7: Renal disorders

Chapter 7 Renal disorders



CHAPTER CONTENTS


Introduction 89

Relevant anatomy and physiology 89
Glomerular filtration 90

Selective tubular reabsorption 90

Tubular secretion 91

Other functions of the nephron 91

Other structures of the renal system 92

Pathophysiology 92
Aetiology 92

Pathophysiology 93

Signs and symptoms 94

Diagnosis 95

Treatment 96

The renal system in pregnancy 96
Introduction 96

Physiological changes in pregnancy 97

Incidence 97

Effect on fetal outcome 99

Midwife’s management 99

References 100


INTRODUCTION


Urinary tract infections are much more common in women than men. One of the main reasons for this is the comparatively short urethra. Poor hygiene and sexual intercourse can both result in ascending infection from a contaminated perineum. The causative organisms are commonly of gastrointestinal origin. In pregnancy, this is aggravated by the mechanical pressure of the enlarging uterus on the renal system and the relaxing effect of the hormones on the smooth muscles of the urinary tract.


Additionally, the kidneys can be compromised before a pregnancy even begins. Recurrent infections or other disease processes can both affect renal function. Major pathophysiological events during the childbearing process, such as haemorrhagic or endotoxic shock can also result in renal disease or failure. The midwife must be able to recognize signs of renal compromise and understand the effect this has on the childbearing process.



RELEVANT ANATOMY AND PHYSIOLOGY


The renal system is composed of (Fig. 7.1):


• Two kidneys, which produce urine

• Two ureters, which transport urine to the bladder

• The bladder, which stores urine and

• The urethra, which disposes of urine at a convenient time.

image

Figure 7.1 Gross structure of the renal system.


The kidney contains the functional units of the renal system – the nephrons. These microscopic structures, of which there are approximately 1 million, are responsible for several related functions. They filter and excrete water, electrolytes and nitrogenous waste products and maintain acid-base balance. They also play a major role in blood pressure control via the renin–angiotensin pathway, and produce erythropoietin factor, a substance involved in the production of red blood cells.


To achieve these major functions, the kidneys receive 20–25% of arterial blood directly fromthe aorta. The majority of this blood is transported directly to the nephrons where urine is produced. The anatomy of the nephron is shown in Figure 7.2. Three major processes are involved in urine production:


1. Glomerular filtration

2. Selective tubular absorption

3. Tubular secretion.

image

Figure 7.2 Structure of nephron indicating processes involved in urine production.



Glomerular filtration


Filtration is the process by which water and dissolved substances move across a membrane under pressure. The afferent arteriole of the Bowman’s capsule has a comparatively larger diameter than the efferent arteriole which leads away from the capsule. This anatomical arrangement produces an increased pressure in the capillaries contained within the glomerulus thus providing the pressurerequired to drive filtration. Approximately 10% of the blood entering the glomerulus is filtered including the majority of the components of blood. Only those substances which are too large to pass through the walls of the capsule and capillaries, such as blood cells and proteins are normally retained in the circulating blood.


Net filtration pressure is the driving force of this process and is made up of three components: hydrostatic pressure in both the glomerular capillaries and Bowman’s capsule, and osmotic pressure (Fig. 7.3). The filtrate that is produced is very dilute and contains both useful and waste products.


image

Figure 7.3 Pressures involved in the process of filtration.



Selective tubular reabsorption


The second process reclaims substances useful or necessary to the body. Tubular reabsorption occurs along the length of the tubule of the nephron but there are three distinct sections that undertake different aspects of this process:


1. The proximal convoluted tubule reabsorbs water and solutes by both active and passive processes.

2. The loop of Henle passes through a concentration gradient which concentrates the urine without loss of valuable solutes.

3. The distal convoluted tubule further reabsorbs substances required by the body.


Tubular secretion


This third process is responsible for actively transporting substances from blood into the urine. Thus, excess hydrogen and potassium ions, and certain drugs that have escaped the filtration process are removed from the body. This process plays a vital role in acid-base balance.



Other functions of the nephron



Concentration of urine


The nephron is involved in several additional physiological processes. Fluid and electrolyte balance is achieved by the action of aldosterone and antidiuretic hormone. Aldosterone controls sodium reabsorption in the distal tubule. Antidiuretic hormone is responsible for the permeability of the distal tubule and collecting duct to water, and thus regulates the concentration of urine.



Blood pressure regulation


Another physiological process, in which the nephron plays a vital role, is in blood pressure regulation. Anatomically, the final section of the ascending limb of the loop of Henle comes into close proximity with the afferent arteriole of the nephron (Fig. 7.4). At this point, the walls of both structures contain specialized cells forming the juxtaglomerular apparatus. The cells in the nephron make up the macula densa – a compact collection of cells that measure the amount of sodium in the filtrate. The corresponding cells in the arteriole, the juxtaglomerular cells, contain muscle fibres and granules of renin.


image

Figure 7.4 The juxtaglomerular apparatus (JGA). (a) Position of JGA. (b) Microstructure of JGA.


Renin is released into the bloodstream when the juxtaglomerular apparatus detects a decrease in circulating blood and/or decreased sodium levels in the glomerular filtrate. Renin reacts with an inactive fraction of plasma globulin, angiotensin I, which is then converted in the lungs into angiotensin II. This substance circulates in the blood acting on the smooth muscle of arterioles resulting in vasoconstriction. It also stimulates the secretion of aldosterone (thus retaining sodium in the kidneys) and to a lesser extent glucocorticoids. Glucocorticoids are involved in the production of glucose when blood sugar levels are low.



