32 Patho

Structure and Function of the Kidney

Overview of Kidney Structure and Location

The kidneys are vital organs responsible for filtering blood, maintaining fluid and electrolyte balance, and excreting waste products. Their structure and anatomical location are essential for their function.

• Shape and Structure: The kidneys are bean-shaped organs, each composed of multiple lobes (up to 18). Each lobe contains nephrons, which are the functional units of the kidney.

• Location: The kidneys are located outside the peritoneal cavity, in the back of the upper abdomen, on each side of the spine. They are positioned between the 12th thoracic (T12) and 3rd lumbar (L3) vertebrae.

Function of the Nephron

The nephron is the microscopic structural and functional unit of the kidney. Each kidney contains about one million nephrons, which are responsible for urine formation through three main processes:

• Glomerular Filtration: Blood is filtered through three layers in the glomerulus, allowing water and small solutes to pass while retaining blood cells and large proteins.

• Reabsorption: Water, electrolytes, and other needed substances are reabsorbed from the filtrate back into the bloodstream.

• Secretion: Waste and unneeded materials are secreted into the tubular fluid to be eliminated as urine.

Capillary Systems Supplying the Nephron

Two main capillary systems supply the nephron, each with distinct roles in filtration and reabsorption:

• Glomerulus Capillary System: Functions as a high-pressure filter between the afferent and efferent arterioles. The arterioles can dilate or constrict to control filtration pressure.

• Peritubular Capillary System: A low-pressure system arising from the efferent arteriole, responsible for reabsorbing needed substances back into the blood.

Major Segments of the Nephron Tubule

The nephron tubule is divided into several segments, each with specialized functions in urine formation:

• Proximal Convoluted Tubule (PCT): The coiled section immediately after Bowman's capsule where most filtrate reabsorption occurs.

• Loop of Henle: A U-shaped, looped section that dips into the medulla of the kidney, crucial for concentrating urine.

• Distal Convoluted Tubule (DCT): A coiled section farther along the nephron that prepares filtrate for final processing.

• Collecting Tubule: Collects filtrate from several nephrons and serves as the final pathway before urine exits the kidney.

Tubular Filtration

• As urine filtrate flows through the nephron's tubules:

  • Reabsorption: Water and solutes are reabsorbed into the blood.

  • Secretion: Wastes and unneeded substances are added into the tubule for removal.

Transport Mechanisms in the Nephron

Transport of substances across nephron membranes is driven by several mechanisms, primarily involving sodium gradients:

• Na+/K+ ATPase Pump: Maintains low sodium concentration inside tubular cells, creating a gradient for sodium movement.

• Cotransport (Symport): Sodium moves in with another substance in the same direction.

• Countertransport (Antiport): Sodium moves in while another substance moves out in the opposite direction.

• Ion Exchange: Swapping of ions across the membrane using sodium's gradient.

Regulation of Urine Concentration

The kidneys regulate urine concentration in response to changes in body fluid osmolarity, primarily through the action of hormones:

• Normal Renal Osmolarity: Approximately 1200 mOsm.

• Antidiuretic Hormone (ADH): Increases the permeability of the collecting ducts, allowing more water reabsorption when body fluids are too concentrated (high osmolarity).

• When osmolarity is high, ADH secretion increases, leading to more water reabsorption and dilution of body fluids.