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Learning Resources - Renal System - Revision Notes

Main excretory system:

Consist of:

2 kidneys – which secrete urine

2 ureters – which convey the urine from the kidneys to the urinary bladder

The urinary bladder where urine collects and is temporarily stored

The urethra – where urine is discharged from the urinary bladder to the exterior

 

Renal system plays a vital part in maintaining homeostasis of water and electrolyte concentrations within the body.

 

The kidneys produce urine that contains metabolic waste products:

Nitrogenous compounds

Urea

Uric acid

Excess ions

Drugs

 

Main functions of the kidneys:

Formation and secretion of urine

Production and secretion of erythroprotein – hormone that controls formation of red blood cells

Production and secretion of renin – an important enzyme in the control of blood pressure

 

Kidneys:

Lie on posterior abdominal wall – one each side of the vertebral column, behind the peritoneum and below the diaphragm:

Extend from the level of the 12th thoracic vertebra to the 3rd lumbar vertebra

Has some protection from the lower rib cage

Right kidney is slightly lower than the left – die to the large space the liver takes up

Bean-shaped organs

About 11cm long

About 6cm wide

About 3cm thick

Held in position by a mass of fat

A sheath of fibroelastic renal fascia encloses the kidney and the renal fat

 

Organs Associated with the Kidneys

Right kidney:

Superiorly – right adrenal gland

Anteriorly – right lobe of liver, duodenum, hepatic flexure of the colon

Posteriorly – diaphragm, muscles of the posterior abdominal wall

 

Left kidney:

Superiorly – left adrenal gland

Anteriorly – spleen, stomach, pancreas, jejunum, splenic flexure of the colon

Posteriorly – diaphragm, muscles of the posterior abdominal wall

 

Cross Structure of the Kidney:

Three areas of tissue that can be distinguished when a longitudinal section of the kidney is viewed with the naked eye:

A fibrous capsule – surrounds the kidney

Cortex – reddish-brown layer, immediately below the capsule and outside they pyramids

Medulla – innermost layer, consists of pale conical-shaped striations, renal pyramids

Hilum – concave medial border of the kidney where renal blood and lymph vessels, the ureter and nerves enter

Renal pelvis – funnel-shaped structure that acts as a receptacle for the urine formed by the kidney

Has a number of distal branches (calyces) – surrounds the apex of a renal pyramid

Urine formed in the kidney passes through a papilla at the apex of a pyramid into a minor calyx – then into a major calyx before passing through the pelvis into the ureter

Walls of the pelvis contain smooth muscle and are lined with transitional epithelium

Peristalsis of the smooth muscle originating in pacemaker cells in the walls of the calyces propels urine through the pelvis and ureters to the bladder

This intrinsic property of the smooth muscle is not under nerve control

 

Microscopic Structure of the Kidney:

Composed of about 1 million functional parts – nephrons, and a smaller number of collecting ducts

The ducts transport urine through the pyramids to the renal pelvis – gives them their striped appearance

Tubules are supported by a small amount of connective tissue – containing blood vessels, lymph vessels and nerves

 

The Nephron:

Consists of a tubule closed at one end – the other end opens into a collecting tubule

 

The closed end is indented to form a cup-shaped glomerular capsule (Bowman’s capsule)

Almost completely encloses a network of arterial capillaries (glomerulus)

 

Remainder of the nephron is about 3cm long and is described in three parts

Proximal convoluted tubule

Medullary loop

Distal convoluted tubule – this leads into a collecting duct

 

The collecting ducts unite forming larger ducts that empty into the minor calyces

 

After entering the kidney at the hilum the renal artery divides into smaller arteries and arterioles

 

In the cortex an arteriole (afferent arteriole) enters each glomerular capsule and then subdivides into a cluster of capillaries forming the glomerulus

 

Between the capillary loops are connective tissue phagocyte mesangial cells

Part of the monocyte-macrophage system

 

The blood vessel leading away from the glomerulus is the efferent arteriole

Breaks up into a second capillary network

Exchange across capillary walls regulates the composition of the blood and supplies local tissues with oxygen and nutrients

 

Venous blood drained from this capillary bed eventually leaves the kidney in the renal vein which empties into the inferior vena cava

 

Blood pressure in the glomerulus is higher than in other capillaries because the diameter of the afferent arteriole is greater than that of the efferent arteriole

 

Walls of the glomerulus and glomerular capsule consist of a single layer of flattened epithelial cells

Glomerular walls are more permeable than those of other capillaries

 

The remainder of the nephron and the collecting tubule are formed by a single layer of highly specialised cells

 

Blood vessels of the kidney are supplied by both sympathetic and parasympathetic nerves

The presence of both branches of the autonomic nervous system controls renal blood vessel diameter and renal blood flow independently of autoregulation

 

Functions of the Kidney:

Formation of urine - Filtration:

