33.1 Introduction to the Renal System

The renal system filters blood, removes waste, and regulates fluid and electrolyte balance. For nurses, understanding how the kidneys work is foundational to pharmacology because so many drugs are cleared renally, and kidney dysfunction changes how you dose and monitor nearly everything.

This section covers the anatomy of the renal system, common kidney conditions, and the process of glomerular filtration, including how GFR is measured and what factors change it.

Anatomy and Physiology of the Renal System

Structure and function of the renal system

The renal system has two main jobs: filter waste from the blood and regulate fluid/electrolyte balance. Here's how each structure contributes.

Kidneys

The kidneys are bean-shaped organs located against the posterior abdominal wall, just below the rib cage. Beyond filtering waste products like urea and creatinine, they also produce important hormones:

• Erythropoietin stimulates red blood cell production
• Renin helps regulate blood pressure through the renin-angiotensin-aldosterone system (RAAS)
• Calcitriol (active vitamin D) promotes calcium absorption in the gut

Nephrons

Each kidney contains about 1 million nephrons, the functional filtering units. A nephron has two main parts:

• Renal corpuscle: the glomerulus (a capillary tuft) surrounded by Bowman's capsule, where blood is initially filtered
• Renal tubule: a series of segments that modify the filtrate before it becomes urine. The segments, in order, are:
  1. Proximal convoluted tubule (PCT)
  2. Loop of Henle (descending and ascending limbs)
  3. Distal convoluted tubule (DCT)
  4. Collecting duct

Each segment reabsorbs or secretes different substances, and many diuretics and other drugs target specific segments. You'll see this come up repeatedly in pharmacology.

Renal blood supply

The kidneys receive about 20-25% of cardiac output, which is a huge proportion for organs their size. Blood flows through the kidney in this sequence:

1. The renal artery enters at the hilum and branches into interlobar arteries, then arcuate arteries, then afferent arterioles
2. Glomerular capillaries form a network inside Bowman's capsule where filtration occurs
3. Blood exits via efferent arterioles into peritubular capillaries, which surround the tubules and handle reabsorption and secretion
4. The renal vein collects blood and returns it to the inferior vena cava

The fact that the kidney has two capillary beds in series (glomerular and peritubular) is unique and directly relevant to how drugs like ACE inhibitors affect kidney function.

Lower urinary tract

• Ureters: muscular tubes (25-30 cm) that move urine from the renal pelvis to the bladder via peristaltic contractions
• Bladder: a hollow, muscular organ in the pelvic cavity that stores 300-500 mL of urine. The detrusor muscle contracts during urination while the internal and external sphincters relax
• Urethra: carries urine from the bladder out of the body during micturition. It's about 20 cm in males (passing through the prostate and penis) and about 4 cm in females. This anatomical difference is clinically significant for infection risk.

Renal Pathophysiology and Glomerular Filtration

Common conditions affecting kidney function

Acute kidney injury (AKI)

AKI is a sudden decline in kidney function over hours to days. Causes are grouped into three categories:

• Prerenal: decreased blood flow to the kidneys (hypotension, sepsis, hypovolemia)
• Intrinsic renal: direct damage to kidney tissue (nephrotoxic drugs, contrast dye, acute glomerulonephritis)
• Postrenal: obstruction of urine outflow (kidney stones, tumors, enlarged prostate)

AKI leads to accumulation of waste products (azotemia), dangerous electrolyte imbalances (especially hyperkalemia), and metabolic acidosis. For nurses, recognizing early signs and monitoring urine output is critical because many medications can contribute to or worsen AKI.

Chronic kidney disease (CKD)

CKD is a progressive loss of kidney function over months to years, defined as a GFR below 60 mL/min/1.73 m² for 3 months or longer. The most common causes are:

• Diabetes (the leading cause)
• Hypertension
• Glomerulonephritis
• Polycystic kidney disease

CKD can progress to end-stage renal disease (ESRD), requiring dialysis or transplantation. Because CKD affects drug clearance, you'll need to adjust doses of renally cleared medications as GFR declines.

Urinary tract infections (UTIs)

UTIs are bacterial infections most commonly affecting the bladder (cystitis) or kidneys (pyelonephritis). Symptoms include dysuria (painful urination), frequency, urgency, suprapubic pain, and hematuria (blood in urine).

Risk factors include female anatomy (shorter urethra), sexual activity, indwelling urinary catheters, and diabetes. Pyelonephritis is more serious and can present with fever, flank pain, and costovertebral angle tenderness.

Nephrolithiasis (kidney stones)

Kidney stones are solid deposits that form in the urinary tract, most commonly calcium oxalate or calcium phosphate. They can cause severe flank pain, renal colic (pain radiating to the groin), hematuria, and nausea/vomiting.

