The Anatomy & Physiology of Circulation
Week 4
Context
Once problems with airway and breathing are addressed we move onto circulation. At the most simple level, the function of the human circulatory system is to to transport blood and oxygen from the lungs to the various tissues of the body. The heart pumps the blood throughout the body. In order to assess circulation we need to have an understanding of the underlying anatomy and physiology at a level appropriate to our knowledge and experience. This section covers a basic overview of the anatomy of the circulatory system. There are more advanced sections for registered health care practitioners to refresh their knowledge and understanding if required.
Content
The circulatory system is composed of the heart, the blood vessels that transport blood, and the constituents of blood (which might vary).
The Heart
The heart is a muscular organ about the size of a closed fist that functions as the body’s circulatory pump. It takes in deoxygenated blood through the veins and delivers it to the lungs for oxygenation before pumping it into the various arteries (which provide oxygen and nutrients to body tissues by transporting the blood throughout the body). The heart is located in the thoracic cavity medial to the lungs and posterior to the sternum. On its superior end, the base of the heart is attached to the aorta, pulmonary arteries and veins, and the vena cava. The inferior tip of the heart, known as the apex, rests just superior to the diaphragm. The base of the heart is located along the body’s midline with the apex pointing toward the left side. Because the heart points to the left, about 2/3 of the heart’s mass is found on the left side of the body and the other 1/3 is on the right.
Heart Anatomy
A double-layered membrane called the pericardium surrounds the heart like a sac. The outer layer of the pericardium surrounds the roots of the heart's major blood vessels and is attached by ligaments to the spinal column and diaphragm. The inner layer of the pericardium is attached to the heart muscle. A coating of fluid separates the two layers of membrane, letting the heart move as it beats.
The heart has 4 chambers. The upper chambers are called the left and right atria, and the lower chambers are called the left and right ventricles. A wall of muscle called the septum separates the left and right atria and the left and right ventricles. The left ventricle is the largest and strongest chamber in the heart. Four valves regulate blood flow through your heart:
- The tricuspid valve regulates blood flow between the right atrium and right ventricle.
- The pulmonary valve controls blood flow from the right ventricle into the pulmonary arteries, which carry blood to the lungs to pick up oxygen.
- The mitral valve lets oxygen-rich blood from the lungs pass from the left atrium into the left ventricle.
- The aortic valve opens the way for oxygen-rich blood to pass from the left ventricle into the aorta, the largest artery.
Heart Regulation
The heart is regulated via a conduction system. This is described in the Electrocardiograph (ECG) (Advanced) section.
Blood Vessels
Blood vessels are divided into Arteries, Arterioloes, Caplilliaries, Venules, and Veins.
Structure and Function
These all have different structures and functions as set out below:
| Structure | Function | |
| Arteries | The walls (outer structure) of arteries contain smooth muscle fibre that contract and relax under the instructions of the sympathetic nervous system. | Transport blood away from the heart Transport oxygenated blood only (except in the case of the pulmonary artery). |
| Arterioles | Arterioles are tiny branches of arteries that lead to capillaries. These are also under the control of the sympathetic nervous system, and constrict and dialate, to regulate blood flow. | Transport blood from arteries to capillaries Arterioles are the main regulators of blood flow and pressure. |
| Caplilliaries | Capillaries are tiny (extremely narrow) blood vessels, of approximately 5-20 micro-metres diameter. There are networks of capillaries in most of the organs and tissues of the body. These capillaries are supplied with blood by arterioles and drained by venules. Capillary walls are only one cell thick, which permits exchanges of material between the contents of the capillary and the surrounding tissue. | To supply the tissues of the body with the components of blood To remove waste from the surrounding cells Exchange of oxygen, carbon dioxide, water, and salts, between the blood and the surrounding body tissues. |
| Venules | Venules are minute vessels that drain blood from capillaries and into veins. Many venules unite to form a vein. | Drain blood from capillaries into veins, for return to the heart. |
| Veins | The walls (outer structure) of veins consist of three layers of tissues that are thinner and less elastic than the corresponding layers of aerteries. Veins include valves that aid the return of blood to the heart by preventing blood from flowing in the reverse direction. | Transport blood towards the heart. Transport deoxygenated blood only (except in the case of the pulmonary vein). |
Major Vessels
You can see the major arteries and veins in this diagram:
Circulatory Shock
Circulatory shock is defined as an inadequate blood flow throughout the body. In the absence of mechanisms that function to maintain blood pressure within a normal range of values, blood pressure decreases dramatically. As a consequence, tissues can suffer from damage as a result of too little delivery of oxygen to cells. Severe circulatory shock can damage vital body tissues to the extent that death of the individual occurs.
Depending on its severity, shock can be divided into three separate stages:
- the nonprogressive or compensated stage,
- the progressive stage,
- the irreversible stage.
