Urinary Tests, Ureters, Bladder, and Micturition Study Notes

Blood Urea Nitrogen (BUN): This test measures the level of nitrogen in the blood that comes from urea.
- Source: Urea is a byproduct of the deamination of amino acids.
- Pathology: Urea is produced in the liver and is subsequently eliminated from the body by the kidneys.
Creatinine: This is another nitrogenous waste product measured in the blood to assess kidney function.
- Source: Creatinine is produced from the metabolic activity of skeletal muscle.
Measurement Objective: Both BUN and creatinine are clinical tools used to quantify the levels of nitrogenous wastes circulating in the blood, reflecting the efficiency of renal filtration.

Definition of Clearance: Clearance measures the amount of a specific substance found in the urine relative to the amount of that same substance in the blood plasma.
Clearance Formula: The relationship is calculated using the ratio of concentration in Urine ( $U$ ) to concentration in Plasma ( $P$ ), multiplied by the total urine volume over a specific period (typically 24 hours).
- $\text{Clearance} = (U/P) \times \text{Volume (24 hours)}$
Interpretation of Results:
- High Clearance: Occurs if there is a large quantity of the substance detected in the urine relative to the plasma.
- Low Clearance: Occurs if there is very little of the substance detected in the urine relative to the plasma.
Clinical Application: Measuring clearance is essential for determining the appropriate dosages for drugs that are primarily eliminated by the kidneys to prevent toxicity or sub-therapeutic levels.

Visual and Physical Characteristics:
- Color: Normal urine is yellow, a color derived from the pigment urochrome.
- Clarity: Normal urine should appear clear.
- pH: Urine is typically acidic, with a pH value of $< 6$ .
- Specific Gravity: This is a measurement of the total amount of solutes and "stuff" (particulate matter) contained within the urine.
Chemical and Electrolyte Composition:
- Ions: Normal urine contains various ions including Potassium ( $K^+$ ), Sodium ( $Na^+$ ), Chloride ( $Cl^-$ ), Hydrogen ( $H^+$ ), and bicarbonate.
- Glucose: Under normal conditions, there should be no glucose in the urine because it is $100\%$ reabsorbed in the renal tubules. The presence of glucose in urine is termed glucosuria.
- Amino Acids: These should be absent from the urine as they are $100\%$ reabsorbed.
- Protein: Professional filtration usually excludes proteins because they are too large to pass through the filtration membrane. The presence of protein in urine is called proteinuria.
- Ketones: The presence of ketones in the urine, known as ketonuria, occurs when the body is breaking down fats for energy.
- Red Blood Cells (RBCs): RBCs are normally too large to be filtered into the urine. If they are present, the condition is called hematuria.

Physical Characteristics:
- Position: The ureters are located in the retroperitoneal space.
- Length: Each ureter is approximately $12 \, \text{inches}$ long.
- Entry Point: They enter into the posterior side of the urinary bladder.
Histological Layers of the Ureters:
- Transitional Epithelium: This innermost layer allows for stretching as urine passes through.
- Lamina Propria: A layer of connective tissue; notably, the ureters have no submucosa.
- Muscularis: Consists of an inner longitudinal layer and an outer circular layer of smooth muscle, which work together to move urine via peristalsis.
- Adventitia: The outermost layer of connective tissue.

The Trigone Region: A triangular area at the base of the bladder defined by three points: two openings for the ureters and one opening leading to the urethra.
Reflux Prevention:
- Ureters do not possess anatomical valves where they meet the bladder.
- Instead, they have "functional" valves. As the bladder fills and expands, the slit-like openings of the ureters are compressed and closed, prohibiting the backward flow (reflux) of urine.
Tissue Layers of the Bladder:
- Transitional Epithelium (Urothelium): Lines the bladder and includes specialized protective cells on the free surface.
- Rugae: These are folds in the bladder wall that allow for massive expansion as the bladder fills.
- Lamina Propria: Connective tissue layer (no submucosa present).
- Muscularis (Detrusor Muscle): A thick muscular layer consisting of three distinct layers of smooth muscle.
- Outer Covering: Depending on the specific part of the bladder, the outer layer is either a serosa or an adventitia.

Histology of the Urethra: In both sexes, the lining is transitional epithelium near the bladder and transitions to stratified squamous epithelium near the external urethral opening.
Female Anatomy:
- Length: Approximately $1.5 \, \text{inches}$ long.
- Positioning: On a female model, the uterus is located between the bladder and the rectum.
- Clinical Note: The short length of the female urethra makes women more susceptible to bladder infections, even though the bladder is normally a sterile environment.
Male Anatomy:
- Length: Significantly longer than the female urethra—about $5 \times$ as long, measuring approximately $8 \, \text{inches}$ .
- Segments of the Male Urethra:
  1. Prostatic Urethra: The portion encircled by the prostate gland.
  2. Membranous Urethra: The portion passing through the pelvic floor.
  3. Spongy or Penile Urethra: The portion running through the length of the penis.

Trigger Mechanism: When the bladder contains between $200 \, \text{--} \, 400 \, ml$ of urine, the bladder wall stretches.
Neural Pathway: Stretch receptors send impulses to the sacral spinal cord levels $S2 \text{--} S3$ .
Muscle Response:
- Parasympathetic Stimulation: Nerves signal the detrusor muscle to contract.
- Internal Urethral Sphincter: Made of smooth muscle; it relaxes involuntarily.
- External Urethral Sphincter: Located at the pelvic floor; made of skeletal muscle, allowing for voluntary control over urination.

Incontinence in Females:
- The female pelvic floor is composed of soft tissue, including skeletal muscle and connective tissue.
- As these tissues lose elasticity over time, the urethra may shift into a more vertical orientation.
- Sudden increases in abdominal pressure (e.g., coughing, sneezing) can then lead to a leakage of urine.
- The involuntary loss of urine is formally termed incontinence and can often be corrected through surgery.
Urinary Retention in Males:
- Males may suffer from urinary retention due to the enlargement of the prostate gland.
- As the prostate grows, it narrows the prostatic urethra, making it difficult to empty the bladder completely.
- This results in residual urine remaining in the bladder after the act of urination is finished.