Chapter 16: Urinary System & Venipuncture

Detailed Chapter Overview

Chapter 16 provides a comprehensive exploration of the urinary system and the essential clinical skill of venipuncture required for its primary imaging examination, the Intravenous Urogram (IVU). This chapter is pivotal as it merges profound anatomical and physiological knowledge with a critical, invasive procedural competency. An in-depth understanding of the urinary tract's macro- and microanatomy—from the kidneys and their complex internal structures down to the bladder—is foundational. The chapter thoroughly details the journey of blood filtration through the microscopic nephron, explaining how this functional unit produces urine. The central theme of the chapter is the safe and effective administration of intravenous contrast media to opacify and visualize the urinary system. It meticulously examines the different classes of iodinated contrast, emphasizing the superior safety profile of nonionic agents. A significant portion is dedicated to patient safety, covering comprehensive patient screening, understanding contraindications such as renal insufficiency, and the prompt recognition and management of adverse contrast reactions, from mild side effects to severe anaphylaxis. The technique of venipuncture is broken down into a detailed, step-by-step process, stressing aseptic technique, site selection, and best practices. The IVU procedure itself is detailed from start to finish, explaining the rationale behind the timed sequence of images that capture the physiological phases of kidney function. It provides explicit instructions for all projections, including specialized views like tomograms and obliques, and details other procedures like retrograde urography and cystography, ensuring the radiographer is prepared for a variety of clinical scenarios.

In-Depth Study Guide

Anatomy and Physiology of the Urinary System

The urinary system filters waste from the blood, regulates blood volume and pressure, and controls levels of electrolytes and metabolites. It is a vital system for maintaining homeostasis.

The Kidneys: Filtration Powerhouses

• Location and Structure: The kidneys are retroperitoneal organs located on either side of the spine around the T12-L3 level. The right kidney is typically lower than the left due to the liver's position. A fatty capsule surrounds each kidney, which is visible on a quality radiograph.
• Internal Anatomy: The outer layer is the **renal cortex**, which contains the glomeruli. The inner layer is the **renal medulla**, composed of 8-18 cone-shaped **renal pyramids**. The apex of each pyramid is called the renal papilla.
• The Nephron - The Functional Unit: The nephron is the microscopic workhorse of the kidney. Each kidney contains over a million nephrons.
  • Renal Corpuscle: Consists of the **glomerulus**, a tangled capillary network, and **Bowman's capsule**, a double-walled cup that surrounds it. Blood is filtered under pressure in the glomerulus, forcing water and small solutes into Bowman's capsule to form the filtrate.
  • Renal Tubule: The filtrate travels through a long, convoluted tubule where essential substances (water, glucose, vitamins, amino acids) are reabsorbed back into the blood, while certain waste products are actively secreted into the tubule.
• Urine Drainage Pathway: From the renal tubules, urine drains into collecting ducts -> renal papilla -> **minor calyces** -> **major calyces** -> **renal pelvis** -> ureter.

Ureters, Bladder, and Urethra: The Drainage System

• Ureters: These two muscular tubes connect the renal pelvis of each kidney to the urinary bladder. They transport urine via peristalsis. There are three points of natural constriction where kidney stones (renal calculi) are most likely to lodge: the **Ureteropelvic Junction (UPJ)**, the **Pelvic Brim**, and the **Ureterovesical Junction (UVJ)**.
• Urinary Bladder: A hollow, muscular, distensible organ that serves as a reservoir for urine, located in the anterior pelvis. The desire to urinate (micturition) typically occurs when the bladder contains about 250 mL of urine. The triangular area on the bladder floor formed by the two ureteral openings and the urethral opening is called the **trigone**.
• Urethra: The tube that conveys urine from the bladder to the exterior of the body.

Venipuncture and Intravenous Contrast Media

The Skill of Venipuncture

Venipuncture is the process of puncturing a vein to administer medication, IV fluids, or, in radiography, contrast media. Aseptic technique is paramount.

