Urinary System

The Urinary System

B. Pimentel, M.D. University of Makati College of Nursing

The Urinary System STRUCTURE

FUNCTION

Kidney

Filtration, regulation of blood volume, regulation of solutes, pH of extra cellular fluid, RBC synthesis, Vit. D synthesis

Ureters

Urine flows from kidneys to urinary bladder

Urinary bladder

Storage of urine

Urethra

Transports urine out from the urinary bladder

The Urinary System

Kidneys (Anatomy) • Bean shaped, retroperitoneal, and located in the superior lumbar region, approx. size 11 x 5 x 3 cm. • Renal capsule – layer of fibrous connective tissue • Perirenal fat (Adipose capsule) – dense layer of adipose tissue, engulfs the renal capsule. (Shock absorber) • Renal fascia – anchors the kidneys and surrounding adipose tissues to the abdominal wall.

Kidneys (Anatomy) • Hilum – medial side of kidney where the renal artery and nerve enter; renal vein and urethra exit. • Renal sinus – a cavity at the opening of the Hilum filled with fat and connective tissue. • Adrenal (or suprarenal) gland – sits on top of both kidneys

Kidneys (Anatomy)

Kidneys (Internal Anatomy) A. Cortex B. Medulla C. Pelvis

Kidneys (Internal Anatomy) A. Cortex –

Superficial region

B. Inner medulla –

Renal pyramids – cone shaped structures that make up the medulla.

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Medullary rays – extend from the renal pyramids into the cortex.

Kidneys (Internal Anatomy) – Renal columns – consists of the same tissue as the cortex that projects between the renal pyramids. • The bases of the pyramids form the boundary between the cortex and the medulla.

– Renal papillae – tips of the pyramids project toward renal sinus.

Kidneys (Internal Anatomy) C. Pelvis •

Occupies a large portion of the renal sinus

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An open space which forms the ureter as it exits the kidney

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Forms the major and minor calyces which enclose the papillae of the renal pyramids

Kidneys (Internal Anatomy) – Minor calyces – funnel shaped chambers into which the renal papillae extend. – Major calyces – a larger funnel formed by the minor calyces of several pyramids.

Kidneys (Internal Anatomy) – Each kidney contains 8 to 20 minor calyces and 2 to 3 major calyces. – Renal pelvis – convergence of the major calyces to form this enlarged chamber. – Ureter – narrowed renal pelvis into a small diameter tube exits at the Hilum and connects to urinary bladder.

Kidneys (Blood Supply)

Kidneys (Blood Supply) • Renal artery - branches from abdominal aorta. • Segmental arteries - diverge from renal arteries. • Lobar arteries - branch from segmental arteries and ascend within the renal columns toward the renal cortex. • Interlobular arteries - branch to form arcuate arteries which project into cortex.

Kidneys (Blood Supply) • Arcuate arteries - branch from interlobar arteries, diverge near the base of each renal pyramid and arch over the bases of the pyramids. • Afferent arterioles - branch from interlobular arteries supply blood to the glomerular capillaries. • Efferent arteriole - arise from glomerular capillaries and carry blood away from the glomeruli. • Peritubular capillaries - as efferent arterioles exits the glomerulus joins a plexus of capillaries around the proximal and distal tubules. Low pressure capillaries which are porous aswell.

Kidneys (Blood Supply)

Kidneys (Blood Supply) • Vasa recta - specialized parts of the peritubullar capillaries that course into the medulla along the loops of Henle and then back toward the cortex. • Interlobular veins - drainage of peritubullar capillaries. • Arcuate veins - drainage of the interlobular veins. • Interlobar veins - drainage of arcuate veins. • Renal vein - drainage of interlobar veins. Exits the kidney at the hilum empties into inferior vena cava.

Kidneys (Blood Supply)

Kidneys (Histology)

Kidneys (Histology) Nephron •

Histological and functional unit of the kidney.

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Tube like structure which serves a the site for filtration, reabsorption, and secretion

Kidneys (Histology) •

Juxtamedullary nephron – nephrons whose renal corpuscles lie near the medulla. With long loops of Henle extending deep into the medulla, account for 15% of nephrons.

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Cortical nephrons – loops of Henle do not extend deep into medulla.

Kidneys (Histology) •

Renal corpuscle – consists of the enlarged end of the nephron (Bowman’s capsule), and a network of capillaries (glomerulus).

