18.06.2019 - Safir Şehir Portalı & Firma Rehberi Teması

Blood

Blood
Reklam

The body usually contains about 5 litres of blood. Its constituents include a
fluid known as plasma and a variety of cells. As well as the various exogenous
cells carried in the blood (nutrients, oxygen, etc.), it produces its own cells.
These are manufactured by stem cells in the bone marrow. Three different
types of cell are produced:

  1. Erythrocytes (or red blood cells) transport oxygen around the body. In
    them, oxygen combines with haemoglobin in the lungs and is transported
    to cells in need of oxygen, where it is released, allowing cell respiration.
  2. Phagocytes and lymphocytes (or white blood cells; see above) include the
    immune system’s B cells and T cells described earlier in the chapter.
  3. Platelets are cells that respond to damage to the circulatory system. They
    aggregate (form a clot) around the site of any damage and prevent loss of
    blood from the system. They are also involved in repair to damage within
    the arteries themselves and contribute to the development of atheroma.
    We consider this process later in the chapter.
    The heart
    The heart has two separate pumps operating in parallel. The right side of the
    heart is involved in the transportation of blood to the lungs; the left side
    pumps blood to the rest of the body (Figure 8.6). Each side of the heart has
    two chambers (Figure 8.7), known as atria and ventricles. The right atrium
    Figure 8.6 The flow of blood through the heart.
    exogenous
    relating to things
    outside the body.
    erythrocyte
    a mature blood cell that
    contains haemoglobin to
    carry oxygen to the
    bodily tissues.
    platelets
    tiny bits of protoplasm
    found in the blood that
    are essential for blood
    clotting. These cells bind
    together to form a clot
    and prevent bleeding at
    the site of injury.
    228 CHAPTER 8 • THE BODY IN HEALTH AND ILLNESS
    takes deoxygenated blood from veins known as the superior and inferior
    vena cava and pumps it into the right ventricle. Blood is then pumped into
    the pulmonary artery, taking it to the lungs, where it picks up oxygen in its
    haemoglobin cells. Oxygen-laden blood then returns to the heart, entering
    through the left atrium. It is then pumped into the left ventricle, and then
    into the main artery, known as the aorta, which carries blood to the rest of
    the body.
    The rhythm of the heart is controlled by an electrical system. This is initiated
    by an electrical impulse generated in a region of the right atrium called
    the sinoatrial node. This impulse causes the muscles of both atria to contract.
    As the wave of electricity progresses through the heart muscle and nerves, it
    reaches an area at the junction of the atria and ventricles known as the atrioventricular
    node. This second node then fires a further electrical discharge
    along a system of nerves including the bundle of His and Purkinje fibres (see
    Figure 8.7), triggering the muscles of both ventricles to contract, completing
    the cycle. Although the sinoatrial node has an intrinsic rhythm, its activity is
    largely influenced by the autonomic nervous system.
    The electrocardiogram (ECG) is used to measure the activity of the heart.
    Electrodes are placed over the heart and can detect each of the nodes firing
    and recharging. Figure 8.8 shows an ECG of a normal heart, indicating the
    electrical activity at each stage of the heart’s cycle.
    n The P wave indicates the electrical activity of the atria firing – the time
    needed for an electrical impulse from the sinoatrial node to spread
    throughout the atrial musculature.
    n The QRS complex represents the electrical activity of the ventricles
    compressing.
    n The T wave represents the repolarization of the ventricles.
    Figure 8.7 Electrical conduction and control of the heart rhythm.
    aorta
    the main trunk of the
    systemic arteries,
    carrying blood from the
    left side of the heart to
    the arteries of all limbs
    and organs except the
    lungs.
    THE CARDIOVASCULAR SYSTEM 229
    When the heart stops beating or its electrical rhythm is completely irregular
    and no blood is being pushed around the body, doctors may use a
    defibrillator to stimulate a normal (sinusoidal) rhythm.
    Blood pressure
    Blood pressure has two components:
  4. the degree of pressure imposed on the blood as a result of its constriction
    within the arteries and veins – known as the diastolic blood pressure
    (DBP);
  5. an additional pressure as the wave of blood pushed out from the heart
    forms flows through the system (our pulse) – known as the systolic blood
    pressure (SBP).
    This pressure is measured in millimetres of mercury (mmHg), representing
