Anatomy Relevant to the Urinary System

by Adam Lawson BA, MSc and Terra Doucette Hiller BA, BSN, RN

The urinary system has a variety of functions which include:

Regulating blood pressure by adjusting the volume of water or urine in the blood, releasing renin and erythropoietin.
Regulating plasma concentrations by:
- excreting sodium, potassium, chloride and other ions.
- synthesizing calcitriol (active Vitamin D) to regulate calcium absorption.
Stabilizing blood pH by controlling the excretion of hydrogen and bicarbonate through urination.
Conserving nutrients while excreting urea and uric acid.
Assisting the liver in detoxifying poisons.

Kidneys

Location
- The kidneys are located on either side of the vertebral column between T12 and L3.
- The right kidney lies behind the liver, hepatic flexure of the colon, and the duodenum.
- The left kidney lies behind the stomach, pancreas, jejunum and splenic flexure of the colon.
- An adrenal gland sits on the superior surface of each kidney.
Support
- The kidneys are supported by a renal fat capsule, a membrane of peritoneum, adjacent visceral organs, and supporting connective tissue. This support prevents normal bodily movements and bumps from disturbing of normal kidney function.
Blood supply - They receive 20-25% of total cardiac output.
Blood flow and pressure is important for the functioning of the kidneys. A mean arterial pressure (MAP) of less than 60mmHg will decrease blood flow to the kidneys; this loss of perfusion will decrease urine production and lead to:
- a buildup of wastes in the blood stream
- electrolyte imbalances
Kidneys will adjust their functioning for patients with consistently high blood pressure. If blood pressure is suddenly reduced in such patients, their kidneys may be damaged by the sudden loss of perfusion pressure.
- Each kidney receives blood from a renal artery which originates from the abdominal aorta, near the superior mesenteric arterybranch.
  
  Abdominal Vascular Auscultation
  (Click to view animation)
  
  Auscultation of the renal arteries for a bruit (in French: "noise") may indicate stenosis of the renal artery. A bruit auscultated over the upper portion of the abdominal aorta may indicate an aneurysm or stenotic aorta.
  
  Stenosis in the aorta or renal arteries can reduce blood flow to the kidneys and enable the development of acute or chronic kidney dysfunction.
  
  The VHD cadaver also had accessory renal arteries arising from the aorta. These vessel can also arise from the iliac arteries or not arise at all. The left accessory renal artery is more commonly found than the right.
  
  When an individual has these multiple renal arteries, each one will supply a certain segment of the kidney; there is no collateral perfusion. Occlusion of one artery will result in infarction of its associated segment.
  
  The accessory artery will supply either the superior or inferior pole of the kidney; supply to the inferior pole is associated with obstruction of the ureter by this artery and subsequent hydronephrosis.
- The right renal artery is longer than the left branch since the aorta tends to be more left of midline.
- The renal arteries will divide into 5 segmental arteries that enter the hilum of the kidney.
  Although the segmental arteries are not labeled individually in this program, it is easier to see each branch in the cross-section. Scroll up and down the cross-section to get a better sense of the branching of the arteries.
- The segmental arteries branch further in this order:
  1. Lobar arteries - beyond the kidney hilum
  2. Interlobar arteries
  3. Arcuate arteries
  4. Afferent arterioles
- Afferent arteriole divide into glomerulus capillaries when it enters the glomerular capsule. The capillary tuft and the capsule together form the glomerulus.
  The wall of the afferent arterioles contain renin-secreting juxtaglomerular cells. The juxtaglomerular cells seem to have mechanoreceptors that secrete renin in response to falling blood pressure. Renin causes smooth muscle of the afferent arterioles to modify itself in response to the blood pressure.
- Just adjacent to the juxtaglomerular cells lies an area known as macula densa ("dense spot"), which acts as a chemoreceptor for monitoring solute concentrations in filtrate. Low solute concentrations cause cells of the macula densa to signal the juxtaglomerular cells to secrete renin.
- Renin initiates several hormone chains which increase blood solute concentration and blood volume by causing the retention of water; these changes affect blood pressure.
- Filtration of the blood occurs in the glomerulus capillaries as they meet with the glomerular capsule.
  Arterioles are highly resistant vessels, which allow for the blood pressure in the glomerulus to be extraordinarily high for a capillary bed. This high pressure pushes filtrate out of the blood and into the glomerular capsule.
- Blood exits the glomerulus through efferent arterioles.
- Efferent arterioles flow into the peritubular capillaries. They run closely by the convoluted tubules and are low pressure porous capillaries to allow for the reabsorption of solutes and water from the tubule cells.
- Blood from the efferent arterioles also enters the vasa recta, which is a thin-walled vessel which runs alongside the loops of Henle. These vessels help concentrate urine.
- From the peritubular capillaries and vasa recta the vessels follow the same venous path as the arteries, but backwards, draining in this order:
  1. intralobular vein
  2. arcuate veins
  3. interlobar veins
  4. renal vein
- The renal vein exits the kidney at the hilum and drains into the inferior vena cava.
Surgical incision locations
- Kidneys are usually approached surgically from the posterolateral abdominal wall. This approach usually leads to less damage to muscles, vessels and nerves.
- The kidney lies closest to the body surface at this angle.
- The incision should be lower than T12 to avoid puncturing the pleural cavity and causing a pneumothorax.
Refer to the cross-section to identify the structures surrounding the kidney that can potentially be cut during a posteriorlateral, posterior or anterior surgical approach.

