THE KIDNEYS

The kidneys are two bean-shaped organs situated behind the peritoneum on the posterior wall of the abdominal cavity, on either side of the vertebral column. The kidneys extend from about the twelfth thoracic vertebra to the third lumbar vertebra. The right kidney is situated a little lower than the left because of the space occupied by the liver.

GROSS STRUCTURE OF THE KIDNEY

A kidney (Figure 32.2) weighs 120–170 g, is 10–13 cm in length, 5–6 cm wide and 3–4 cm thick. The kidneys are protected and supported by renal fascia and by layers of perirenal fat. Three layers of tissue surround each kidney:

Figure 32.2 Macroscopic structure of the kidney

The kidney consists of three general regions (Figure 32.2):

A deep indentation, the hilus, is present on the medial border of the kidney. It is from the hilus that the ureter emerges. The renal vein and the renal artery also pass through the hilus. The renal artery branches from the aorta to transport oxygenated blood to the kidney. The renal vein transports deoxygenated blood from the kidney to the inferior vena cava.

MICROSCOPIC STRUCTURE OF THE KIDNEY

The nephron

Each kidney contains at least 1 million nephrons, together with their collecting tubules or ducts. The nephron is the functional unit of the kidney. Each nephron (Figure 32.3) is composed of a vascular and tubular system that allows for the formation of urine. The nephrons are located in the renal tissue, with most in the cortex and some extending deep into the medulla of the kidney. A nephron consists of several anatomically distinct regions:

• The glomerulus, located in the cortex and composed of a tuft of capillaries inserted into the horseshoe-shaped origin of the renal tube (the Bowman’s capsule). The outer wall of the Bowman’s capsule is composed of flattened squamous epithelium. The inner layer is made up of very specialised cells that form a semi-permeable membrane surrounding the glomerulus

• The proximal convoluted tubule — the first part of the renal tubule, situated in the cortex

• The loop of Henle, which consists of the straight portion of the proximal tubule, the straight part of the distal tubule, and the loop that joins them

• The distal convoluted tubule — the last portion of the renal tubule, which empties into a straight collecting tubule in the cortex

• The collecting tubule, which joins with other collecting tubules in the inner medulla of the kidney to eventually open into the calyces.

Figure 32.3 The nephron

Division of the efferent vessels (the arterioles that carry blood away from the capillaries) forms a second set of capillaries. The veins that collect blood from these capillaries unite to eventually form the renal vein.

FUNCTION OF THE KIDNEYS

The functions of the kidneys are to maintain homeostasis by:

In carrying out these functions the kidneys excrete urine.

THE URETERS

The ureters are two narrow, thick-walled muscular tubes, 25–30 cm long, which originate in the renal pelvis of each kidney. They pass down the posterior abdominal wall and into the pelvic cavity to enter the posterior base of the bladder. The ureters enter the bladder at an oblique angle so that, as the bladder fills and contracts, urine is not forced back towards the kidneys. The walls of the ureters consist of an outer fibrous coat that is continuous with the renal capsule, a middle coat of involuntary muscle, and a lining of transitional epithelium. The function of the ureters is to carry urine from the kidneys to the bladder, by means of peristaltic action. The involuntary muscle layer in the walls of the ureters contract from 1–5 times per minute to force urine into the bladder.

THE URINARY BLADDER

The urinary bladder is a hollow muscular organ that acts as a reservoir for urine. It sits in the pelvic cavity behind the symphysis pubis. In the female it is in front of the uterus and in the male it sits in front of the rectum. The walls of the bladder consist of:

The trigone of the bladder is a triangle formed by the two ureteric orifices, where the ureters enter the bladder, and the urethral orifice, where the urethra leaves. Below the neck of the bladder lies the striated muscle that constitutes an external sphincter.

THE URETHRA

The urethra (Figure 32.4) is a muscular tube extending from the neck of the bladder to the external meatus. In the female the urethra is about 2.5–4 cm long and opens at the external urethral orifice in front of the vaginal opening. The external sphincter guards this opening. In the male, the urethra is about 15–20 cm long and opens at the tip of the penis. The male urethra has a double function: it forms a passage for urine as well as semen. It is guarded by an external sphincter immediately below the prostatic portion of the urethra. Near to the bladder, the urethra is lined with transitional epithelium that gives way to squamous epithelium. The function of the urethra is to provide a passage for urine from the bladder, out of the body.

