The Kidney and Urinary System: Functions, Anatomy, and Regulation

Main Functions of the Kidney

Excretion of Toxins and Nitrogenous Wastes: The kidneys are crucial for filtering and removing harmful substances and metabolic byproducts from the blood.
Regulation of Blood Chemical Levels: They maintain the precise balance of various chemicals, electrolytes, and nutrients in the blood.
Maintenance of Water Balance: Kidneys play a vital role in regulating the body's water content, preventing dehydration or overhydration.
Regulation of Blood Pressure and Volume: Through various mechanisms, the kidneys help control systemic blood pressure and the total volume of blood circulating in the body.
Regulation of Red Blood Cell Production: The kidneys secrete a hormone called Erythropoietin (EPO), which is essential for stimulating red blood cell formation.

Erythropoietin (EPO)

Production Site: Predominantly produced by specialized interstitial cells within the kidney.
Action on Red Blood Cells: Protects existing red blood cells from destruction.
Stimulation of Production: Stimulates stem cells in the bone marrow to increase the production of new red blood cells.

Gross Anatomy of the Kidney

Appearance: Reddish-brown, bean-shaped organs.
Normal Adult Size: Approximately 11 cm x 7 cm x 3 cm.

Microscopic Anatomy: The Nephron

The nephron is the functional unit of the kidney, responsible for filtering blood and forming urine. It consists of a renal corpuscle and a renal tubule.

Renal Tubule Segments

Proximal Convoluted Tubule (PCT): This is the first segment of the renal tubule, directly extending from the Bowman's capsule.
Nephron Loop (Loop of Henle): An extension of the proximal tubule, it consists of three main parts:
- Descending limb
- The loop itself
- Ascending limb
Distal Convoluted Tubule (DCT): An extension of the ascending limb of the nephron loop.
Collecting Duct (CD): Straight extensions of the distal tubule, which collect urine from multiple nephrons.

Location of Nephrons

Cortical Nephrons: Constitute about 85% of the total nephron population. Most of their structure, including the renal corpuscle and the majority of the renal tubule, is located within the renal cortex.

Glomerular Filtration

Process: Glomerular blood pressure forces water and various dissolved substances (solutes) to filter out of the glomerulus and into the glomerular capsule (also known as Bowman’s capsule). This initial filtrate is essentially plasma without large proteins or blood cells.
Normal Glomerular Filtration Rate (GFR): The normal rate at which filtrate is formed is 125 ml/min.

Hormonal Regulation of Blood Volume and Blood Pressure

Several hormones play critical roles in regulating the body's fluid balance and blood pressure by acting on the kidneys.

Aldosterone:
- Effect: Has a strong affinity for Na+ (sodium ions). It promotes the reabsorption of Na+ from the renal tubules back into the blood.
- Consequence: Water follows sodium, leading to increased water reabsorption into the blood. This ultimately increases blood volume and, consequently, increases blood pressure.
Antidiuretic Hormone (ADH):
- Effect: Also known as vasopressin, ADH

Comprehensive Overview of Kidney Functions and Urinary System

Comprehensive Overview of Kidney Functions and Urinary System