Occurs through semipermeable walls of the glomerulus and glomerular capsule

Water and other small molecules pass through

Blood cells, plasma proteins, and other larger molecules are too large to pass through and therefore remain in the capillaries

Filtration is assisted by the difference in blood pressure in the glomerulus and the glomerular capsule

Pressure is opposed by the osmotic pressure of the blood and by filtrate hydrostatic pressure in the glomerular capsule

 

Filtration pressure is:

55 – (30+15) =10mmHg

Volume of filtrate formed by both kidneys each minute is called the glomerular filtration rate (GFR)

In adults 125ml/min

 

Selective reabsorption:

Process by which the composition and volume of the glomerular filtrate are altered during its passage through the convoluted tubules

Enables reabsorption, into the blood, of those filtrate constituents needed to maintain fluid and electrolyte balance and the pH of the blood

 

Secretion:

Substances such as substances that are not required by the body, foreign material and some drugs are cleared by secretion into the convoluted tubules and are excreted from the body in the urine

Tubular secretion of hydrogen ions is important for maintaining normal blood pH

 

Composition of urine:

Water 96%

Urea 2%

Other 2% includes

Uric acid

Creatinine

Ammonia

Sodium

Potassium

Chlorides

Phosphates

Sulphates

Oxalates

 

Urine is clear and amber in colour due to the presence of urobilin:

A bile pigment altered in the intestine

Reabsorbed and then excreted by the kidneys

 

Specific gravity is between 1020-1030 and pH is around 6 (normal pH range is between 4.5-8)

 

Healthy adult passes between 1000-1500ml of urine per day

 

Amount of urine produced and the specific gravity vary according to fluid intake and the amount excreted

 

Urine production is decreased during sleep and exercise

 

Water Balance and Urine Output:

Source of most body water us:

Dietary food

Dietary fluid

Small amount of metabolic water

Formed by metabolic processes

 

Water is excreted as the main constituent of:

Urine

Expired air

Faeces

Through the skin as sweat

 

Amount of water lost in expired air and faeces is fairly constant

 

Amount of sweat produced is associated with environmental and body temperatures

 

Balance between fluid intake and output is controlled by the kidneys

 

The minimum urinary output (smallest volume required to excrete body waste products) is about 500ml per day

Urinary volume in excess of this is controlled mainly by antidiuretic hormone (ADH) released into the blood by the posterior lobe of the pituitary gland

 

Sensory nerve cells in the hypothalamus (osmoreceptors) detect changes in the osmotic pressure of the blood

 

Nerve impulses from the osmoreceptors stimulate the posterior pituitary to release ADH

 

When the osmotic pressure is raised, ADH output is increased and as a result, water reabsorption by the cells in distal convoluted tubules and collecting ducts is increased – reducing the blood osmotic pressure and ADH output

 

This is a negative feedback mechanism:

Maintains blood and the blood osmotic pressure

In turn it also controls sodium and water concentrations

Mechanism may be suppressed when there is an excessive amount of a dissolved substance in the blood

 

When blood volume is increased stretch receptors in the atria of the heart release atrial natriuretic hormone (ANP):

Reduces reabsorption of sodium and water by proximal convoluted tubules and collecting ducts

More sodium and water are excreted

Lowers blood volume

Reduces atrial stretching and through the negative feedback mechanism ANP secretion is switched off

Raised ANP levels also inhibit secretion of ADH and aldosterone

Promoting loss of sodium and water

 

Electrolyte Balance:

Changes in the concentration of electrolytes in the body fluids may be due to changes in:

Body water content

Electrolyte levels

 

Several mechanisms that maintain the balance between water and electrolyte concentration

 

Sodium and Potassium Balance:

Sodium is the most common cation (positively charged ion) in extracellular fluid

 

Potassium is the most common intracellular cation

 

Sodium is a constituent of almost all foods and is often added to food during cooking:

Intake is usually in excess of the body’s needs

Excreted mainly in urine and sweat

 

Sodium and potassium occur in high concentrations in digestive juices:

Sodium in gastric juice

Potassium in pancreatic and intestinal juice

Normally these ions are reabsorbed by the colon

Following prolonged diarrhoea they may be excreted in large quantities

Results in electrolyte balance

 

Calcium Balance:

Regulation of calcium levels – achieved by coordinated secretion of parathyroid hormone and calcitonin

 

The distal collecting tubules reabsorb more calcium in response to PTH secretion and reabsorb less calcium in response to secretion of calcitonin

 

pH Balance:

To maintain the normal blood pH the cells of the proximal convoluted tubules secrete hydrogen ions

 

In the filtrate they combine with buffers:

Bicarbonate – forming carbonic acid

(H+ + HCO-3 --> H2CO3­)

Ammonia – forming ammonium ions

(H+ + NH3 --> NH+4)

Hydrogen phosphate – forming dihydrogen phosphate

(H+ + HPO2/3- --> H2PO-3)

