Chapter 25

Function of Kidneys

The kidneys regulate water volume, solute concentration, ion concentrations, acid-base balance, excrete wastes, and produce erythropoietin and renin. As well as Activating vitamin D and carrying out gluconeogenesis.

Urinary System Components which involve the kidney

The urinary system includes ureters (transport), urinary bladder (storage), and urethra (transport) for urine transport and storage.

Location & External Anatomy of Kidneys

Located retroperitoneal and the right kidney will be crowded by the liver so it will be lower than the left. There are three layers of supportive tissue, Renal Fascia, Perirenal fat capsule and fibrous capsule

Renal Fascia (supportive tissue)

Anchoring outer layer of dense fibrous connective tissue

Perirenal fat capsule (supportive tissue)

Fatty cushion

Fibrous capsule (supportive tissue)

Transparent capsule that prevents spread of infection to kidney

Gross Anatomy of Kidneys

The kidneys have a renal cortex, renal medulla with pyramids, renal pelvis, and calyces for urine collection and flow.

Minor calyces

Cup-shaped areas that collect urine draining from pyramidal papillae

Major Calyces

Areas that collect urine from minor calyces and Empty urine into renal pelvis

Blood Supply

Kidneys cleanse blood and adjust its composition, so it has a rich blood supply. Renal arteries deliver about one-fourth (1200 ml) of cardiac output to kidneys each minute

Arterial Flow

Renal → segmental → interlobar → arcuate → cortical radiate (interlobular)

Venous Flow

Cortical radiate → arcuate → interlobar → renal veins

• No segmental veins

Nephrons

These are the functional units of the kidney, consisting of renal corpuscle and renal tubule for urine formation.

Renal Corpuscle

Two parts of renal corpuscle, the glomerulus and the glomerular capsule

Glomerulus

1. Tuft of capillaries composed of fenestrated endothelium

2. Highly porous capillaries

3. Allows for efficient filtrate formation

4. Filtrate: plasma-derived fluid that renal tubules process to form urine

Glomerular Capsule

Also called Bowman’s capsule: cup-shaped, hollow structure surrounding glomerulus. Has two layers parietal layer and visceral layer.

Parietal layer

Simple squamous epithelium

Visceral Layer

Clings to glomerular capillaries; branching epithelial podocytes

Renal Tubule (Description)

• This is about 3 cm (1.2 in.) long

• Consists of single layer of epithelial cells, but each region has its own unique histology and function

Renal Tubule (Three Major part)

1. Proximal Convoluted Tubule

2. Distal Convoluted Tubule

3. Nephron Loop

Proximal Convoluted Tubule

• Closest to renal corpuscle

• Cuboidal cells with dense microvilli that form brush border. Increase surface area and also have large mitochondria.

• Functions in reabsorption and secretion

• 65% of Na+ and water reabsorbed, many ions, uric acids, half of urea

• Confined to cortex

Nephron loop (Henle)

U-shaped structure consisting of both the descending and ascending limb.

• Thin segment is passive to Na+ movement

• Thick segment has Na+-K+-2Cl– symporters and Na+-H+ antiporters that transport Na+ into cell

Distal convoluted tubule and collecting duct (hormones)

Reabsorption is hormonally regulated in these areas

1. Antidiuretic Hormone

2. Aldosterone

3. Atrial natriuretic peptide

4. Parathyroid hormone

Antidiuretic Hormone (ADH)

Released by posterior pituitary gland and an increased ADH levels cause an increase in water reabsorption

Aldosterone

Targets collecting ducts (principal cells) and distal DCT and will cause little Na+ to leave the body. Without aldosterone, daily loss of filtered Na+ would be 2%, which is incompatible with life. Functions to increase blood pressure and decrease K+ levels.

Atrial natriuretic peptide

1. Reduces blood Na+, resulting in decreased blood volume and blood pressure. Released by cardiac atrial cells if blood volume or pressure elevated

Parathyroid hormone

Acts on DCT to increase Ca2+ reabsorption

Descending Limb

Proximal part of descending limb is continuous with proximal tubule. Distal portion also called descending thin limb; simple squamous epithelium. H2O cannot leave while solutes can leave.

