the closure of the semi-lunar valve.
- Cardiac output (CO) is the amount of blood pumped out of each side of the heart ( meaning the ventricles ) in one minute
- The value of CO is calculated as the product of the Heart Rate ( HR) multiplied by the Stroke Volume (SV)
- SV generally increases as the force of the ventricular contractions increases.
- CO = HR x SV
- CO = 75 beats per min x 70mls per beat
- CO = 5250 ml/min
- Generally speaking, blood starting in the heart should complete a circuit of the body and arrive back in the heart in one minute.
- According to Starling’s Law of the heart, stroke volume is influenced by the amount of stretching force applied to the muscle cells of the heart.
- The more the cells are stretched, the great the contraction will be
Starling’s Law of the Heart
- A healthy, efficient heart only pumps about 60% of the volume of blood in the ventricles.
- Venous return is critical to the amount of stretching the heart undergoes.
- If one side of the heart begins to pump more blood than the other, the increase in venous return to the opposite ventricle will force it to pump a larger volume of blood.
- This helps prevent back ups and increased pressures in the system.
- Anything that increases the volume or speed of venous return will also increase the SV and CO.
- The squeezing actions of skeletal muscles on veins also plays a major role in increasing venous return.
Regulation of Heart Rate
- Coronary contraction does not depend on stimulus from the nervous system, because the heart has it’s own intrinsic pacemakers ( SA and AV nodes)
- HOWEVER—heart rate can be temporarily influenced by stimulus from the autonomic nerves.
- Sympathetic nerves can act to increase or decrease the heart by stimulating the SA or AV nodes.
- Parasympathetic nerves act to decrease the heart rate
- Being excited or startled can cause a rapid increase in heart rate.
- Heart rate can also be influenced by hormones
- Epinephrine and Thyroxine mimic the effects of the sympathetic nerves.
- Ion concentrations also has a rofound influence on the heart.
- Decreased Ca+ will depress the heart rate
- Excessive Ca+ will cause prolonged contractions, and possibly stop the heart from beating.
CHF and Blood Vessels
- When the pumping efficiency of the heart is depressed so that circulation is inadequate to meet tissues needs, congestive heart failure (CHF) results.
- CHF is usually a progressive condition, and in most cases reflects diminished coronary circulation by coronary atherosclerosis
- Each side of the heart can fail independently
- If the left side fails, pulmonary congestion will occur.
- The right side continues to pump blood, but the left side is unable to push oxygenated blood into systemic circulation
- The blood vessels of the lungs become swollen with blood, and the increased pressure forces fluids to leak from circulation and into the lung tissue, resulting in pulmonary edema.
- Suffocation will result if this condition is untreated.
- If the right heart fails, blood is not moved out of the systemic circulation, and peripheral congestion will occur.
- This results in edema in distal parts of the body, such as the feet, ankles and hands.
- Failure of one side of the heart places increased strain on the opposite side of the heart—eventually leading to whole heart failure
Blood Vessels
- Blood circulates through the body within the vasculature.
- In decreasing size order, vessels carrying blood away from the heart are, arteries, arterioles, capillary beds.
- In increasing size order, vessels carrying blood back towards the heart from the capillary beds are, venules, and veins
- The walls of most blood vessels consist of three layers called tunics.
- The innermost layer is called the tunica intima.
- Tunica intima consists of a thin layer of closely fitted endothelial cells.
- It is very smooth, to allow for smooth flow of blood
- The tunica media is the middle layer of tissue, and is primarily smooth muscle tissue and elastic connective tissue.
- This smooth muscle is controlled by the sympathetic nervous system, which changes the diameter of the blood vessels
- The tunica externa is the outer-most layer, and is mostly fibrous connective tissue that supports and protects the vessel.
- The walls of arteries are usually thicker than those of the veins.
- Veins are subject to less pressure than ateries, but move blood against gravity.
- Veins contain one way valves to prevent backflow of blood