Ureters

The ureters are a narrowing continuation of the renal pelvis.
Ureters start at the level of L2 and descend retroperitoneally through the abdomen. They enter the pelvic girdle region near the bifurcation of the iliac arteries and meet the posterolateral aspect of the external bladder.
The ureters will run medially within the posterior bladder wall before opening into the bladder's interior. This prevents the backflow of urine into the bladder due to any increase of pressure within the bladder compressing the bladder wall, closing off the ureters.
Gravity and muscular portions of the ureters will contract with a peristaltic wave to propel urine to the bladder.

Urinary Bladder

The bladder is a muscular sac that stores and expels urine.
It lies on the pelvic floor just posterior to the pelvic symphysis
In males: it lies just anterior of the rectum and prostate.
In females: it lies just anterior to the vagina and uterus.
Notice how close the bladder is to the uterus. When a woman is pregnant, the pressure of the fetus on the bladder will reduce the bladder's capacity and result in the need for frequent urination.
The bladder is surrounded by a muscular layer called the detrusor muscle. The detrusor muscle contracts to force urine out through the urethra. It relaxes to allow the bladder to fill.
At the inferior surface there is a region of the bladder wall known as trigone ("triangle"). The triangle is defined by the two openings of the ureters and the opening of the urethra. This sensitive region will stretch with the filling of the bladder send a nervous signal of urgent fullness to the brain.
The trigone a common location for urinary tract infections. Such infections produce a constant urge to urinate.
Blood supply
- The bladder is supplied blood from branches of the internal iliac arteries, known as the superior vesical arteries and the inferior vesical arteries.
- Veins draining the bladder form a plexus on the bladder's inferior and posterior surfaces that empties into the internal iliac veins.

Urethra

The urethra is a thin-walled tube that drains urine from the bladder out of the body.
The urethra in males is generally much longer then the urethra in females.
Females are more prone to urinary tract infections due to the shorter length of the urethra and the proximity of the vagina and the rectum to the urethra. The vaginal lining is highly susceptible to infection. Fecal bacteria also can easily enter the urethra. Bacteria will therefore travel a much shorter distance to reach areas like the trigone. Females should always wipe from front to back to help prevent these infections.
The emptying of urine from the bladder to the urethra is controlled by the internal urethral sphincter, which is a thickening of the detrusor muscle. The smooth muscle of the internal urethral sphincter involuntary keeps the urethra closed and prevents dribbling of urine in between voids.
The external urethral sphincter surrounds the urethra. It is made of skeletal muscle that is used to voluntarily inhibit urination until the proper time.
The levator ani muscle also serves in the voluntary constriction of the urethra.
In males the prostate lies directly inferior to the bladder and surrounds the urethra.
Males with an enlarged prostate, due to benign prostatic hypertrophy (BPH) or prostate cancer, often times have difficulty starting urination, have a weak stream, may dribble after urination, have a strong or sudden urge to urinate, or have pain with urination. The prostate puts pressure on the bladder and urethra because of its location; pressure on the trigone will cause the strong or sudden urge to urinate; pressure on the urethra causes difficulty starting urination, weak stream, dribbling, or incontinence.

When a male has an enlarged prostate, insertion of a regular Foley catheter can be difficult, painful and near impossible. A coude catheter can be used in these cases; it has curved tip for getting around the prostate. (Coude means bent like an elbow in French).

A transurethral resection of the prostate (TURP) procedure is often done to treat the symptoms of severe BPH. This procedure involves removing the prostate tissue through the urethra using a resectoscope. A three-way indwelling catheter is inserted into the bladder after the procedure to facilitate urinary drainage over the resection site and to allow continuous or intermittent bladder irrigations that prevent obstruction from blood clots or mucus.
In males, the urethra can also propel semen out of the body.
- Semen and urine will not be released simultaneously.
- Urethral glands will secrete a mucus that helps lubricate the urethra just before ejaculation.

References