Ascending Limb

Thick ascending limb, thin in some nephrons. H2O can leave however solutes can not.

Distal Convoluted Tubule (DCT)

Cuboidal cells with very few microvilli. Function more in secretion than reabsorption. This is also confined to cortex.

Collecting ducts

Two cell types principal cells and intercalated cells. These receive filtrate from many nephrons, Run through medullary pyramids, Give pyramids their striped appearance. The ducts fuse together to deliver urine through papillae into minor calyces.

Principal Cells (Collect duct)

1. Sparse with short microvilli

2. Maintain water and Na+ balance

Intercalated Cells (Collecting ducts)

1. Cuboidal cells with abundant microvilli

2. Two types of intercalated cells

3. A and B: both help maintain acid-base balance of blood

Classes of Nephrons

Two major groups of nephrons, cortical nephrons and juxtamedullary nephrons

Cortical Nephrons

1. Make up 85% of nephrons

2. Almost entirely in cortex

Juxtamedullary Nephrons

1. Long nephron loops deeply invade medulla

2. Ascending limbs have thick and thin segments

3. Important in production of concentrated urine

Nephron Capillary Beds

Renal tubules are associated with two capillary beds, the glomerulus and capillaries. Juxtamedullary nephrons are associated with the vasa recta

Glomerulus Capillaries

These are specialized for filtration. Different from other capillary beds because they are fed and drained by arteriole

Glomerulus (Afferent Arteriole)

• Enters glomerulus and leaves via efferent arteriole

• Arises from cortical radiate arteries

Glomerulus (Efferent Arteriole)

Feeds into either peritubular capillaries or vasa recta

Blood pressure is higher in the Glomerulus due to..

• Afferent arterioles are larger in diameter than efferent arterioles

• Arterioles are high-resistance vessels

Peritubular Capillaries

Low-pressure, porous capillaries adapted for absorption of water and solutes. Arise from efferent arterioles and cling to adjacent renal tubules in cortex. Empty into venules.

Vasa Recta

Long, thin-walled vessels parallel to long nephron loops of juxtamedullary nephrons. Arise from efferent arterioles serving juxtamedullary nephrons. Function in formation of concentrated urine.

Juxtaglomerular Complex (JGC)

Each nephron has one juxtaglomerular complex. Involves modified portions of:

1. Distal portion of ascending limb of nephron loop

2. Afferent (sometimes efferent) arteriole

3. Important in regulating rate of filtrate formation and blood pressure

3 cell populations are seen in JGC

1. Macula Densa

2. Granular cells

3. Extraglomerular mesangial Cells

Macula Densa (cells in JGC)

Tall, closely packed cells of ascending limb, contain chemoreceptors that sense NaCl content of filtrate

Granular Cells/ Juxtaglomerular cells (JGC cells)

Enlarged, smooth muscle cells of arteriole. Act as mechanoreceptors to sense blood pressure in afferent arteriole. Contain secretory granules that contain enzyme renin.

Extraglomerular mesangial cells

Located between arteriole and tubule cells and interconnected with gap junctions. May pass signals between macula densa and granular cells

Physiology of Kidney

• 180 L of fluid processed daily, but only 1.5 L of urine is formed

• Kidneys filter body’s entire plasma volume 60 times each day

• Consume 20–25% of oxygen used by body at rest

• Filtrate (produced by glomerular filtration) is basically blood plasma minus proteins

• Urine is produced from filtrate

Urine

• <1% of original filtrate

• Contains metabolic wastes and unneeded substances

Glomerular Filtration

The glomerulus filters blood plasma to form filtrate in the Bowman's capsule through hydrostatic pressure.

