BIOS3070 Week 29 Lecture 1 and 2- Kidneys

BIOS3070 Human Physiology and Disease 2

Instructor: Dr. Emma HargreavesContact: e.j.mead@kent.ac.uk

Week 29: The Kidneys

Focus: Exploration of kidney anatomy, their multifaceted functions, hormonal control mechanisms, and associated diseases and disorders affecting renal health.

Chapter Overview

Reference: Human Physiology, An Integrated Approach, 8th Edition, Global Edition; Chapters 19/20. Topics covered in this chapter include:

• Hormonal control mechanisms involved in fluid and electrolyte balance.

• Comprehensive reviews of kidney diseases and disorders, including their pathophysiology, diagnostic criteria, and management strategies.

Learning Outcomes

Upon completion of this week’s lessons, students should be able to:

• Describe and explain the multiple functions of the kidneys including their role in homeostasis.

• Understand the regulation of:

  • Extracellular fluid volume

  • Blood pressure, including mechanisms of regulation such as the renin-angiotensin-aldosterone system

  • Osmolarity of body fluids

  • Ion balance, particularly sodium, potassium, and calcium levels

  • pH regulation through bicarbonate conservation and hydrogen ion secretion

  • Waste excretion processes including urinary output and composition

  • Endocrine functions through hormone production, specifically erythropoietin and calcitriol.

Anatomy of the Urinary System

Gross Anatomy:

• Kidneys: Located in a retroperitoneal position, typically just beneath the lower ribs (T12-L3 vertebrae), encapsulated by a protective renal fascia.

• Adrenal Glands: Situated atop each kidney, these glands produce hormones that influence metabolic processes, including cortisol and adrenaline.

• Ureters: Muscular tubes that transport urine from the kidneys to the bladder via peristaltic movements.

• Bladder: A muscular sac that stores urine before it is expelled during micturition.

Nephron Anatomy

Nephron as the Functional Unit:

• Each kidney contains approximately one million nephrons. Each nephron comprises:

  • Renal Corpuscle: Includes the glomerulus (a ball of capillaries) and Bowman's capsule where filtering occurs.

  • Renal Tubule: Consists of proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct, which modify the filtrate through reabsorption and secretion processes.

Kidney Function

Key Processes:

1. Filtration:

   • Occurs at the glomerulus where blood pressure forces plasma and solutes into Bowman's capsule, forming the initial filtrate, which is isosmotic to plasma.

2. Reabsorption:

   • Essential substances (water, ions, glucose) are selectively returned to the bloodstream at various segments of the nephron, particularly the proximal tubule (70% of filtrate volume).

   • Main Sites: Proximal tubule, loop of Henle, distal tubule, and collecting duct, making adjustments based on the body's demands.

3. Secretion:

   • Additional toxins and excess ions are actively secreted into the tubular fluid from the blood, participating in homeostatic balance of electrolytes and metabolic waste.

Mechanism of Filtration

Filtration Fraction:

• About 20% of the plasma enters the nephron at the glomerulus, with less than 1% ultimately being excreted as urine.

Glomerular Filtration Rate (GFR):

• A critical measure of kidney function, indicating the volume of filtrate produced by both kidneys per minute, influenced by net filtration pressure and renal perfusion.

• Normal GFR ranges from 90 to 120 mL/min in healthy adults.

Juxtaglomerular Apparatus (JGA)

• A specialized structure that regulates GFR through sensing changes in tubular fluid flow and sodium concentration; it releases renin that catalyzes the production of angiotensin II, impacting blood pressure and electrolyte balance.

Reabsorption Processes

Types of Transport:

• Active Transport: Predominantly sodium ions through sodium-potassium pumps.

• Passive Reabsorption: Driven by concentration gradients, affecting urea and water recapture.

• Transport Pathways: Consist of transepithelial (involving cell membranes) and paracellular routes (between cells).

Examples of Renal Handling:

• Sodium: Reabsorbed variably along the nephron (80% in proximal tubule, 20% in loop of Henle).

• Glucose: Nearly 100% reabsorption under normal conditions via sodium-glucose cotransporters.

• Potassium: Flexible handling based on dietary intake; can be secreted or reabsorbed as needed.

• Urea: Partially secreted and reabsorbed, contributing to osmolarity in the medulla.

Micturition Process

• Urine is stored in the bladder until autonomic and voluntary control mechanisms coordinate its expulsion via the urethra. The process involves reflex arcs and higher brain functions to facilitate appropriate timing.

Hormonal Regulation

1. Vasopressin (ADH):

   • Facilitates water reabsorption in the collecting ducts, concentrating urine when osmolarity is high or blood pressure is low.

2. Aldosterone:

   • Regulates sodium and potassium levels; released in response to low blood pressure to enhance sodium retention and potassium excretion.

3. Renin-Angiotensin Pathway:

   • Activated by decreased renal perfusion, leading to increased blood pressure and fluid retention via the systemic effects of angiotensin II.

Acid-Base Balance

• Normal blood pH maintained within 7.38-7.42, utilizing renal mechanisms for hydrogen ion secretion and bicarbonate reclamation to mitigate acidosis and alkalosis.

Urinalysis

Common Indicators:

• Evaluating specific gravity for hydration status, while the presence of glucose, ketones, or proteins can signal pathophysiological conditions including diabetes and nephrotic syndrome.

Urinary System Disorders

Common Issues:

• Infections: Particularly urinary tract infections (UTIs), disproportionately affecting females due to shorter urethra.

• Diabetes Insipidus: Characterized by an inability to concentrate urine due to inadequate vasopressin response, resulting in polyuria and thirst.

• Kidney Stones: Formed from concentrated minerals and can obstruct urine flow, leading to hydronephrosis if untreated.

• Chronic Renal Failure: A progressive decline in renal function, necessitating dialysis or possible kidney transplantation for survival.