Chapter 5: The cardiovascular system Flashcards
The cardiovascular system is divided how?
The cardiovascular system ensures a continuous flow of blood to all body cells, and its function is subject to continual physiological adjustments to maintain an adequate blood supply.
- the heart, whose pumping action ensures constant circulation of the blood
- the blood vessels, which form a lengthy network through which the blood flows.
How does the heart pump blood into separate systems of blood vessels?
The right side of the heart pumps blood to the lungs (the pulmonary circulation) where gas exchange occurs, i.e., the blood collects oxygen from the air sacs and excess carbon dioxide diffuses into the air sacs for exhalation.
The left side of the heart pumps blood into the systemic circulation, which supplies the rest of the body. Here, tissue wastes are passed into the blood for excretion, and body cells extract nutrients and oxygen.
Blood vessels
A tube through which the blood circulates in the body. Blood vessels include a network of arteries, arterioles, capillaries, venules, and veins.
Arteries and arterioles
These blood vessels transport blood away from the heart
Anastomoses and end-arteries
Anastomoses are arteries that form a link between main arteries supplying an area.
An end-artery is an artery that is the sole source of blood to a tissue, e.g., the branches from the circulus arteriosus (circle of Willis) in the brain or the central artery to the retina of the eye
Capillaries and sinusoids
Sinusoids refer to the small, irregularly-shaped blood vessels found in certain organs, especially the liver, while capillaries refer to any of the fine branching blood vessels, which form a network between the arterioles and venules.
Capillary refill time
It is a simple test to measure the time taken for color to return to an external capillary bed after pressure is applied, typically by pressing the end of a finger with the thumb and forefinger. Normal capillary refill time is usually 2 seconds or less.
Veins and venules
A blood vessel that carries blood that is low in oxygen content from the body back to the heart.
The smallest veins are called venules.
Blood supply
The outer layers of tissue of thick-walled blood vessels receive their blood supply via a network of blood vessels called the vasa vasorum. Thin-walled vessels and the endothelium of the others receive oxygen and nutrients by diffusion from the blood passing through them.
Control of blood vessel diameter
Blood vessel diameter is controlled by a smooth muscle of tunica media which is supplied by sympathetic nerves of the autonomic nervous system.
Blood vessel diameter and blood flow
Resistance to flow of fluids along a tube is determined by three factors: the diameter of the tube; the length of the tube; and the viscosity of the fluid. The most important factor determining how easily the blood flows through blood vessels in the first of these variables, that is, the diameter of the resistance vessels (the peripheral resistance).
Local regulation of blood flow
The greatest change in blood pressure and velocity of blood flow occurs at the transition of arterioles to capillaries.
This reduces the pressure and velocity of flow for gas and nutrient exchange to occur within the capillaries.
As such arterioles are the main part of the circulatory system in which local control of blood flow occurs.
Capillary exchange
Refers to the exchange of material from the blood into the tissues in the capillary. There are three mechanisms that facilitate capillary exchange: diffusion, transcytosis and bulk flow.
Capillary fluid dynamics
Capillary dynamics are controlled by the four Starling forces. The oncotic pressure is a form of osmotic pressure exerted by proteins either in the blood plasma or interstitial fluid. Hydrostatic pressure is a force generated by the pressure of the fluid on the capillary walls either by the blood plasma or interstitial fluid.
Heart
The heart is a muscular organ, which pumps blood through the blood vessels of the circulatory system.
Position of the heart
The heart is located behind and slightly left of the sternum, between the two lungs, and enclosed in a sac called the pericardium. In the human being the heart lies just behind and slightly to the left of the sternum (breastbone) in a double-layered sac called the pericardium.
Organs associated with the heart
Inferiorly – the apex rests on the central tendon of the diaphragm
Superiorly – the great blood vessels, i.e., the aorta, superior vena cava, pulmonary artery, and pulmonary veins
Posteriorly – the esophagus, trachea, left and right bronchus, descending aorta, inferior vena cava, and thoracic vertebrae
Laterally – the lungs – the left lung overlaps the left side of the heart
Anteriorly – the sternum, ribs, and intercostal muscles.
Structure of the heart wall
The heart wall is composed of three layers of tissue: pericardium, myocardium and endocardium.
Pericardium
The membrane enclosing the heart, consisting of an outer fibrous layer and an inner double layer of serous membrane.
Myocardium
the muscular tissue of the heart.
Fibrous tissue in the heart
The myocardium is supported by a network of fine fibers that run through all the heart muscle. This is called the fibrous skeleton of the heart. In addition, the atria and the ventricles are separated by a ring of fibrous tissue, which does not conduct electrical impulses.
Endocardium
The endocardium is a thin, smooth tissue that makes up the lining of the chambers and valves of the heart. The innermost layer of the heart’s walls, it serves as a barrier between cardiac muscles and the bloodstream and contains necessary blood vessels.
Interior of the heart
The right and left sides of your heart are divided by an internal wall of tissue called the septum. The area of the septum that divides the two upper chambers (atria) of your heart is called the atrial or interatrial septum. The area of the septum that divides the two lower chambers (ventricles) of your heart is called the ventricular or interventricular septum.