Figure 32.4 Position of the male and female urethra

MICTURITION

Micturition (or voiding) is the act of passing urine. Urine is carried down the ureters by peristaltic waves, each of which sends a spurt of urine into the bladder. While the normal capacity of the adult urinary bladder is about 450 mL, the bladder is capable of expanding to hold larger amounts. The bladder fills slowly over a period of time and, when it holds about 300–500 mL, a desire to empty the bladder is experienced. A reflex initiated by impulses from stretch receptors in the bladder wall regulates the process of micturition. The sensory neurons transmit the impulses to the spinal cord, where they are relayed to the brain, which in turn stimulates parasympathetic neurons that innervate the bladder wall. When a client is ready to pass urine, these impulses cause the bladder muscles to contract, the urethral sphincters relax and urine is voided into the urethra and excreted. Urine is excreted in amounts of 1–2 L in 24 hours. Micturition is normally under voluntary control by about 2–3 years of age.

URINE COLLECTION

The nurse may be required to collect urine for observation or testing in the ward, or so that a specimen can be sent to the pathology laboratory for analysis. Key aspects related to the collection of urine include:

Observation and urinalysis

If urine is to be collected for observation or testing in the ward, the client is requested to void into a clean bedpan or urinal (Figure 32.5). The urine should not be contaminated by faeces or menstrual blood, as a false assessment may result. The urine is transferred from the toilet utensil into a clean container in preparation for observation or testing. (For more detail regarding performing a urinalysis, see Chapter 22.)

Figure 32.5 Positioning a bedpan A: Another method of placing a bedpan is used when the client is unable to help. The client lies on one side and the bedpan is placed firmly against the buttocks B: The nurse pushes down on the bedpan and towards the client C: The client is positioned on the bedpan so that the urethra and anus are directly over the opening

ALTERED AMOUNTS

The term oliguria is used to describe decreased urine output of below 500 mL/day. This condition is caused by fluid and electrolyte imbalances, kidney dysfunction or urinary tract obstruction. Polyuria is the term used to describe the excretion of greatly increased amounts of urine, for example, over 2500 mL/day. It may occur as a result of a large fluid input, diuretic medications, or certain disease states such as diabetes insipidus.

SUPPRESSION OF URINE FORMATION

The term anuria refers to the cessation of urine excretion by the kidneys, or the production of less than 100 mL/day, and may result from kidney dysfunction or failure.

URINE RETENTION

When there is an accumulation of urine in the bladder and the client is unable to void, the condition is referred to as retention of urine. Retention of urine is commonly due to an obstruction of the bladder outlet or urethra, and it may also occur as a result of tension in the urethral sphincter, caused by anxiety or pain. Retention of urine with overflow occurs when the accumulation of urine in the bladder leads to stretching of the urethral orifice and, although small amounts of urine dribble away, the bladder remains full and distended.

INCONTINENCE OF URINE

Incontinence is the inability to control the excretion of urine and may result from a variety of local or generalised conditions that include:

DYSURIA

Dysuria is the term used to describe painful micturition and is commonly due to a UTI or obstruction.

FREQUENCY OF MICTURITION

Frequency of micturition is when a client experiences the need to void more often than normal and commonly voids small amounts of urine each time. This condition is commonly associated with a UTI or may occur as a result of anxiety or stress.

INSERTION OF A CATHETER

The procedure of catheter insertion is called catheterisation and is performed using sterile equipment and aseptic technique. As a catheter can cause trauma to the urethral or bladder mucosa and is a potential source of UTI, it is inserted only when absolutely necessary and only by an RN or a medical officer. Because the male urethra is longer and more curved than the female urethra and catheterisation is often more difficult, male clients are usually catheterised by a health worker skilled in this procedure. A suggested procedure for female catheterisation is outlined in Table 32.2. Nurses should be aware of their role and responsibilities regarding catheter insertion in the workplace. The basic requirements are:

TABLE 32.2 GUIDELINES FOR CATHETERISATION OF FEMALES^*

* This procedure is only performed by an RN or a medical officer

If the catheter is to be indwelling, the following are also needed:

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