Cardiovascular disorders

Chapter 26 Cardiovascular disorders



Cardiovascular disease is an enormous health problem in the Western world and is high in the list of causes of death. Evidence from clinical trials suggests that prevention of hypertension and lipidaemia (see Chapter 27 ‘Problems with lipid metabolism’, p. 203) can significantly reduce cardiovascular disease and deaths.



Physiology of the Heart


The cardiovascular system is effectively a closed system with control mechanisms that normally prevent the pressure becoming too high or too low. The heart is controlled by two main factors:





Hydrostatic Pressure


A constant hydrostatic pressure is maintained in two main ways:



2. Varying the volume of blood pumped out by the heart (inotropic activity): after the heart has contracted, there is negative pressure. Because the cardiovascular system is a pressurized system, the blood flows into the heart under pressure and starts to stretch the muscles of the heart wall (Figure 26.1). The amount of stretch created by the venous return of the blood is called preload. This elicits a response from the cardiac muscle wall, which encourages the heart stroke. The more stretch, the greater the volume of blood pumped out by the heart. If the heart muscle is stretched too much it becomes dilated and the force of contraction decreases and the heart goes into failure.


Cardiac glycosides in digitalis, digoxin and Crategus (hawthorn) increase the force of contraction of the heart (see Chapter 24 ‘Glycosides’, p. 183), which is important in patients with heart failure.



• How the Kidneys can Affect Blood Pressure


The kidneys perform two functions for the cardiovascular system:




The two are interlinked.



Stages in the Removal of Water from the Body


Figure 26.2 shows the structure of a nephron, the functional unit of the kidney. The numbers in the figure are explained below.




(i) Filtration

Blood enters the glomerulus of the kidney under pressure. This pressure forces the water and small molecules (e.g. sodium, potassium, glucose) that are present in the blood through the basement membrane of the glomerulus and into the Bowman’s capsule (Figure 26.2). At this stage, the filtered fluid is mainly blood plasma minus largely all the plasma proteins (there is some seepage of these, but not much). Factors such as heavy exercise can increase the amount of protein found in the urine, but a large amount of protein in the urine indicates kidney pathology. The fluid flows from the Bowman’s capsule into the proximal tubule.






(v) Collecting Tubule

This is where the final balancing of water and electrolytes takes place, before the fluid is removed from the body. Potassium-sparing diuretics (Figure 26.5) work here, as well as in the distal convoluted tubules. The collecting tubule is normally impermeable to water, but it can become permeable in the presence of antidiuretic hormone (ADH), which is secreted by the pituitary gland (see Chapter 37 ‘Metabolic disorders’, p. 288). As much as 75% of water in the urine can be reabsorbed as it leaves the collecting duct by osmosis. A failure to produce ADH (due to a fault in the posterior pituitary gland) is seen in the condition known as diabetes insipidus, which involves excretion of copious amounts of urine, despite reduced consumption of fluids.




Jul 22, 2016 | Posted by in PHARMACY | Comments Off on Cardiovascular disorders

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