#13350 - Urinary Histology - Organisation of the Body

Urinary system

Structure : consists of 2 kidney, 2 ureters, 1 bladder, 1 ureter

Function:

Fromation of urine: Through filtration, active absorption, passive absorption, secretion, excretion

-remove toxins, and eliminate waste products

-Regulates body fluid osmolarity and electrolyte balance

Endocrine function

-regulates blood pressure and volume via the juxataglomerular apparatus and secrtion of Renin

-Erythrocyte production through production of hormone erythropoietin in response to hypoxia

-calcium homeostasis- hydroxylation of vitamin D to active form

Structure of the kidney

-Red bean shaped organ on the posterior abdominal wall- concave border which is the hilum and is where the renal vein, renal artery, renal pelvis, nerves enter and leave and convex lateral surface. The pelvis is divided into 2/3 major calyces, which arise from minor calyces

-Seperated into an outer cortex, dark brown and granular- subdivided into the cortical labyrinth and medullary rays and medulla, contains 10-18 renal pyramids- each constitutes a lobe of the kidney- apex of each pyramid is perforated by 15-20 papillary ducts of Bellini at the area criborasa

-Region of medulla between neighbouring renal pyramids is occupied by cortical like material known as renal coloumns

-So urine flows through renal pyramids, calyces, renal pelvis, ureter

-Fibrous capsule- smooth muscle, collagen, elastic fibres

Arterial blood supply:

Each kidney supplied by the renal artery-direct branch of the abdominal aorta

Subdivides into interlobar arteries which pass between neighbouring pyramids towards the cortex– at the corticomedullary junction - these give rise to arcuate arteries which follow the base of the renal pyramid

-Small interlobular arteries derived from arcuate arteries enter cortical labyrinth to reach renal capsule

-Along the extent of the interlobulatr arteries- smaller vessels- afferent glomerular arterioles arise and are enveloped by Bowman’s capsule and form capillary plexus- glomerulus

These give rise to interlobular arteries which supply the cortical labyrinth and give rise to afferent glomerular arterioles

draisn into the efferent arteriole into peritubular capillaries – vasa recta

Venous blood supply

Interstitium and capsule are drained by interlobular veins these enter arcuate veins enters the interlobar veins renal vein inferior vena cava

Functional unit of the kidney: Uriniferous tubule – 2 parts each derived from different regions of Intermediate mesoderm

-Nephron: (1-3million present) produces the plasma filtrate, 85% lies in the cortical region, 15% lies in the juxtamedullary

-Nephric tubule/collecting tubule: Modifies the volume and composition of filtrate that produces urine

-Uriniferous tubule are densely packed , epithelial in nature, separated from the connective tissue/stroma by basal lamina, most of the connective tissue is richly vascularised

Structure of a nephron

A) RENAL CORPUSCLE

-Each nephron begins with an oval structure known as the renal corpuscle/Malphigian body which consists of a bowman’s capsule-dilated pouch at proximal end of renal tubule and glomerulus which is a tuft of capillaries invaginated into the bowman’s capsule

-Surrounding the bowman’s capsule is a parietal layer made of simple squamous epithelium, visceral layer envelops the capillaries and has highly specialised podocytes. Bowmans space is between the layers.

-The renal corpuscle has a vascular pole through which arterioles enter and leave- the glomerulus, network of fenestrated capillaries, is supplied by the afferent arteriole and is drained by efferent arteriole . Proximal convoluted tubule leaves at the urinary pole

Function: Ultrafiltration of the glomerulus to form ultrafiltrate

-Filtration barrier: Fenestrated capillary, basal lamina, podocyte slit pore .

-Fenestrated capillary-endothelial cells are attenuated, pores are 70-90nm- so very permeable: filter to macromolecules such as proteins and Blood cells

-However permeability can be altered due to mesangial cells

Glomerulus: connective tissue consists of mesangial cells, specialised smooth muscle-function is to regulate blood flow through the capillaries

-Extraglomerular mesangial cells/Lacis cells are found on vascular pole and relay info

-Intraglomerular mesangial cells found in the renal corpuscle, pericyte (smooth muscle) like cells phagocytic, respond to vasoactiv hormones and contraction narrow capillaries, synthesis of NO in intercellular signalling systems

-Basal lamina filters blood leaving the capillaries- made of three layer, Lamina rara interna, Lamina densa (type 4 collagen), Lamina rara externa (Heparin sulfate). The collagen sperates molecules by size, heparin sulphate seperates by charge so larger negatively charged molecules are filtered less easily