Glomerular Filtration Rate (GFR)

This is the volume of filtrate formed per minute by both kidneys (normal = 120–125 ml/min)

GFR (Proportional to)

1. Net filtration pressure (NFP): Primary pressure is glomerular hydrostatic pressure

2. Total surface area available for filtration: Glomerular mesangial cells control by contracting

3. Filtration membrane permeability: Much more permeable than other capillaries

Regulation of Glomerular Filtration

• Constant GFR is important as it allows kidneys to make filtrate and maintain extracellular homeostasis

  • Goal of local intrinsic controls (renal autoregulation): maintain GFR in kidney

GFR affects systemic blood pressure

• Increased GFR causes increased urine output, which lowers blood pressure, and vice versa

• Goal of extrinsic controls: maintain systemic blood pressure

Extrinsic controls

These are both neural and hormonal mechanisms. The purpose of extrinsic controls is to regulate GFR to maintain systemic blood pressure. These will override renal intrinsic controls if blood volume needs to be increased.

Renin-angiotensin-aldosterone mechanism

Main mechanism for increasing blood pressure

Three pathways to renin release by granular cells

• Direct stimulation of granular cells by sympathetic nervous system

• Stimulation by activated macula densa cells when filtrate NaCl concentration is low

• Reduced stretch of granular cells

Tubular Reabsorption

Reabsorption process in renal tubules returns nutrients and ions to the blood, regulated by hormones like ADH and aldosterone.

Transcellular Route

Solute enters apical membrane of tubule cells. Travels through cytosol of tubule cells. Exits basolateral membrane of tubule cells. Enters blood through endothelium of peritubular capillaries

Paracellular Route

Between tubule cells. Limited by tight junctions, but leaky in proximal nephron. Water, Ca2+, Mg2+, K+, and some Na+ in the PCT move via this route

Sodium concentrate effects…

the water that is input into the cells

Tubular Secretion

Tubular secretion moves substances from blood to filtrate, including K+, H+, and creatinine, to adjust blood composition.

There are two types of countercurrent mechanisms

Countercurrent multiplier & countercurrent exchanger

Countercurrent multiplier

Interaction of filtrate flow in ascending/descending limbs of nephron loops of juxtamedullary nephrons.

Countercurrent exchanger

Blood flow in ascending/descending limbs of vasa recta.

Gradient in the Kidney

Runs from 300 mOsm in cortex to 1200 mOsm at the bottom of the medulla, allowing more water movement deeper into the kidneys.

Urine’s chemical composition

Consists of 95% water, nitrogenous wastes (urea, uric acid, creatinine), and normal solutes (Na+, K+, PO43–, SO42–, Ca2+, Mg2+, HCO3–).

Urine’s physical characteristics

Includes color (pale to deep yellow), transparency (should be clear), and odor (slightly aromatic when fresh, may develop ammonia odor upon standing).

pH of Urine

Slightly acidic (~pH 6, range of 4.5 to 8.0), influenced by diet and health conditions.

Specific gravity

Ratio of mass of substance to mass of equal volume of water, ranging from 1.001 to 1.035 due to water and solutes in urine.

Ureters

Slender tubes conveying urine from kidneys to bladder, preventing backflow as bladder pressure increases.

Urinary bladder anatomy

Muscular sac for temporary urine storage, located retroperitoneally, with specific structures in males and females.

Urethra anatomy

Includes internal urethral sphincter (involuntary) and external urethral sphincter (voluntary), with differences in male and female urethras.

Female urethra (3-4cm)

• Tightly bound to anterior vaginal wall

• External urethral orifice: anterior to vaginal opening; posterior to clitoris

Male Urethra (three regions)

• Prostatic urethra (2.5 cm): within prostate

• Intermediate part of the urethra (membranous urethra) (2 cm): passes through urogenital diaphragm from prostate to beginning of penis

• Spongy urethra (15 cm): passes through penis; opens via external urethral orifice

Trigone

Smooth triangular area outlined by openings for ureters and urethra. Infections tend to persist in this region

Internal urethral sphincter

Involuntary (smooth muscle) at bladder-urethra junction and contracts to open

External urethral sphincter

Voluntary (skeletal) muscle surrounding urethra as it passes through pelvic floor

Micturition

Also known as urination or voiding, the process of expelling urine from the bladder.