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Bowman’s capsule – enlarged terminal end of the nephron. Indented to form a double walled chamber.

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Glomerulus – capillary within Bowman’s capsule.

Kidneys (Histology) Filtration membrane •

Filtration slits – consists of capillary endothelial fenestrae and podocyte cells of the visceral layer of the Bowman’s capsule which wrap around the glomerular capillaries.

Kidneys (Histology) Afferent arteriole – supplies blood to the glomerulus. Efferent arteriole – drains blood from the capillary.

Kidneys (Histology) Proximal tubule – simple cuboidal epithelium with microvilli projecting from the luminal surface. Loops of Henle – continuations of the proximal tubules, consists of two loops: – descending and ascending loops.

Kidneys (Histology) Descending loop of Henle – first part is similar in structure to the proximal tubule the loop that extends into the medulla become very thin near the end of the loop. – There is an abrupt change to simple squamous epithelium.

Kidneys (Histology) Ascending loop – continuous in diameter of the descending loop then thickens with simple cuboidal cells near the renal corpuscle.

Kidneys (Histology) Distal tubule – shorter than proximal tubules, simple cuboidal epithelium, – do not posses a large number of microvilli.

Kidneys (Histology) Collecting ducts – connection of many distal tubules of many nephrons. Forms much of the medullary rays and extend through the medulla towards the tips of the renal pyramids.

Urine Production • Filtration – movement of fluid across the filtration membrane as a result of a pressure difference. – Filtrate – fluid entering the nephron.

• Reabsorption – movement of substances from the filtrate back into the blood. • Secretion – active transport of solutes into the nephron

Urine Production

Urine Production (Filtration) • Renal fraction – the part of the cardiac output that passes through the kidneys. Average 21% of cardiac output. • Filtration fraction – part of the plasma flowing through the kidney that is filtered through the filtration membrane into the lumen of Bowmen’s capsule to become filtrate. • Glomerular filtration rate – the amount of filtrate produced each minute. 120-125ml/min. – 1 to 2 liters of urine are produced per day.

Urine Production (Filtration) • Filtration pressure – pressure gradient which forces fluid from the glomerular capillary across the filtration membrane into the lumen of Bowmen’s capsule. – Smooth muscle in the walls of the afferent and efferent arterioles can alter the vessel diameter and the glomerular filtration pressure. – The diameter of the afferent arteriole is greater than the diameter of the efferent arteriole. – High hydrostatic pressure – Glumerular endothelium is highly permeable

Urine Production (Filtration)

Urine Production (Reabsorption) • 99% of the filtrate leaves the nephron (proximal and distal tubules, and loop of Henle) to enter the interstitial fluid. • These substances then enter the low pressure peritubullar capillaries and flow through the renal veins back to general circulation. • Solutes reabsorbed from the lumen of the nephron to the interstitial fluid include; Potassium, Sodium, Calcium, Magnesium, Chloride, Phosphate • Organic nutrients: Glucose and Amino acids

Urine Production (Reabsorption)

Urine Production (Reabsorption) • Sodium reabsorption – Active transport via Na+/K pump

• Water reabsorption – Obligatory due to osmotic gradient created by Na+ reabsorption

Urine Production (Reabsorption) • Glucose reabsorption – secondary active transport or cotransport

Urine Production (Reabsorption)

Urine Production (Reabsorption) • Proximal convoluted tubule – Highest concentration of reabsortion – NaCl and H2O

• Loop of Henle – NaCl and H2Oreabsorption will be crucial for the creation of an osmotic gradient between the renal cortex and renal medulla.

• DCT – NaCl and H2O reabsorption influenced by the presence of aldosterone.

• Collecting duct – reabsorption of H2O due to ADH

Urine Production (Secretion) • The movement of some substances, such as by products of metabolism that become toxic in high concentrations and drugs or molecules not normally produced by the body, into the nephron. • Some of the secreted products include: – Ammonia, Potassium, Hydroxybenzoates, Neurotransmitters, Bile pigments, Uric acid, Drugs, Toxins

Thinking back… 1.

Glumerular Filtration Rate (GFR)

3.

Filtration Pressure

5.

Filtration Membrane

Thinking back…

Thinking new!