 

Carbonic acid is converted to carbon dioxide and water:

Carbon dioxide is reabsorbed

Maintains the buffering capacity of the blood

Hydrogen ions are excreted in the urine as ammonium salts and hydrogen phosphate

Normal pH of urine is usually between 4.5 to 8

Depends on

Diet

Time of day

 

Ureters:

Tubes that convey urine from the kidneys to the urinary bladder

 

About 25-30cm long

 

Have a diameter of about 3mm

 

Ureter is continuous with the funnel-shaped renal pelvis:

Passes downwards through the abdominal cavity

Behind the peritoneum

In front of the psoas muscle into the pelvic cavity

Passes obliquely through the posterior wall of the bladder

Because of this arrangement when urine accumulates and the pressure rises in the bladder, the ureters are compressed and the opening are occluded

Prevents reflux of urine into the ureters (towards the kidneys) as the bladder fills and during micturition – when pressure increases as the muscular bladder wall contracts

 

Structure:

Ureters consist of three layers of tissue:

Outer covering of fibrous tissue - Continuous with the fibrous capsule of the kidney

A middle muscular later:

Consisting of interlacing smooth muscle fibres that form a functional unit spiralling around the ureter

Some clockwise

Some anticlockwise

Additional longitudinal layer in the lower third:

Inner layer – mucosa - Composed of transitional epithelium

 

Function:

Ureters propel urine from the kidneys into the bladder by peristaltic contraction of the smooth muscle layer:

An intrinsic property of smooth muscle

Not under autonomic control

 

Peristalsis originates in a pacemaker in the minor calyces:

Peristaltic waves occur several times a minute

Increasing in frequency with the volume of urine produced

Send little spurts of urine into the bladder

 

Urinary Bladder:

A reservoir for urine

Lies in the pelvic cavity

Size and position vary

Depends on the volume of urine it contains

When distended the bladder rises into the abdominal cavity

 

Structure:

The bladder is roughly pear shaped

Becomes more oval as it fills with urine

Posterior surface is the base

The bladder opens into the urethra at its lowest point – the neck

The peritoneum covers only the superior surface before it turns upwards as the parietal peritoneum, lining the anterior abdominal wall

Posteriorly it surrounds the ureters in the female and the rectum in the male

 

Bladder wall has three layers:

Outer layer

Loose connective tissue - Contains:

Blood vessels

Lymph vessels

Nerves

 

Covered on upper surface by the peritoneum:

Middle layer

Consists of a mass of interlacing smooth muscle fibres and elastic tissue loosely arranged in three layers

Called the detrusor muscle

When it contracts it empties the bladder

Mucosa

Composed of transitional epithelium

 

When the bladder is empty the inner lining is arranged in folds – gradually disappear as it fills

 

The bladder is distensible but when it contains 300-400ml of urine, the awareness of the need to pass urine is felt

 

Total capacity is rarely more than 600ml

 

Three orifices in the bladder wall form a triangle or trigone:

Upper two orifices on the posterior wall are the openings of the ureters

Lower orifice is the opening to the urethra

 

The internal urethral sphincter – a thickening of the urethral smooth muscle layer in the upper part of the urethra – controls outflow of urine from the bladder:

Not under voluntary control

 

Urethra:

A canal extending from the neck of the bladder to the exterior – at the external urethral orifice

 

Longer in males than in females

 

Male urethra is associated with the urinary and the reproductive systems

 

Female urethra is approximately 4cm long

Runs downwards behind the symphysis pubis

Opens at the external urethral orifice just in front of the vagina

The external urethral orifice is guarded by the external urethral sphincter

Under voluntary control

 

Walls consist of three layers of tissue:

Muscle layer

Continuous with that of the bladder

At its origin is the internal urethral sphincter, consisting mainly of elastic tissue and smooth muscle fibres

Under autonomic control

Slow continuous contraction of this sphincter keeps the urethra closed

In the middle third is skeletal muscle surrounding the urethra

Under voluntary nerve control

Forms the external urethral sphincter

 

Submucosa - A spongy layer

Contains blood vessels and nerves

 

Mucosa - Continuous with that of the bladder in the upper part of the urethra

In the lower part the lining consists of stratified squamous epithelium – continuous with the skin of the vulva

 

Micturition:

When the bladder has accumulated 300-400ml of urine afferent autonomic nerve fibres in the bladder wall – sensitive to stretch – are stimulated

 

Micturition occurs when autonomic efferent fibres convey impulses to the bladder causing contraction of the detrusor muscle and relaxation of the internal urethral sphincter

 

When the nervous system is fully developed the micturition reflex is stimulated but sensory impulses also pass upwards to the brain and there is the awareness of the need to pass urine

 

By learned and conscious effort contraction of the external urethral sphincter and muscles of the pelvic floor will inhibit micturition for a limited period

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