    the height of a tube of mercury in millilitres that the pressure can push up
    (using a now old-fashioned sphygmomanometer). This varies around the
    body, being at its highest close to the heart and at its lowest as the blood
    re-enters the heart. To standardise the measurement of blood pressure, it is
    usually measured at the top of the arm. Healthy levels of blood pressure
    are an SBP below 130–140 mmHg and a DBP below 90 mmHg (written as
    130/90 mmHg: see also the discussion of hypertension later in the chapter).
    A number of physiological processes are involved in controlling blood
    pressure. Those of particular interest to psychologists involve the autonomic
    nervous system. The brainstem (and the hypothalamus) receives continuous
    information from pressure-sensitive nerve endings called baroreceptors
    situated in the carotid arteries and aorta. This information is relayed to a
    centre in the brainstem known as the vasomotor centre. Reductions in blood
    pressure or physical demands such as exercise that require increased blood
    pressure causes activation of the sympathetic nervous system. Sympathetic
    activation results in increases in the strength and frequency of heart contractions
    (via the activity of the sinoatrial and atrioventricular nodes) and a
    Figure 8.8 An electrocardiograph of the electrical activity of the heart (see text for
    explanation).
    defibrillator
    a machine that uses
    an electric current to
    stop any irregular and
    dangerous activity of the
    heart’s muscles. It can
    be used when the heart
    has stopped (cardiac
    arrest) or when it is
    beating in a highly
    irregular (and
    ineffective) manner.
    baroreceptors
    sensory nerve endings
    that are stimulated by
    changes in pressure.
    Located in the walls of
    blood vessels such as
    the carotid sinus.
    carotid artery
    the main artery that
    takes blood from the
    heart via the neck to
    the brain.
    230 CHAPTER 8 • THE BODY IN HEALTH AND ILLNESS
    contraction of the smooth muscle in the arteries. Together, these actions
    result in an increase in blood pressure, and allow sustained flow of blood to
    organs such as the muscles at times of high activity. Parasympathetic activity
    results in an opposing reaction.
    What do
    YOU think?
    How do caffeine, smoking and drinking alcohol influence blood pressure?
    People who drink significant amounts of alcohol have a one and a half to two
    times greater risk of developing hypertension than those who do not drink.
    The association between alcohol and high blood pressure is particularly
    noticeable when the alcohol intake exceeds five drinks per day. In addition,
    the connection is a dose-related phenomenon. In other words, the more
    alcohol that is consumed, the stronger is the link with hypertension (e.g.
    Saremi, Hanson, Tulloch-Reid et al. 2004).
    Smoking also increases the risk of heart disease and stroke in people who
    already have hypertension, although it is not strongly associated with the
    development of hypertension. Nevertheless, smoking a cigarette can produce
    an immediate, temporary rise in the blood pressure of 5 to 10 mmHg.
    However, steady smokers may actually may have a lower blood pressure
    than non-smokers. This is because the nicotine in the cigarettes causes a
    decrease in appetite, which leads to weight loss. This, in turn, lowers the
    blood pressure. Stopping smoking can lead to increased food consumption
    and increased weight (e.g. Janzon, Heblad, Berglund et al. 2004).
    Finally, there is extensive evidence that normal levels of caffeine can
    increase blood pressure. Overall, the impact of dietary caffeine on population
    BP levels is likely to be modest – probably about 4 mmHg of diastolic
    blood pressure and 2 mmHg of systolic blood pressure. Despite this relatively
    modest increase in blood pressure, James (2004) estimated that they
    could contribute to 14 percent of premature deaths resulting from coronary
    heart disease and 20 percent of those from a stroke.
    Are the risks associated with these types of behaviour something that
    those involved in public health should act upon? Should we place health
    warnings on coffee, tea, alcohol? Should advertising warn that coffee – and
    other caffeine-containing drinks – can do you harm? What are the limits to
    health promotion and how the state advises our consumption of potentially
    harmful substances – tea, coffee, caffeine supplements, alcohol, cigarettes,
    marijuana? After all, any limit is going to be arbitrary, and something we
    should all consider.
Reklam
BU KONUYU SOSYAL MEDYA HESAPLARINDA PAYLAŞ
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