• Vein Selection: The most common and ideal veins for IV contrast injection are located in the antecubital fossa of the elbow, specifically the **median cubital vein** (most common), the **cephalic vein** (lateral), and the **basilic vein** (medial).
• Needle/Catheter Selection: A butterfly needle or an over-the-needle IV catheter can be used. Catheters are often preferred as they are more secure and flexible once in the vein.
• Detailed Procedure: 1. Verify patient ID and explain the procedure. 2. Wash hands and don gloves. 3. Apply a tourniquet 3-4 inches above the site. 4. Palpate and select a suitable vein. 5. Cleanse the site vigorously with an alcohol wipe. 6. Anchor the vein by pulling the skin taut. 7. Insert the needle, bevel up, at a 20-45 degree angle until you feel a "pop" and see a blood return ("flashback"). 8. Advance the catheter slightly, then release the tourniquet. 9. Remove the needle while occluding the vein proximal to the catheter tip. 10. Connect the syringe or tubing and secure the catheter.

Understanding Iodinated Contrast Media

• Function: Water-soluble, iodinated compounds are injected into the bloodstream. The kidneys filter this contrast from the blood, and as it passes through the urinary tract, it makes the system radiopaque (visible on x-rays).
• Ionic vs. Nonionic:
  • Ionic Contrast: Older agents that dissociate into charged particles (ions) in a solution. This creates a high osmolality (a high concentration of particles), which is responsible for a higher incidence of adverse reactions.
  • Nonionic Contrast: Modern agents that do not dissociate into charged particles. They have a much lower osmolality, are better tolerated by the body, and have a significantly lower risk of causing an adverse reaction. They are the standard of care.
• Patient Screening and Safety: This is a critical responsibility. Before any injection, the radiographer must screen the patient for:
  • History of allergies (especially to iodine or previous contrast).
  • History of asthma (increases risk of reaction).
  • Kidney disease or diabetes (impaired renal function can hinder contrast excretion). Current lab values for **BUN (Blood Urea Nitrogen)** and **Creatinine** should be checked. Elevated levels are a major contraindication.
  • Use of metformin (Glucophage), a diabetes medication that must be withheld after the procedure to prevent renal complications.
• Managing Contrast Reactions: The radiographer must be able to recognize and respond to reactions.
  • Expected Side Effects: A transient feeling of warmth and a metallic taste are common and not allergic reactions.
  • Mild (Local): Pain, redness, swelling at the injection site.
  • Moderate (Systemic): Widespread hives (urticaria), facial edema, bronchospasm (wheezing). Require intervention.
  • Severe (Anaphylaxis): A life-threatening emergency characterized by severe respiratory distress, profound hypotension (shock), and potential cardiac arrest. Requires immediate activation of a code team.

Radiographic Procedures of the Urinary System

Intravenous Urogram (IVU)

A functional study of the urinary system. A timed series of radiographs demonstrates the kidneys filtering the contrast and the subsequent drainage through the ureters into the bladder.

• Patient Prep: Mild laxative and NPO for several hours to prevent gas and fecal matter from obscuring the kidneys.
• Image Sequence:
  • Scout KUB: A preliminary image to check technique and look for calculi.
  • Nephrogram (or Nephrotomogram): An image taken within 1 minute of injection. The contrast fills the capillaries and nephrons, causing a "blush" of the entire renal parenchyma. Tomography (taking slices) at this stage provides the best view of the renal outline.
  • 5-Minute KUB: Shows the contrast opacifying the calyces and renal pelvis.
  • 10-15-Minute Obliques (RPO/LPO): A 30-degree oblique is used. In the **RPO**, the left kidney is parallel to the IR and the right ureter is visualized away from the spine. In the **LPO**, the right kidney is parallel to the IR and the left ureter is visualized.
  • 20-Minute KUB: Shows contrast filling the bladder.
  • Postvoid: An erect or supine film taken after the patient urinates to assess bladder emptying.

Other Procedures

• Retrograde Urography: A non-functional study performed in surgery. A urologist uses a cystoscope to pass catheters up the ureters and injects contrast directly into the renal pelvis. This provides excellent anatomical detail of the collecting system when an IVU is contraindicated.
• Cystography: A study of the urinary bladder where contrast is introduced via a urinary catheter. Used to diagnose tumors, diverticula, or vesicoureteral reflux.