Manufacture of erythrocytes


Erythropoietin factor is produced in the kidneys. This substance is secreted into the bloodstream in response to hypoxia where it reacts with a plasma protein to produce erythropoietin. Erythropoietin stimulates the bone marrow to produce erythrocytes.



Synthesis of vitamin D


Calcitriol is activated in the kidneys. This substance is a component of vitamin D and requires conversion in the kidney to an active form before it can fulfil its role of stimulating calcium absorption in the gut.



Other structures of the renal system


Two ureters lead from the kidneys and drain urine into the bladder where it is stored. The bladder can hold approximately 600 mL but the desire to micturate is conveyed to the nervous system once 300 mL of urine is present. Finally, the urethra leaves the base of the bladder passing through the muscles of the perineum to the urethral orifice. As indicated above, the female urethra is considerably shorter than in the male which predisposes women to ascending urinary tract infections.



PATHOPHYSIOLOGY


Generally humans can survive with only one fully functional kidney. Only when disease processes damage a large proportion of the functional units, the nephrons, will renal function become impaired.


Disease processes of the renal system can be considered from two different aspects; infection of tissue with the potential for damage to the renal system, and actual damage to tissue from a disease process. For ease of understanding, pathophysiology will be discussed from these two viewpoints although it must be clearly understood by the midwife that each condition can lead to the other.



Aetiology



Infection


Bacteria from the gastrointestinal tract are the commonest causative agents in urinary tract infection. Often this is Escherichia coli. As a result of the close proximity of the urethral meatus to the anus, contamination of the perineum spreads easily into the urethra particularly if personal hygiene is poor. Abrasion of the urinary meatus during sexual intercourse can also result in entry of pathogens to the renal system. The short length of the urethra then enables infective organisms to reach the bladder easily and proliferate. Infection in the bladder may spread to the ureters and on into the kidney itself causing inflammation, pyelonephritis and possible damage to the nephrons themselves.



Chronic renal disease


Damage to renal tissue can be caused by a variety of events. Recurrent infections can lead to loss of functional tissue over time. Renal damage can occur secondary to another disorder, such as diabetes or hypertension. Structural defects of the renal system such as congenital abnormality or abnormalities such as tumours or polycystic kidneys will also cause long-term dysfunction. Glomerulonephritis is an autoimmune disease which may occur without obvious cause or secondary to a viral infection.



Acute renal failure


The kidneys are particularly vulnerable during any event where there is circulatory shut down (Perkins & Kisel 2005). Acute blood loss, toxic shock from overwhelming infection can both lead to acute renal failure as can a urinary obstruction or damage resulting in backflow of urine.



Pathophysiology


As already discussed, bacteria can enter the renal tract comparatively easily in women. Infection of the urinary tract will cause inflammation of affected tissues. Inflammation to the ureters, bladder and urethra will result in discomfort and pain. However, these structures are largely responsible for transport and storage of urine and thus inflammation will not have a major impact on renal function. If the infection reaches the kidneys however, this will result in disruption of the normal processes taking place in the nephrons.


Two important processes may be compromised by inflammation in the renal system. Fluid and electrolyte imbalance may occur as the inflamed tubules of the nephron become less efficient at reabsorbing sodium, which is then retained in body tissues and fluids. With retention of sodium, water is also retained, resulting in oedema and/or increased cardiac workload. Acid-base balance will also be affected as secretion of buffering substances is compromised and hydrogen ions are retained in the body (Box 7.1).



Box 7.1 Acid-base balance


Health and good body functioning are dependent both on fluid and electrolyte balance and acid-base balance. The renal system plays a vital role in the latter. Biochemical reactions in the cells of the body are very sensitive to even small changes in the pH of extracellular fluids (ECF). Normal pH of plasma and ECF is 7.35–7.45, which is slightly alkaline. Yet the foods and fluids we take into our bodies, and those present in some organs of the body, have a very different pH. Gastric juices in the stomach range from 1.2–3. Orange juice is 3.5; coffee is 5 and antacids 9–10.


The renal organs as well as other systems of the body therefore have the responsibility of maintaining the acid-base balance and the renal system is a vital part of this process.


Acid-base balance is maintained through three systems:


1. Buffering: A buffer is a chemical ‘sponge’ soaking up excess H+ or releasing them. Examples of body buffers are bicarbonate, phosphate, proteins. Buffers work within a matter of seconds.

2. Respiration: Excess H+ combine with othersubstances in the blood to produce CO2, whichis then excreted through an increase in ventilation rate. This process corrects imbalances in a matter of minutes.

3. The renal system acts over hours and days tostabilize a prolonged imbalance. The distaltubules remove excess H+.

If there is renal dysfunction therefore, acid-basebalance will be disturbed causing illness andpotentially death.


Chronic renal disease is usually the result of secondary damage to the kidneys as a result of another disease process. Commonly, this is due to hypertension but other causes are polycystic kidneys and glomerulonephritis. Examples of these related to pregnancy are:


• Diabetes in which the high level of circulating glucose encourages atherosclerosis resulting in diminished blood flow to the kidneys. The juxtaglomerular apparatus detects this, releases renin, vasoconstriction occurs and blood pressure rises. This further damages the glomerulus. Additionally, any infective organism given access to the renal system will proliferate in the tissues due to the increased glucose levels and this will further compromise the situation.
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Jul 11, 2016 | Posted by in MIDWIFERY | Comments Off on 7: Renal disorders

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