INTRODUCTION TO EXCRETORY SYSTEM

Dr.U.Rathnakumari MBBS.,

MD Physiology I yr PG

Gross Anatomy of the Kidney

Major Functions of the Kidneys

1. Regulation of HOMEOSTASIS: • body fluid osmolarity and volume• electrolyte balance• acid-base balance• blood pressure

2. EXCRETION of • metabolic products• toxic substances (drugs,pesticides, etc.)• excess substance (water, etc)

3. SECRETION of :

• Erythropoietin• 1,25-dihydroxy cholecalciferol

vitamin D3 (vitamin D activation)• Renin• prostaglandin

Physiological Anatomy of the Kidney

Nephron

• The functional unit of the kidney

• Each kidney is made up of about 1 million nephrons

• Each nephron has two major components:

1. A glomerulus (a tuft of capillaries)2. A long tube (tubule)

The juxtaglomerular apparatus

• Where the distal tubule lies against the afferent (sometimes efferent) arteriole

• Arteriole walls have juxtaglomerular (JG) cells– Enlarged, smooth muscle cells – Have secretory granules containing RENIN– Act as mechanoreceptors

• Macula densa– Tall, closely packed distal tubule cells – Lie adjacent to JG cells – Function as chemoreceptors or

osmoreceptors

• Mesanglial cells: – Have phagocytic and contractile properties– Influence capillary filtration

Renin Angiotensin system

Renal Tubule Proximal convoluted tubule (PCT) • cuboidal cells with numerous microvilli and mitochondria

• Reabsorbs water and solutes from filtrate and secretes substances into it

• Loop of Henle – a hairpin-shaped loop of the renal tubule– Proximal part is similar to the proximal convoluted

tubule– followed by the thin segment (simple squamous cells)

and the thick segment (cuboidal to columnar cells)

Distal convoluted tubule (DCT) • cuboidal cells without microvilli that

function more in secretion than reabsorption

Collecting Tubules• Two important cell types are found here

– Intercalated cells• Cuboidal cells with microvilli • Maintainin acid-base balance of the body

– Principal cells• Cuboidal cells without microvilli• maintain the body’s water and salt balance

Types of nephrons

• Cortical nephrons – 85% of nephrons; located in the cortex

• Juxtamedullary nephrons:– Are located at the cortex-medulla junction– Have loops of Henle that deeply invade the

medulla – Have extensive thin segments– Are involved in the production of concentrated

urine

renal blood flow

Characteristics of renal blood flow

high blood flow. • 1200 ml/min, or 21 percent of the cardiac

output. • 94% to the cortex Two capillary beds• High hydrostatic pressure in glomerular

capillary (about 60 mmHg) and low hydrostatic pressure in peritubular capillaries (about 13 mmHg)

Capillary Beds

Vasa recta – long, straight efferent arterioles of juxtamedullary nephrons

Functions of the Nephron

1. Filtration

1. Reabsorbtion

1. Secretion

1. Excretion

Glomerular filtration

• First step in urine formation– Bulk transport of

fluid from blood to kidney tubule

• Isosmotic filtrate• Blood cells and

proteins don’t filter

– Result of hydrostatic pressure

– GFR = 180 L/day

filtration membrane

filtration membrane

• Filter that lies between the blood and the interior of the glomerular capsule

• It is composed of three layers– Fenestrated endothelium of the glomerular

capillaries– Visceral membrane of the glomerular capsule

(podocytes)– Basement membrane composed of fused

basal laminae of the other layers

filtration membrane

Glomerular filtration rate (GFR)

• Amount of filtrate produced in the kidneys each minute. 125mL/min = 180L/day

• Factors that alter filtration pressure change GFR. These include: – Increased renal blood flow -- Increased GFR– Decreased plasma protein -- Increased GFR.

Causes edema.– Hemorrhage -- Decreased capillary BP --

Decreased GFR

Tubular Reabsorbtion

– Process of returning filtered material to bloodstream

– 99% of what is filtered– May involve transport protein(s)– Normally glucose is totally reabsorbed

Tubular Secretion

– Material added to lumen of kidney from blood– Active transport (usually) of toxins and foreign

substances, drugs etc• Saccharine• Penicillin

Mechanisms of Urine Formation

involves three major processes

1.Glomerular filtration2.Tubular

reabsorption3.Secretion

QUESTIONS

1. What are the parts of the excretory system?

2 . name the functions of the kidneys.

3.How is the kidney related to hematopoiesis?

• 4.How does the kidney regulate blood calcium level?

5.What is the total no .of nephrons in the human body?

• 6.What are the types of nephrons you know?

7.What is a renal corpuscle?

name its parts.

8.Describe the epithelium of PCT.

9.Name the parts of JGA- The juxtaglomerular apparatus.

10.Define GFR. what is its normal value?

11.What is the normal volume of urine excreted in one day?

• what is ‘POLKISSEN’?