Micturition

Introduction

Micturition is the process by which the urinary bladder empties its contents, and it is more commonly known as urination. This process involves two steps: (1) the bladder fills until wall tension exceeds a certain threshold level and (2) a nervous reflex known as the micturition reflex occurs, and the bladder empties. The micturition reflex is an autonomic spinal cord reflex, occurring independently of signals higher up in the central nervous system. Micturition can be inhibited or facilitated by centers in the cerebral cortex or the brain stem. An individual may consciously suppress the desire to urinate created by the micturition reflex or may attempt to urinate even without such a feeling of urgency.

System structure: Anatomy of the urinary system

The fluid that pools in the renal calyces is the same urine that exits the body during micturition. No further reabsorption or secretion of solutes, and no further transport of water across cell membranes, occurs once the urine passes through the collecting ducts of the kidney. Urine is transported from the kidneys to the urinary bladder via two muscular tubes known as ureters ( Fig. 23.1 ). The bladder is a smooth muscle chamber that has two main parts. The body, also known as the fundus, is the main part of the bladder and acts as a storage chamber for the urine. The bladder neck, also known as the posterior urethra, is a funnel-shaped-shaped extension of the body of the bladder. This extension continues as the urethra, which opens to the outside of the body.

The smooth muscle of the bladder is known as the detrusor muscle. These muscle fibers extend in all directions around the bladder—longitudinally, radially, and spirally—without distinct layers, and the muscle cells themselves are fused together, similar to the syncytium of cardiac muscle. These muscle patterns create low-resistance electrical pathways between the cells and allow for a given action potential to spread quickly throughout the entire detrusor muscle, thereby producing a synchronous bladder contraction for micturition.

The mucosa of the bladder consists of a transitional epithelium, which includes basal columnar cells on the outside, intermediate cuboidal cells, and superficial squamous cells on the inside ( Fig. 23.2 A). When the bladder is empty or underfilled, even the superficial cells are slightly rounded. When the bladder is filled, these superficial cells stretch and flatten into the classic squamous shape. In addition to this distensibility at the histologic level, the bladder mucosa also maintains an anatomic level of distensibility ( Fig. 23.2 B). The mucosal surface at rest is grossly folded into rugae, similar to that of the stomach, and can also stretch and flatten to accommodate increases in urine volume. These two levels of distensibility enable the bladder to expand in volume without significantly increasing the pressure inside.

Urine enters the bladder through two ureters and leaves through a single urethra. These three ports make up the angles of the trigone, a small triangular area on the posterior wall of the bladder immediately above the neck (see Figs. 23.1 B and C). The mucosa of the trigone is distinct from that of the rest of the bladder. Whereas the majority of the bladder mucosa is folded into rugae, the trigone mucosa is smooth, regardless of the volume of urine. The ureters enter obliquely through the detrusor muscle, coursing 1 to 2 cm beneath the bladder mucosa before emptying into the bladder along the two upper angles of the trigone. This oblique extended course through the bladder wall helps prevent vesicoureteral reflux, the retrograde flow of urine from the bladder into the ureters. The posterior urethra constitutes the lowermost apex of the trigone. In the male, the posterior urethra leads to the anterior urethra, which extends through the penis before opening to the outside of the body at the external meatus. In the female, the urinary tract nearly ends with the posterior urethra as it leads almost directly to the outside of the body. Females have higher risk of urinary tract infections because of the short urethra (see Clinical Correlation Box 23.1 ).

The bladder neck consists of the inferior 2 to 3 cm of the bladder. Its wall, like the wall of the bladder body, comprises the detrusor muscle; however, the muscle here is organized into distinct layers to form the internal sphincter of the bladder. Sympathetic tone supplied to the internal sphincter through the hypogastric nerve keeps the internal sphincter tonically contracted. Additional control of micturition is located beyond the bladder neck as the urethra passes through the urogenital diaphragm. Here, the external sphincter of the bladder, a voluntary muscle layer as opposed to the smooth muscle of the body and neck of the bladder, can be consciously controlled to prevent or interrupt urination (see Fig. 23.2 ).