Risk factors include dehydration, high-oxalate diets (spinach, rhubarb), and certain medications (topiramate, acyclovir). Obstruction from stones can lead to hydronephrosis and impaired kidney function if not addressed.

Glomerulonephritis

Glomerulonephritis is inflammation of the glomeruli, usually from immune-mediated mechanisms such as IgA nephropathy or lupus nephritis. It can also result from systemic diseases like diabetes and hypertension.

Clinical findings include hematuria, proteinuria (protein in urine), edema, hypertension, and declining kidney function. Diagnosis often requires a kidney biopsy to identify the specific type and guide treatment.

Process of glomerular filtration and factors influencing glomerular filtration rate

How glomerular filtration works

Filtration is a passive process driven by Starling forces:

• Glomerular hydrostatic pressure (from blood pressure) pushes fluid out of the capillaries into Bowman's space. This is the main force favoring filtration.
• Oncotic pressure (from plasma proteins like albumin) pulls fluid back into the capillaries. This opposes filtration.

The net result is that fluid and small solutes move from the glomerular capillaries into Bowman's space, forming an ultrafiltrate.

The filtration barrier has three layers:

1. Fenestrated endothelial cells (capillary wall with small pores)
2. Glomerular basement membrane
3. Epithelial podocytes with filtration slits

This barrier allows water, electrolytes, glucose, amino acids, and small molecules (less than ~7 nm) to pass through while blocking larger proteins like albumin and blood cells. When this barrier is damaged (as in glomerulonephritis), protein leaks into the urine.

Glomerular filtration rate (GFR)

GFR is the volume of fluid filtered by the glomeruli per unit time and is the single best indicator of overall kidney function. Normal GFR in a healthy adult is approximately 125 mL/min, which works out to about 180 L/day. Most of that filtrate is reabsorbed by the tubules; only about 1-2 L becomes urine.

Factors affecting GFR

• Renal blood flow: Increased flow (exercise, pregnancy) raises GFR. Decreased flow (hypovolemia, hypotension, renal artery stenosis) lowers it.
• Afferent arteriolar resistance: Constriction of the afferent arteriole (by norepinephrine, endothelin) decreases GFR because less blood reaches the glomerulus, lowering hydrostatic pressure.
• Efferent arteriolar resistance: Constriction of the efferent arteriole (by angiotensin II) increases GFR because blood backs up in the glomerulus, raising hydrostatic pressure. This is why ACE inhibitors and ARBs, which dilate the efferent arteriole, can lower GFR and must be monitored in patients with renal impairment.
• Oncotic pressure: Dehydration or conditions that raise plasma protein concentration increase oncotic pressure, which opposes filtration and decreases GFR.
• Autoregulation: The kidneys maintain a relatively constant GFR across a mean arterial pressure range of about 80-180 mmHg through two mechanisms:
  • Myogenic response: afferent arteriolar smooth muscle contracts in response to increased pressure
  • Tubuloglomerular feedback: the macula densa (in the DCT) senses NaCl delivery and signals the afferent arteriole to adjust tone

Estimating GFR in clinical practice

GFR can't be measured directly at the bedside, so it's estimated using serum creatinine. Creatinine is a waste product of muscle metabolism that is freely filtered by the glomerulus and not significantly reabsorbed. This makes it a useful (though imperfect) marker.

A key relationship to remember: a doubling of serum creatinine reflects roughly a 50% reduction in GFR. So even a modest rise in creatinine can signal significant kidney function loss.

Three common methods for estimating GFR:

• Creatinine clearance (from a 24-hour urine collection):

$CrCl \text{ (mL/min)} = \frac{U_{Cr} \times V}{P_{Cr}}$

where $U_{Cr}$ is urine creatinine (mg/dL), $V$ is urine flow rate (mL/min), and $P_{Cr}$ is plasma creatinine (mg/dL)

• Cockcroft-Gault equation (estimates creatinine clearance using serum creatinine, age, weight, and sex):

$CrCl \text{ (male)} = \frac{(140 - \text{age}) \times \text{weight (kg)}}{72 \times \text{serum creatinine (mg/dL)}}$

For females, multiply the result by 0.85. This equation is still widely used for drug dosing adjustments.

• MDRD equation (estimates GFR based on serum creatinine, age, sex, and race):

$\text{GFR} = 175 \times (\text{serum creatinine})^{-1.154} \times (\text{age})^{-0.203} \times (0.742 \text{ if female}) \times (1.212 \text{ if African American})$

Note: Many institutions are now transitioning to the CKD-EPI equation, which is more accurate at higher GFR values and has been updated to remove the race variable. Check which equation your facility uses, as this affects drug dosing decisions.