-Podocytes – adjacent podocytes interdigitate to cover the basal lamina- primary processes have numerous tentacle like cytoplasmic extensions which form secondary processes /pedicels which completely envelop the most of the capillaries – they attach to the laminin of the basement membrane by ingtegrins attached to alpha actinin and talin

- Filtration slits formed by interdigitating secondary processes/pedicels are covered by thin slit diaphragm made up of a mesh of negative nephrin molecules- these retard the passage of molecules crossing the endothelial fenestrations

B) TUBULAR SYSTEM OF NEPHRON

Function: Reabsorption

Structure:

Proximal tubule: has two regions, pars convuluta (tortuous region), Pars recta (straight portion which fuses with the descending limb of Henle)

-Made of cuboidal epithelium-abundant microvilli on luminal surface forming brush border, have canaliculi, lateral and basal membrane are also convoluted

-Basal membrane is rich in mitochondria that provide ATP for active ion transport , abundant lysosomes for endocytosis and break down of muscle

-Function:

Selective reabsorption- 80% of water, Na, CL, 100% of glucose, amino acids, vitamins move from the lumen into interstitium through diffusion and back into blood by active transport. Reabsorbed materials are returned into the peritubular capillary network of the cortical labyrinth for distribution to the remainder of the body

Salt and water are absorbed in equimolar concentrations so osmolarity of the ultrafiltrate is not altered and remains the same as that in blood

Henle’s loop: Descending thin limb, hair pin fold, ascending thin limb, asending thick limb

-Squamous epithelium

a) Thin descending loop of Henle

-In juxtaglomerular nephrons it extends to the apex of the medullary pyramid where it forms a hairpin loop and continues toward the cortex as the ascending thin limb of Henle’s loop

-Verypermeable to water – prescence of AQP1, So water passes to intersitium as ultra filtrate attemps to equilibriate its osmolarity with the interstituim. Low permeability to urea and ions

-Thin segments are lined with squamous epithelium

b) Thin ascending limb

-Not permeable to water due to abscence of AQP but permeable to Na+ , Cl- so filtrate gets more dilate and hypertonicity in the interstituim due to high concentrations of Na+ and Cl- in medulla

c) Vasa rectra

-Water and solutes are reabsopred by vasa recta – branch of the glomerular artery and surround the loop of Henle

-Essential in the maintainence of the a hypertonic medullay interstituim by counter current exchange- requires a very slow flow.

-Made of continuous endothelial cells on the descending limb of vasorecta and Fenestrated endothelial cells on ascending limb

Distal tubule

Function: Ion exchange and maintaining acid base balance of blood

Components:

Pars Recta – asending thick limp of Henle

Function: Form concentrated urine

-impermeable to water but has a Na/k/2CL- cotransporter on luminal surface and actively pumps Na, Cl from lumen into cell

-Basally located Na/K ATPase pump transfers Na, Cl out of cell into renal interstium

-Water can’t leave or enter lumen ultrafiltrate is hypoosmotic by the time it reaches the macula densa region

-Lined with simple cuboidal cells

-cells that contact the afferent glomerular arterioles are Macula densa of the distal tubule- they are thin, tall, cuboidal cells whose nuclei are close togethers- monitor osmolarity and volume of ultrafiltrate

-If either of these are elevated- macula densa cells via gap junctions induce the juxtaglomerular cells (modified smooth muscle cells of the afferent and efferent glomerular arterioles) to release stored proteolytic enzyme, Renin into blood stream. Renin cleaves amino acids from circulating decapapeptide angiotensinogen changing it to angiotenin 1 which is then cleaved by converting enzyme located on luminal surfaces of capillaries forming angiotenin 2- powerful vasoconstrictor and promotes release of aldosterone

-Aldosterone- binds to receptors on cells of the distal convultued tubules- prompting them to resorb Na, Cl- from ultrafiltrate

Pars convulata (distal convoluted tubule)

-Cuboidal cells are shorter than the proximal collecting tubules and have no brush border

-Elaborate basal membrane invaginations- for ion transport

-Cells have ion pumps for active transport

-NaCl is reabsorbed, K+ secreted

-Cells posses aldoesterone receptors which when activated resorbs sodium ions and secretes H+, K+, NH4+ into ultrafiltrate

Collecting duct

-several distal convulted tubules join each collecting duct

-Collecting duct - descend from the medullary rays of the cortex through the renal pyramids- as the ultrafiltrate passes down it is subject to increasing osmotic gradients of the renal interstituim-in the absence of ADH the cells of...