Urinary Histology Notes

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...

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