Basic science of relevance to urological practice

Physiology of bladder and urethra 660

Renal anatomy: renal blood flow and renal function 661 Renal physiology: regulation of water balance 664 Renal physiology: regulation of sodium and potassium

excretion 665

Renal physiology: acid–base balance 666

660 CHAPTER 17 Basic science in urological practice

Physiology of bladder and urethra

Bladder

The bladder consists of an endothelial lining (urothelium) on a connective tissue base (lamina propria), surrounded by smooth muscle (the detrusor), with an outer connective tissue, adventitia.

The urothelium consists of a multilayered transitional epithelium. It has numerous tight junctions that render it impermeable to water and solutes. The detrusor muscle is a homogeneous mass of smooth muscle bundles.

The bladder base is known as the trigone—a triangular area with the two ureteric orifices and the internal urinary meatus forming the corners. Intravesical pressure during filling is low. The main excitatory input to the bladder is via parasympathetic innervation (S2–4; cholinergic postganglionic fibers that when activated cause contraction; see p. 500).

Urethra

The bladder neck is normally closed during filling. It is composed of a circular smooth muscle (sympathetic innervation). High pressure is generated at the midpoint of the urethra in women, and at the level of the membranous urethra in men, where the urethral wall is composed of a longitudinal and circular smooth muscle coat, surrounded by striated muscle (external sphincter).

The striated part of the sphincter receives motor innervation from the somatic pudendal nerve (S3,4) and has voluntary control (ACh, or acetylcholinesterase mediates contraction).

The smooth muscle component of the sphincter has myogenic tone and receives excitatory and inhibitory innervation from the autonomic nervous system. Contraction is enhanced by sympathetic input (noradrenaline) and ACh. Inhibitory innervation is nitrergic (nitric oxide).

Micturition

Voiding is mediated by the pontine micturition center in the brain. During urine storage, bladder neck and sphincter smooth muscle are constricted, and ganglia in the bladder wall are inhibited by sympathetic input, while somatic innervation causes contraction of the striated sphincter muscle. As the bladder fills, sensory nerves respond to stretch and send information about bladder filling to the CNS.

At a socially acceptable time, the voiding reflex is activated. Stimulation of detrusor smooth muscle by parasympathetic anticholinergic nerves causes the bladder to contract. Simultaneous activation of nitrergic nerves reduces the intraurethral pressure, inhibition of somatic input relaxes the striated sphincter muscle, and sympathetic inhibition causes coordinated bladder neck and sphincter smooth muscle relaxation, resulting in bladder emptying.

RENAL ANATOMY: RENAL BLOOD FLOW AND RENAL FUNCTION 661

Renal anatomy: renal blood flow and renal function

The kidneys and ureters lie within the retroperitoneum (literally behind the peritoneal cavity). The hila of the kidneys lie on the transpyloric plane (vertebral level—L1).

Each kidney is composed of a cortex, surrounding the medulla, which forms projections—papillae, which drain into cup-shaped epithelial-lined pouches called calyces (the calyx draining each papilla is known as a minor calyx, and several minor calyces coalesce to form a major calyx, several of which drain into the central renal pelvis).

The renal artery, which arises from the aorta at vertebral level L1/2, branches to form interlobar arteries, which in turn form arcuate arteries, and then cortical radial arteries from which the afferent arterioles are derived. Venous drainage occurs into the renal vein.

There are two capillary networks in each kidney—a glomerular capillary network (lying within Bowman’s capsule) that drains into a peritubular capillary network, surrounding the tubules (proximal tubule, loop of Henle, distal tubule, and collecting ducts).

The anterior relations of the right kidney are, from top to bottom, the suprarenal gland, the liver, and the hepatic flexure of the colon. Medially, anterior to the right renal pelvis is the second part of the duodenum.

The anterior relations of the left kidney are, from top to bottom, the suprarenal gland, the stomach, the spleen, and the splenic flexure of the colon. Medially lies the tail of the pancreas.

The posterior relations of both kidneys are, superiorly, the diaphragm and lower ribs and, inferiorly (from lateral to medial), transversus abdominis, quadratus lumborum, and psoas major.

Renal blood flow (RBF)

The kidneys represent <0.5% of body weight, but they receive 25% of cardiac output (~1300 mL/min through both kidneys; 650 mL/min per kidney). Combined blood flow in the two renal veins is about 1299 mL/min, and the difference in flow rates represents the urine production rate (i.e., ~1 mL/min).

Autoregulation of RBF

RBF is defined as the pressure difference between the renal artery and renal vein divided by the renal vascular resistance. The glomerular arterioles are the major determinants of vascular resistance.

RBF remains essentially constant over a range of perfusion pressures (~80–180 mmHg) (i.e., RBF is autoregulated).

Autoregulation requires no innervation and probably occurs via the following:

•A myogenic mechanism (increased pressure in the afferent arterioles causes them to contract, thereby preventing a change in RBF)

•Tubuloglomerular feedback—the flow rate of tubular fluid is sensed at the macula densa of the juxtaglomerular apparatus (JGA), and in some way this controls flow through the glomerulus to which the JGA is opposed.