All types of circulatory shock exhibit one or more of these stages, regardless of their causes. There are several causes of shock, but haemorrhagic (due to blood loss), or hypovolaemic (reduction in the amount of blood circulating), shock is used below, to illustrate the characteristics of each stage.
In compensated shock, the blood pressure decreases only a moderate amount, and the mechanisms that regulate blood pressure function successfully to reestablish normal blood pressure and blood flow. The baroreceptor reflexes, chemoreceptor reflexes, and ischaemia within the medulla oblongata, initiate strong sympathetic nervous system responses that result in intense vasoconstriction and increased heart rate. As the blood volume decreases, the stress-relaxation response of blood vessels causes the blood vessels to contract and helps sustain blood pressure. In response to reduced blood flow through the kidneys, increased amounts of renin are released. The elevated renin release results in a greater rate of angiotensin II formation, causing vasoconstriction and increased aldosterone release from the adrenal cortex. The aldosterone, in turn, promotes water and salt retention by the kidneys, conserving water. In addition, Anti-Diuretic Hormone (ADH) is released from the posterior pituitary gland, which also enhances the retention of water by the kidneys. Because of the fluid shift mechanism, water also moves from the interstitial spaces and the intestinal lumen, to restore the normal blood volume. An intense sensation of thirst increases water intake, also helping to elevate normal blood volume. In mild cases of compensated shock, the baroreceptor reflexes can be adequate to compensate for blood loss until the blood volume is restored, but in more severe cases all of the mechanisms described are required to compensate for the blood loss.
In progressive shock, the compensatory mechanisms are not adequate to compensate for the loss of blood volume. As a consequence, a positive-feedback cycle develops in which the blood pressure regulatory mechanisms are not able to compensate for circulatory shock. As circulatory shock becomes worse, regulatory mechanisms become even less able to compensate for the increasing severity of the circulatory shock. The cycle proceeds until the next stage of shock is reached or until medical treatment is applied that assists the regulatory mechanisms in re-establishing adequate blood flow to tissues. During progressive shock, the blood pressure declines to a very low level that is not adequate to maintain blood flow to the cardiac muscle, and the heart begins to deteriorate. Substances that are toxic to the heart are released from tissues that suffer from severe ischaemia. When the blood pressure declines to a very low level, blood begins to clot in the small vessels. Eventually blood vessel dilation begins as a result of decreased sympathetic nervous system activity, and because of the lack of oxygen in capillary beds. Capillary permeability increases under ischaemic conditions, allowing fluid to to leave the blood vessels and enter the interstitial spaces, and finally intense tissue deterioration begins in response to inadequate blood flow.
Without medical intervention, progressive shock leads to irreversible shock. Irreversible shock leads to death, regardless of the amount or type of medical treatment applied. In this stage of shock, the damage to tissues, including cardiac muscle, is so extensive that the patient is destined to die even if adequate blood volume is reestablished and blood pressure is elevated to its normal value. Irreversible shock is characterised by decreasing function of the heart and progressive dilation of peripheral blood vessels.
Patients suffering from shock are normally placed in a horizontal plane, with the head slightly lower than the feet, and given oxygen. Replacement therapy consists of transfusions of whole blood, plasma, artificial solutions called plasma substitutes, and physiological saline solutions, administered to increase blood volume. In some circumstances, drugs that enhance vasoconstriction are also administered. Occasionally, such as in patients in anaphylactic shock, antiiflammatory substances such as glucocorticoids and antihistamines are administered. The basic objective in treating shock is to reverse the condition so that progressive shock is arrested, to prevent it from progressing to the irreversible stage, and to cause the condition to be reversed so that normal blood flow through tissues is reestablished.
References and Further Reading
Garretson, S. and Malberti, S.( 2007). Understanding hypovolaemic, cardiogenic and septic shock. Nursing Standard 50(21): 46–55.
Garrioch, M.A. (2004) The body’s response to blood loss Vox Sanguinis 87(Suppl. 1)S74–S76 [WWW]
Harbrecht, B.G., Alarcon, L.H. and Peitzman, A.B. (2004) Management of shock. In Moore, E.E. et al. (Eds)Trauma (5th Ed) New York: McGraw-Hill, 201–25.
Jowett, N. and Thompson, D. (2007) Comprehensive Cardiac Care (4th Ed) London: Baillière Tindall.
Marieb E.N, & Hoehn K (2013) Human Anatomy and Physiology (9th Ed) London: Pearson Education
Spahn, D.R., Cerny, V., Caoats, T.J. et al. (2007) Management of bleeding following major trauma: a European guideline. Critical Care 11: R17.
Tortora.G, & Derrikson B (2013) Essentials of Human Anatomy and Physiology. Chichester: Wiley