Chapter 6: Elimination
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Urinary elimination
Definition
Anatomy and physiology
The urinary tract (see Figure 6.1) consists of the two kidneys, the two ureters, the bladder and the urethra. The urinary system produces, stores and eliminates urine. The kidneys are responsible for excreting wastes such as urea and ammonium and for the reabsorption of glucose and amino acids. They are involved in the production of hormones including calcitrol, renin and erythropoietin. The kidneys also have homeostatic functions, including the regulation of blood volume and electrolytes, acid‐base balance and blood pressure (Tortora and Derrickson [237]).
Figure 6.1 The (a) male and (b) female genitourinary tracts.
Each kidney excretes urine into a ureter, which arises from the renal pelvis on the medial aspect of each kidney. In adults, the ureters are approximately 25–30 cm long and 2–4 mm in diameter (Tortora and Derrickson [237]). The ureters are muscular tubes that propel urine from the kidneys to the urinary bladder. They enter through the back of the bladder, running within the wall of the bladder for a few centimetres. Ureterovesical valves prevent the backflow of urine from the bladder to the kidneys.
The bladder sits on the pelvic floor and is a hollow, muscular, distensible organ that stores urine until it is convenient to expel it. Urine enters the bladder via the ureters and exits via the urethra. As the bladder fills, stretch receptors in the muscular wall signal the parasympathetic nervous system to contract the bladder, initiating the conscious desire to expel urine. In order for urine to be expelled, both the automatically controlled internal sphincter and the voluntarily controlled external sphincter must be opened.
Urine leaves the bladder via the urethra. In females this lies in front of the anterior wall of the vagina and is approximately 3.8 cm long. In males it passes through the prostate and penis and is approximately 20 cm long (Tortora and Derrickson [237]).
