Unit 10: Kidneys

Primary Function of the Kidneys

Filter blood, remove metabolic wastes, and regulate fluid/electrolyte balance.

Paired Kidney Location

Retroperitoneal, on each side of the body; left kidney is more caudal.

Main Vessels Entering/Exiting Kidney

Renal artery (in), renal vein (out).

Tube Carrying Urine from Kidney to Bladder

Ureter.

Six Key Jobs of the Kidneys

1) Waste/toxin removal. 2) Fluid/electrolyte balance. 3) Acid-base balance. 4) BP regulation (RAAS). 5) RBC production (erythropoietin). 6) Bone/Calcium (Vitamin D).

RAAS System Role

Kidney regulates blood pressure via the Renin-Angiotensin-Aldosterone System.

Hormone for RBC Production

Erythropoietin.

Functional Unit of the Kidney

Nephron.

Four Steps of Urine Formation

Filtration, Reabsorption, Secretion, Excretion.

Step 1: Filtration Location

Glomerulus within Bowman's capsule.

What is Filtered?

Plasma components smaller than albumin (water, ions, small molecules).

Step 2: Reabsorption

Recovery of essential solutes (e.g., glucose, electrolytes) back into blood.

Step 3: Secretion

Active transport of wastes (e.g., H+, drugs) from blood into tubule filtrate.

Step 4: Excretion

Final product (urine) leaves via collecting duct to renal pelvis.

Definition: Renal Disease/Failure

Decreased kidney function; can be acute or chronic.

Definition: Azotemia

Build-up of nitrogenous wastes (urea, creatinine) in the blood.

Two Primary Waste Products in Azotemia

Urea (BUN) and Creatinine.

GFR (Glomerular Filtration Rate) Definition

Rate at which filtrate is formed in glomeruli; proportional to functional kidney mass.

Low GFR Consequence

Leads to buildup of nitrogenous wastes (azotemia).

Three Categories Affecting GFR

Pre-renal, Renal, Post-renal.

Pre-renal Factor Example

Decreased blood flow to kidneys (e.g., dehydration, shock).

Renal Factor Example

Direct kidney damage (e.g., toxins, infection).

Post-renal Factor Example

Obstruction to urine flow (e.g., bladder stone, blocked urethra).

Can Kidneys Regenerate Nephrons?

No. Born with a finite number; nephrons cannot be rebuilt.

Acute Kidney Injury (AKI) - Former Name

Acute Renal Failure (ARF).

AKI Characteristics

Sudden, potentially reversible; can be fatal or progress to chronic disease.

Common AKI Toxin Example

Ethylene glycol (antifreeze) poisoning.

Chronic Renal Failure (CRF) Key Trait

Irreversible, progressive kidney damage.

Species Most Commonly Affected by CRF

Cats.

Common Clinical Sign of CRF

PU/PD (Polyuria/Polydipsia - excessive drinking/urinating).

Four Key Tests for Renal Evaluation

Blood Urea Nitrogen (BUN), Creatinine, SDMA, Urinalysis (U/A).

BUN (Blood Urea Nitrogen) Source

Main waste product of protein breakdown in the liver.

Is BUN Kidney-Specific?

No. Elevated by high protein diet, GI bleeding, dehydration, and renal/post-renal issues.

Creatinine Source

Waste product from creatine metabolism in muscles.

Is Creatinine Diet-Dependent?

No. Relatively stable, influenced by muscle mass and renal function.

Limitation of BUN & Creatinine

Require >66% loss of kidney function to show elevation on bloodwork.

SDMA Full Name

Symmetric Dimethylarginine.

SDMA Source

Produced by all nucleated cells at a constant rate.

SDMA Key Advantage

More sensitive; can detect kidney dysfunction with ~40% loss of function.

Is SDMA Falsely Elevated by Diet/Dehydration?

No. It is filtered by the kidneys.

Urinalysis (U/A) Component - USG

Urine Specific Gravity.

USG Measures

The kidney's ability to concentrate urine.

Tool to Measure USG

Refractometer.

Normal USG for Dogs

1.030.

Normal USG for Cats

1.035 (ideally >1.040).

Low USG Indicates

Dilute urine; seen with PU/PD, renal disease, or after IV fluids/anesthesia.

High USG Indicates

Concentrated urine; typical of dehydration.

Diagnostic Combo for Renal Disease

Elevated SDMA/Creatinine + Low USG.

What is a UPCR?

Urine Protein:Creatinine Ratio; checks for abnormal protein loss in urine.

Cause of False UPCR Elevation

Blood in the urine (from inflammation, infection, etc.).

IRIS Guidelines

International Renal Interest Society staging system for chronic kidney disease.

IRIS Staging Based On

Serum creatinine and SDMA values.

Kidney's Role in Acid-Base Balance

Regulates pH by excreting H+ ions and reabsorbing HCO3- (bicarbonate).

Pre-renal Azotemia

Azotemia caused by factors BEFORE the kidney (e.g., poor blood flow).

Renal Azotemia

Azotemia caused by damage WITHIN the kidney.

Post-renal Azotemia

Azotemia caused by obstruction AFTER the kidney (e.g., blocked urethra).

"Blocked Cat" is an Example of…

Post-renal azotemia/obstruction.

Why is USG Checked with Azotemia?

To determine if low USG (inability to concentrate) is due to renal disease versus dehydration (high USG).

Term for Excessive Drinking & Urinating

PU/PD (Polydipsia/Polyuria).

Vitamin D Activation Site

Kidneys convert Vitamin D to its active form for calcium regulation.