Urine Concentration • Osmolarity – A solution's osmolarity is the number of solute particles dissolved in the volume of solvent. – It's indicative of the solution's ability to cause osmosis. – Normal plasma osmolality: 300 milliosmoles per liter

Urine Concentration • The loops of Henle in the juxtamedullary nephrons creates this osmotic gradient – As filtrate passes through the ascending limb of the loop of Henle, sodium is pumped out into the interstitial fluid. – As the filtrate reaches the distal tubule, the filtrate is dilute or hyposmotic. • Ascending limb of the loop is impermeable to water, so water can't follow the pumped sodium. • The urine osmolarity falls to about 100 mOsm/L. • The salt pumped out raises the ISF osmolarity to as high as 1200 mOsm/L in the medulla of the kidney. – This creates a gradient with the cortex isotonic to plasma (300 mOsm/L) and the medulla very hypertonic (1200 mOsm/L).

Urine Concentration

Urine Concentration

Urine Concentration • When large amounts of water are ingested it is necessary to excrete large amounts of water but not large amounts of solutes. Thus, the urine excreted is of low concentration. • When small amounts of fluid are ingested, the urine must be concentrated with the small amount of water excreted to avoid dehydration.

Urine Concentration • These are accomplished by; – Medullary concentration gradient – maintains a high concentration of solutes in the medulla, this is dependent on the function of the loops of Henle, the vasa recta, and on the distribution of urea. – Countercurrent system – the vasa recta perform this function. It is a system of parallel tubes in which fluid flows in opposite directions. The walls of the vasa recta are permeable to water and solutes. – Urea – responsible for a substantial part of the high osmolality in the medulla.

Regulation (ADH) • Acts on distal tubules and collecting ducts. • Absence of ADH results in impermeability of water at the distal tubule and collecting ducts. • Diabetes insipidus – insufficient secretion of ADH, production of up to 20 liters of dilute urine per day.

Regulation (ADH) • Stimulation of ADH secreting neurons is regulated by osmolality of the blood and the interstitial fluid, and blood pressure. – Reduced osmolality and increasing blood pressure inhibits secretion of ADH. – Increased osmolality and decreased blood pressure increase secretion of ADH.

Regulation (Aldosterone) • Increases the transport of sodium out of the filtrate and back into the blood • Resulting in increased urine volume and increased sodium concentration.

Renin-Angiotensin-Aldoeterone Axis

Renin-Angiotensin-Aldoeterone Axis During low [Na] or low BP •

Juxtaglumerular cells secrete renin which converts angiotensinogen into angiotensin I

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Angiotensin I will be converted into angiotensin II by Angiotensin Converting Enzyme (ACE)

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Angiotensin II is a potent vasoconstrictor and thus increases BP – The increase in BP will increase Filtration pressure and GFR. – Stimulate the release of aldosterone from the adrenal glands and ADH from the posterior pituitary

Regulation (Autoregulation) • Involves changes in the degree of vasoconstriction of the afferent arterioles. • Maintenance, within the kidneys, of a relatively stable glomerular filtration rate. • Glomerular filtration rate is relatively constant as systemic blood pressure changes between 90 and 180 mmhg.

Regulation (Autoregulation) • Increases in blood pressure causes the vasoconstriction of the afferent arterioles to prevent an increase in renal blood flow and the filtration pressure across the filtration membrane. • Decreased blood pressure results in less vasodilation of the afferent arterioles thus preventing a decrease in renal blood flow and filtration pressure across the filtration membrane.

Regulation (Autoregulation)

Regulation (Autoregulation)

Micturation Reflex • Activated by stretching of the urinary bladder wall, resulting elimination of urine. • Micturition center is located in the pons and cerebrum. • Parasympathetic stimulation causes the walls to contract and the external urinary sphincter to relax.

Urethers • Slender tubes which serves to conduct urine from the kidneys to the bladder • Histology: Transitional epithelium with smooth muscles and a fibrous adventitia

Urinary Bladder • A muscular sac which is located anterior to the rectum in males and in females, is anterior to the vagina and uterus. • Trigone: Triangular region within the bladder which forms the opening for 2 urethers and urethra • Histology: Transitional epithelium with a muscular layer (detrusor muscle)

Urinary Bladder and Urethra • A thin walled muscular tube which drains the urine out from the bladder • Histology: – Proximal, transitional epithelium – Middle, pseudostratified columnar epthelium – Distlally, stratified squamous

• Sphincters – Internal – junction between the bladder and urethra (involuntary) – Extrenal - voluntary

Urethra

END