Anatomy and Physiology of the Urinary System


The kidneys lie on either side of the spine in the retroperitoneal space between the parietal peritoneum and the posterior abdominal wall, well protected by muscle, fat, and ribs. They are roughly the size of your fist. The male kidney is typically a bit larger than the female kidney. The kidneys are well vascularized, receiving about 25 percent of the cardiac output at rest. Figure 13.1 displays the location of the kidneys.


Diagram of a human torso showing the location of the kidneys within the torso.
Figure 13.1 Kidneys. The kidneys are slightly protected by the ribs and are surrounded by fat for protection (not shown). From Betts, et al., 2013. Licensed under CC BY 4.0.

Kidneys’ Internal Structure

A frontal section through the kidney reveals an outer region called the renal cortex and an inner region called the medulla (see Figure 13.2). The renal columns are connective tissue extensions that radiate downward from the cortex through the medulla to separate the most characteristic features of the medulla, the renal pyramids and renal papillae. The papillae are bundles of collecting ducts that transport urine made by nephrons to the calyces of the kidney for excretion. The renal columns also serve to divide the kidney into 6–8 lobes and provide a supportive framework for vessels that enter and exit the cortex. The pyramids and renal columns taken together constitute the kidney lobes.


Left kidney diagram. Image description available.
Figure 13.2 Left Kidney. From Betts, et al., 2013. Licensed under CC BY 4.0. [Image description.]

Renal Hilum

The renal hilum is the entry and exit site for structures servicing the kidneys: vessels, nerves, lymphatics, and ureters. The medial-facing hila are tucked into the sweeping convex outline of the cortex. Emerging from the hilum is the renal pelvis, which is formed from the major and minor calyces in the kidney. The smooth muscle in the renal pelvis funnels urine via peristalsis into the ureter. The renal arteries form directly from the descending aorta, whereas the renal veins return cleansed blood directly to the inferior vena cava. The artery, vein, and renal pelvis are arranged in an anterior-to-posterior order.

Nephrons and Vessels

The renal artery first divides into segmental arteries, followed by further branching to form interlobar arteries that pass through the renal columns to reach the cortex (see Figure 13.3). The interlobar arteries, in turn, branch into arcuate arteries, cortical radiate arteries, and then afferent arterioles. The afferent arterioles service about 1.3 million nephrons in each kidney.


This figure shows the network of blood vessels and the blood flow in the kidneys.
Figure 13.3 Blood Flow in the Kidney. From Betts, et al., 2013. Licensed under CC BY 4.0.

Nephrons are the “functional units” of the kidney; they cleanse the blood and balance the constituents of the circulation. The afferent arterioles form a tuft of high-pressure capillaries about 200 µm in diameter, the glomerulus. The rest of the nephron consists of a continuous sophisticated tubule whose proximal end surrounds the glomerulus in an intimate embrace—this is Bowman’s (glomerular) capsule. The glomerulus and Bowman’s capsule together form the renal corpuscle. As mentioned earlier, these glomerular capillaries filter the blood based on particle size. After passing through the renal corpuscle, the capillaries form a second arteriole, the efferent arteriole (see Figure 13.4). These will next form a capillary network around the more distal portions of the nephron tubule, the peritubular capillaries and vasa recta, before returning to the venous system. As the glomerular filtrate progresses through the nephron, these capillary networks recover most of the solutes and water and return them to the circulation.


This image shows the blood vessels and the direction of blood flow in the nephron.
Figure 13.4 Blood Flow in the Nephron. The two capillary beds are clearly shown in this figure. The efferent arteriole is the connecting vessel between the glomerulus and the peritubular capillaries and vasa recta. From Betts, et al., 2013. Licensed under CC BY 4.0.


As urine is formed, it drains into the calyces of the kidney, which merge to form the funnel-shaped renal pelvis in the hilum of each kidney. The hilum narrows to become the ureter of each kidney. As urine passes through the ureter, it does not passively drain into the bladder but rather is propelled by waves of peristalsis. The ureters are approximately 30 cm long.


The urinary bladder collects urine from both ureters (see Figure 13.5). The bladder lies anterior to the uterus in females, posterior to the pubic bone and anterior to the rectum. In males, the anatomy is similar, minus the uterus, and with the addition of the prostate inferior to the bladder. The bladder is partially retroperitoneal (outside the peritoneal cavity) with its peritoneal-covered “dome” projecting into the abdomen when the bladder is distended with urine.


Cross-section of the bladder. Image description available.
Figure 13.5 Bladder. (a) Anterior cross section of the bladder. (b) The detrusor muscle of the bladder (source: monkey tissue) LM × 448. (Micrograph provided by the Regents of the University of Michigan Medical School © 2012.) From Betts, et al., 2013. Licensed under CC BY 4.0. [Image description.]


The urethra transports urine from the bladder to the outside of the body for disposal. The urethra is the only urologic organ that shows any significant anatomic difference between males and females; all other urine transport structures are identical (see Figure 13.6).


Diagrams of female and male genitalia. Image descriptions available.
Figure 13.6 Female and Male Urethras. The urethra transports urine from the bladder to the outside of the body. This image shows (a) a female urethra and (b) a male urethra. From Betts, et al., 2013. Licensed under CC BY 4.0. [Image description.]

The urethra in both males and females begins inferior and central to the two ureteral openings forming the three points of a triangular-shaped area at the base of the bladder called the trigone (Greek tri- = “triangle” and the root of the word “trigonometry”). The urethra tracks posterior and inferior to the pubic symphysis. Voiding is regulated by an involuntary autonomic nervous system-controlled internal urinary sphincter consisting of smooth muscle and voluntary skeletal muscle that forms the external urinary sphincter below it.

Image Descriptions

Figure 13.2 image description: The left panel of this figure shows the location of the kidneys in the abdomen. The right panel shows the cross section of the kidney. [Return to Figure 13.2].

Figure 13.5 image description: The left panel of this figure shows the cross section of the bladder and the major parts are labeled. The right panel shows a micrograph of the bladder. [Return to Figure 13.5].

Figure 13.6 image description: Diagrams of the (a) female and (b) male genitalia highlighting the respective urethras. [Return to Figure 13.6].


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Medical Terminology: An Interactive Approach Copyright © 2022 by LOUIS: The Louisiana Library Network is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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