The flow of blood through the heart
The heart has two upper chambers—the left and right atriums—and two larger lower chambers—the left and right ventricles. A series of valves control blood flow in and out of these chambers.
Electrical impulses, controlled by the cardiac conduction system, make the heart muscle contract and relax, creating the rate and rhythm of your heartbeat.
Venous drainage
Most of the venous blood is collected into several cardiac veins that join to form the coronary sinus, which opens into the right atrium. The remainder passes directly into the heart chambers through little venous channels.
Conducting system of the heart
A cardiac conduction system is a group of specialized cardiac muscle cells in the walls of the heart that send signals to the heart muscle causing it to contract. The main components of the cardiac conduction system are the SA node, AV node, a bundle of His, bundle branches, and Purkinje fibers.
Sinoatrial node (SA node)
A small body of specialized muscle tissue in the wall of the right atrium of the heart that acts as a pacemaker by producing a contractile signal at regular intervals.
Atrioventricular node (AV node)
The electrical relay station between the upper and lower chambers of the heart.
The atrioventricular bundle
The atrioventricular bundle, also called the bundle of His, is a bundle of cardiac muscle fibers located within the septum of the heart. This fiber bundle extends from the AV node and travels down the septum, which divides the left and right ventricles.
Nerve supply to the heart
As mentioned earlier, the heart is influenced by autonomic (sympathetic and parasympathetic) nerves originating in the cardiovascular center in the medulla oblongata.
The vagus nerve (parasympathetic) supplies mainly the SA and AV nodes and atrial muscle.
Sympathetic nerves supply the SA and AV nodes and the myocardium of atria and ventricles, and stimulation increases the rate and force of the heartbeat.
The cardiac cycle
At rest, the healthy adult heart is likely to beat at a rate of 60–80 beats per minute (b.p.m.). During each heartbeat, or cardiac cycle, the heart contracts (systole) and then relaxes (diastole).
Stages of the cardiac cycle
A single cycle of cardiac activity can be divided into two basic phases - diastole and systole.
Diastole is defined as the phase in which the heart, especially the ventricles, is at rest. The relaxed heart allows for blood to fill the cardiac chambers.
Systole is defined as the phase in which the heart, especially the ventricles, is contracting. The contraction allows for blood to flow into the pulmonary circulation and systemic vasculature via the main pulmonary artery and aorta, respectively.
Heart sounds
Blood flow creates vibrations in the heart chambers and valves which produce audible sounds that can be heard through a stethoscope. Smooth, low-resistance blood flow is called a laminar flow. When the flow is rough with high resistance it is known as a turbulent flow.
First sound
When the two ventricles contract and pump out blood into the aorta and pulmonary artery the mitral and tricuspid valves close to prevent the blood flowing back into the atria. The first sound S1 is generated by vibrations created by the closing of these two valves.
Normally the mitral valve closes just before the tricuspid valve, and when the two different sounds are detectable, it is called a “split S1.” A split S1 may be indicative of certain conditions affecting the heart.
Second sound
After pumping the blood, the ventricles relax to receive blood from the atria, and the diastole phase starts. The aortic and pulmonic valves close and cause vibrations, giving rise to the second heart sound, S2. The increase in the intensity of this sound may indicate certain conditions.
When the aortic valve closes just before the pulmonic valve, it may generate a split S2. This may indicate impairment in heart function.
Third sound
The third heart sound is a low-pitched sound audible with the rapid rush of blood from the atrium into the ventricle as it starts relaxing. This may be a normal sound in some people but in people with heart conditions, S3 may indicate heart failure.
Fourth sound
The fourth is a low-intensity sound heard just before S1 in the cardiac cycle. The sudden slowing of blood flow by the ventricle as the atrium contracts causes this sound, which may be a sign of heart disease.
Electrical changes in the heart
The heart is a pump made up of muscle tissue. Like all muscle, the heart needs a source of energy and oxygen to function. The heart’s pumping action is regulated by an electrical conduction system that coordinates the contraction of the various chambers of the heart.
Cardiac output
The cardiac output is the amount of blood ejected from each ventricle every minute. The amount expelled by each contraction of each ventricle is the stroke volume. Cardiac output is expressed in liters per minute (L/min) and is calculated by multiplying the stroke volume by the heart rate (measured in beats per minute):
Cardiac output=Stroke volume × Heart rate.
Stroke volume arterial blood pressure
The stroke volume is determined by the volume of blood in the ventricles immediately before they contract, i.e., the ventricular end-diastolic volume (VEDV), sometimes called preload.
Preload depends on the amount of blood returning to the heart through the superior and inferior vena cava (the venous return).
Blood volume
This is normally kept constant by the kidneys. Should blood volume fall, e.g., through sudden hemorrhage, this can cause stroke volume, cardiac output and venous return to fall. However, the body’s compensatory mechanisms will tend to return these values towards normal, unless the blood loss is too sudden or severe for compensation.
