g

Nephron

functional unit of the kidenys

Overview of Kidney Functions

- Maintain H2O balance in the body - Maintain proper osmolarity of body fluids - Regulate quantity and concentration of most ECF ions - Maintain proper plasma volume - Help maintain proper acid-base balance - Eliminate wastes of bodily metabolism - Excrete foreign compounds - Produce renin - Produce erythropoietin - Convert vitamin D into its active form

Vascular component of nephron

- Glomerulus

- Afferent arterioles

- Efferent arterioles

- Peritubular capillaries

Tubular component of nephron

- Starts at Bowman's capsule

- Cortical and juxtamedullary nephrons

- Glomeruli of cortical nephrons lie in the outer cortex layer

- Glomeruli of juxtamedullary nephrons lie in the inner cortex layer

Afferent arteriole

carries blood to the glomerulus

Glomerulus

a ball of capillaries that filters a protein-free plasma into the tubular component

Efferent arteriole

carries blood from the glomerulus

Peritubular capillaries

supply the renal tissue; involved in exchanges with the fluid in the tubular lumen

Bowman's capsule

collects the glomerular filtrate

Proximal tubule

uncontrolled reabsorption and secretion of selected substances occur here

Loop of Henle (of juxtamedullary nephrons only; not shown)

establishes an osmotic gradient in the renal medulla that is important in the kidney's ability to produce urine of varying concentration

Distal tubule and collecting duct

variable, controlled reabsorption of Na+ and H2O and secretion of K+ and H+ occur here; fluid leaving the collecting duct is urine, which enters the renal pelvis

Juxtaglomerular apparatus (combined vascular/ tubular components)

produces substances involved in the control of kidney function

Basic Renal Processes

Glomerular filtration

Tubular reabsorption

Tubular secretion

Urine excretion

Glomerular filtration

nondiscriminant filtration of a protein- free plasma from the glomerulus into Bowman's capsule

Tubular reabsorption

selective movement of filtered substances from the tubular lumen into the peritubular capillaries

Tubular secretion

selective movement of nonfiltered substances from the peritubular capillaries into the tubular lumen

Glomerular Filtration

- The glomerular membrane is considerably more permeable than capillaries elsewhere. - Glomerular capillary wall consists of a single layer of flattened endothelial cells. - Glomerular capillary blood pressure is the major force that causes glomerular filtration. - Forces involved in glomerular filtration. - Glomerular filtration rate (GFR).

Filtration Process in the Glomerulus

To be filtered, a substance must pass through: - The pores between and the fenestrations within the endothelial cells of the glomerular capillary. - An acellular basement membrane. - The filtration slits between the foot processes of the podocytes in the inner layer of Bowman's capsule.

Glomerular capillairy blood pressure

favors filtration at 55 mmHg

Plasma-colloid osmotic pressure

opposes filtration 30 mmHg

Bowman's capsule hydrostatic pressure

OPPOSES filtration, 15 mmHg

Net filtration pressure

Favors filtration 10 mmHg

Reabsorption in the Kidneys

To be reabsorbed (to move from the filtrate to the plasma), a substance must cross five distinct barriers: 1. The luminal cell membrane 2. The cytosol 3. The basolateral cell membrane 4. The interstitial fluid 5. The capillary wall

Tubular Reabsorption of Na+

-Na+ reabsorption depends on the Na+-K+ ATPase pump in the basolateral membrane.

- Of the Na+ reabsorbed, on average:

- 67% is reabsorbed in the proximal tubule - 25% in the loop of Henle

- 8% in the distal and collecting tubules

- Sodium reabsorption plays different important roles in each of these segments.

Balance Concept

- The internal pool of a substance is the amount of that substance in the ECF (extracellular fluid). - Increased by transferring more in from the external environment or by metabolically producing it within the body. - To maintain stable balance of an ECF constituent, its input must equal its output. - Positive balance: gains exceed losses. - Negative balance: losses exceed gains.

Balanced between the ICF and ECF

Body water