Source: OpenStax (2013) Anatomy and Physiology. Rice University. Available at: https://openstax.org/books/anatomy-and-physiology/pages/1-introduction
Biochemical steps in the RAAS
The RAAS is triggered whenever there is a drop in blood pressure and follows a set series of events, which are highlighted below.
When BP falls, the kidneys release an enzyme termed renin (Chapter 11).
Renin rapidly converts angiotensinogen into the protein angiotensin-I.
Angiotensin-1 is inert and has no biological activity; it circulates freely in the blood until it reaches the lungs.
Within the lung tissue are located the angiotensin-converting enzymes (ACEs). These convert the inert angiotensin-I into its biologically active form angiotensin-II.
Angiotensin-II binds to receptors on smooth muscle cells in the tunica media (muscle layer) of arteries to initiate vasoconstriction. The process of vasoconstriction increases peripheral resistance (PR), helping to restore BP (remember, this mechanism was triggered by a drop in BP).
Angiotensin-II stimulates the release of aldosterone from the adrenal cortex.
Aldosterone promotes sodium retention by the kidneys, increasing plasma sodium levels. Sodium attracts water from the tissues into the blood (water follows sodium) by osmosis, increasing blood volume and pressure.
Angiotensin-II stimulates the release of antidiuretic hormone (ADH) from the posterior pituitary gland. As described previously, ADH increases blood volume by reducing urine output while simultaneously inducing vasoconstriction.
The end result of the RAAS is that blood pressure is restored and maintained.
Now you have completed this chapter, attempt the multiple-choice questions in Activity 3.5 to assess your knowledge.
Activity 3.5 Multiple-choice questions
1 Which of the following layers of the heart is composed predominantly of branched cardiac muscle fibres?a) The endocardiumb) The epicardiumc) The myocardiumd) The visceral pericardium
2 The bicuspid (mitral valve) is locateda) Between the left atrium and left ventricleb) Between the right atrium and right ventriclec) At the origin of the pulmonary arteriesd) At the origin of the aorta
3 During which phase of the cardiac cycle does 70 per cent of the atrial blood volume pass into the ventricles?a) Isovolumetric contractionb) Passive ventricular fillingc) Ejectiond) Atrial systole
4 Which of the following resting heart rates would be referred to as tachycardia?a) 90 bpmb) 45 bpmc) 112 bpmd) 75 bpm
5 Which of the following areas of the cardiac conductive system acts as the heart’s natural pacemaker?a) The atrioventricular bundle (bundle of His)b) The Purkinje fibresc) The atrioventricular node (AVN)d) The sinoatrial node (SAN)
6 Angina pectoris is usually associated with occlusion ofa) The aortab) The coronary arteriesc) The carotid arteriesd) The pulmonary arteries
7 Which of the following blood vessels do not carry oxygenated blood?a) The pulmonary veinsb) The coronary arteriesc) The aortad) The pulmonary arteries
8 The major baroreceptors that continually measure arterial blood pressure are located ina) The vena cavaeb) The pulmonary arteriesc) The jugular veinsd) The aortic arch and carotid sinuses
9 Antidiuretic hormone (ADH), which is also known as vasopressin, is released whena) Blood pressure decreasesb) The blood is diluted by drinking too much waterc) The kidneys need to eliminate calcium (Ca)d) Blood pressure increases
10 Which of the following statements relating to the renin angiotensin aldosterone system (RAAS) is true?a) The system is activated when blood pressure is highb) Angiotensinogen is produced by the kidneyc) Angiotensin-II is a powerful vasoconstrictord) Renin is produced by the liver
Chapter summary
The cardiovascular system, consisting of the heart and blood vessels, functions as the major transport system. Blood acts as the transport medium and is continually circulated throughout the body in the blood vessels. Arteries are muscular, thick-walled vessels that usually carry oxygenated blood under high pressure away from the heart, while veins are thin-walled blood vessels equipped with valves that usually carry deoxygenated blood under low pressure towards the heart.
Capillaries are the smallest blood vessels and found in complex networks termed capillary beds which permeate the tissues of the body and function to distribute blood. The right-hand side of the heart pumps deoxygenated blood to the lungs via the pulmonary circuit, while the left-hand side pumps oxygenated blood to all other areas (organ systems) via the systemic circuit. The heart is anchored in a relatively central position within the thorax (retrosternal) and protected by a compound membrane termed the pericardium. To function as an efficient pump, the chambers of the heart contract and relax in a five-phase sequence termed the cardiac cycle.
The events of the cardiac cycle are precisely timed and coordinated by the cardiac conductive system. The electrical activity of this system can be recorded on an electrocardiogram. Blood pressure (BP) is a product of cardiac output (CO) and the peripheral resistance (PR) afforded by the blood vessels (BP = CO × PR). Ideally, blood pressure is maintained at around 120/80 mmHg by a combination of neural and hormonal mechanisms.
Activities: Brief outline answers
Activity 3.1: Research and revision (page 55)
Sequence of blood flow through major blood vessels of the heart:
Vena Cavae → Right Atrium → Tricuspid Valve → Right Ventricle → Pulmonary Artery → Pulmonary Valve → Lungs → Pulmonary Veins → Left Atrium → Bicuspid Valve → Left Ventricle → Aortic Valve → Aortic Arch
You may find it useful to make up a mnemonic or other memory aid to help remember this sequence.
Activity 3.2: Communication (page 63)
Atrial fibrillation (AF) is an arrhythmia that becomes increasingly common with advancing age. During AF the atria will be contracting rapidly and in an uncoordinated manner, which can result in poor ventricular filling. AF is often asymptomatic when at rest since 70 per cent of ventricular filling occurs passively; however, Gerald has been experiencing symptoms of breathlessness during exercise (when gardening and walking upstairs). It is during exercise that the full filling of the ventricles becomes increasingly important.
With AF the final 30 per cent of ventricular filling may not occur and as a result the cardiac output (CO) may fall, leading to the breathlessness Gerald is experiencing. AF can cause turbulent blood flow in the atria, increasing the risk of thrombosis. It is almost certainly a mobile clot (embolus) that has caused the stroke which Gerald has just suffered. Although Gerald has not recently been compliant in taking his medication, as a nurse it is vital to reinforce how essential apixiban is to managing his AF since it is this medication that will reduce the chances of clots forming and further strokes occurring.
Activity 3.3: Critical thinking (page 70)
Based on Gloria’s prior history, it is clear that she has long-standing coronary artery disease. This would explain why she has experienced chest pain during exertion which is indicative of angina. Her past history of infarction indicates that portions of her heart muscle (myocardium) have been damaged. This new issue of swollen ankles and feet is worrying since peripheral oedema is a clinical feature of right-handed heart failure.
It may be that despite having stents fitted, Gloria has suffered further infarctions or the damage caused