Mammalian Kidney & Nephron Structure

Gross Structure of a Mammalian Kidney

• Fibrous capsule protects the kidney
• Cortex is the outer region and comprises Bowman's capsules, convoluted tubules, and blood vessels
• Medulla is the inner region and comprises of collecting ducts, loops of Henle, and blood vessels

Structure of a Nephron

• Glomerulus (A)
• Bowman's capsule (B)
• Proximal convoluted tubule (C)
• Collecting duct (D)
• Afferent arteriole (E)
• Efferent arteriole (F)
• Loop of Henle (G)
• Distal convoluted tubule (H)

Blood Vessels Associated with a Nephron

• A wide afferent arteriole from the renal artery enters the renal capsule to form the glomerulus
• The glomerulus has a branched knot of capillaries which combine to form a narrow efferent arteriole
• The efferent arteriole branches to form a capillary network that surrounds the tubules

Kidney Histology

• The key components are the glomerulus, proximal tubule, and distal tubule

Sections of a Nephron

• Bowman's capsule is at the start of the nephron
• The capsule is cup-shaped, surrounds the glomerulus, and has an inner layer of podocytes
• The proximal convoluted tubule (PCT) consists of a series of loops surrounded by capillaries
• The walls of the PCT are made of epithelial cells with microvilli
• The Loop of Henle is a hairpin loop extending from the cortex into the medulla
• The Distal convoluted tubule (DCT) is similar to the PCT but has fewer capillaries
• The collecting duct receives fluid from the DCTs of several nephrons
• The collecting duct empties into the pelvis of the kidney

Deamination

• Deamination is the removal of the amino group in an amino acid

Urea

• Urea is a waste molecule produced in the liver
• Urea production is a product of amino acid breakdown
• Urea is excreted in urine

Ultrafiltration

• Ultrafiltration occurs in Bowman's capsule
• High hydrostatic pressure in the glomerulus forces small molecules out of capillary fenestrations against the osmotic gradient
  • The small molecules include urea, water, glucose, and mineral ions
• The basement membrane acts as a filter
• Blood cells and large molecules like proteins remain in the capillary

Bowman's Capsule Adaptations for Ultrafiltration

• Fenestrations are between the epithelial cells of capillaries
• Fluid can pass between and under the folded membrane of podocytes

Selective Reabsorption

• Useful molecules from the glomerular filtrate, such as glucose, are reabsorbed into the blood
• Selective reabsorption happens in the proximal convoluted tubule

Transport Processes in Selective Reabsorption

• Glucose from the glomerular filtrate is co-transported with Na+ ions
• Active transport occurs in cells lining the proximal convoluted tubule
• There are intercellular spaces
• Diffusion occurs in the blood capillary lining tubule

Kidney Urine Production

• The filtrate passes through the Loop of Henle after selective reabsorption
• The Loop of Henle acts as a countercurrent multiplier
• Then the filtrate goes through the distal convoluted tubule, where water and mineral ions are reabsorbed
• More water is reabsorbed in the collecting duct
• The remaining fluid, now urine, contains only waste materials and water

Loop of Henle

• Active transport of Na+ and Cl- occurs out of the ascending limb
• The water potential of the interstitial fluid decreases
• Osmosis of water occurs out of the descending limb, the ascending limb is impermeable to water
• The water potential of the filtrate decreases going down the descending limb
  • The water potential is lowest in the medullary region and highest at the top of the ascending limb

Pituitary Gland

• The pituitary gland is an endocrine gland in the brain
• It secretes regulatory hormones into the bloodstream
• Regulatory hormones either have their own effects or stimulate glands to secrete hormones
• ADH: Antidiuretic hormone; the pituitary gland is important in osmoregulation because it secretes ADH

Antidiuretic Hormone (ADH)

• ADH is secreted by the posterior pituitary gland
• It decreases water loss in urine by increasing the water reuptake capacity of the collecting duct

Aquaporins

• These are a type of intrinsic membrane protein channel
• They allow the passage of water across membranes

How ADH Increases Water Reuptake

• ADH binds to cell membrane receptors on the collecting duct cells
• This triggers an intracellular signaling cascade
• The cascade results in the exocytosis of vesicles that contain aquaporins embedded in their membranes
• Aquaporins become part of the plasma membrane after exocytosis

ADH Secretion by the Pituitary Gland

• Osmoreceptors in the hypothalamus detect low blood water content and generate an action potential
• The action potential travels down the pituitary stalk to the pituitary gland
• The pituitary gland then secretes ADH into the bloodstream

Transcription Factors

• Transcription factors are proteins that bind to regulatory regions of DNA and control DNA transcription
• They can turn genes on or off

Peptide Hormones and DNA Transcription

• Peptide hormones affect DNA transcription
• This occurs by binding to extracellular receptors which produce changes within cells, brought about by intracellular signaling cascades

Peptide Hormones and Extracellular Receptors

• Peptide hormones bind to extracellular receptors because they are hydrophilic
• Due to being hydrophilic, they cannot pass through the plasma membrane

Steroid Hormones and DNA Transcription

• Steroid hormones diffuse into the nucleus
• Then bind to nuclear or cytoplasmic receptors, directly altering DNA transcription by binding to regulatory sections

Example of a Peptide Hormone

• Insulin is an example of a peptide hormone
• It binds to membrane-bound insulin receptors
• Insulin is secreted by the beta cells of the pancreas
• It regulates blood glucose concentration

Example of a Steroid Hormone

• Testosterone is an example of a steroid hormone
• It binds to a nuclear receptor called the androgen receptor, which is a transcription factor
• Testosterone is secreted by the testes
• It is the primary male sex hormone involved in the development of male sex organs

Function of Transcription Factors

• Transcription factors bind to DNA
• They either promote or inhibit transcription by blocking or enhancing the activity of RNA polymerase

Explore the gross structure of the mammalian kidney, including the cortex and medulla. Learn about the detailed structure of a nephron, covering the glomerulus, Bowman's capsule, and convoluted tubules. Understand the associated blood vessels and kidney histology.