Anatomy and Physiology of the Renal System

Table of Contents

Anatomy of the Renal System

The urinary system is the main excretory system eliminating waste products from blood through urine. It’s anatomy consists of two kidneys, each joined to the bladder by the tube called ureter, which conveys urine from the kidneys to the bladder for storage. Following bladder contraction, urine is
expelled through the urethra.

The Kidneys

There are two kidneys which lie behind the peritoneum on either side of the vertebral column. In adults, they measure approximately 12 to 14cm
The urine is formed in the kidney by the nephrons.
Each kidney has approximately one million nephrons.

Role of the Kidneys
• Influence blood pressure control
• Release renin to activate the renin-angiotensin system
• Leads to increased blood pressure and volume
• Increased blood flow to the kidney
• Can lead to water retention or excretion

Organs of the Urinary Tract

Ureters

These carry urine from kidney pelvises to the urinary bladder

Urinary bladder

This lies posterior to the symphysis pubis and anterior to the rectum
It stores urine before it emptied from the urinary system
It is made up of four layers i.e.

Mucosa; this is the inner most layer with rugae that allows its distention.
Sub mucosa which provides rich vascular supply
Smooth muscle layer/ detrusor muscle
Serosa: a continuation of peritoneum
The bladder has a triangular area called trigone with three openings at its angles i.e two for ureters laterally and one for the urethra at the apex

Urethra

This conveys urine from urinary bladder to outside of the body.
The internal sphincter of smooth muscle and external urethral sphincter of skeletal muscles constricts the lumen of the urethra causing bladder to fill.

Female urethra is 4cm long and male urethra is 20 cm
In male it is divided into three i.e. prostatic in the prostate, membranous in the root of the penis, and spongy in the shaft of the penis

Nephron

This is a functional (urine) forming units of the kidneys

Components of the Nephron

Bowman’s Capsule a cup-like structure made of squamous epithelium and inner layer has modified cell (podocytes) closely associated with glomerular capillaries
Glomerulus made of highly permeable capillary network
Proximal convoluted tubule, made of cuboidal epithelium with microvilli. It is a primary site.
of tubular reabsorption and secretion mechanism.
Loop of Henle; both ascending and descending loops are involved in urine concentration
Distal Convoluted tubule; this is shorter than the distal and contains macula denser specialized sensory cells which monitor NaCl concentrations it’s a site of tubular reabsorption and secretion
Collecting Ducts; these empty urine into the renal pyramides

Physiology of the urinary system

The volume of the urine excreted per day is about 1500m/s or roughly 1ml /min.
The processes responsible for urine formation are ultra filtration at the glomeruli and reabsorption in the tubules of the nephrons.
The kidneys are largely responsible for maintaining this constancy and the excretion of waste products of metabolism.
For example, urea which is a waste product of protein metabolism is excreted in a large quality.
Various renal functions are illustrated below

FUNCTIONS OF THE RENAL SYSTEM

Regulation of the water content of the body: About 2/3 of water filtered by the glomeruli is reabsorbed in the proximal tubules iso-osmotically.
The remaining water is reabsorbed in distal tubules and collecting duct; under the influence of anti diuretic hormone (ADH).
Regulation of normal acid-base balance of the blood.
The kidneys help to maintain normal internal environment by preventing body fluids from becoming too acidic or too alkaline.
Regulation of electrolyte content of the body.
A large part of sodium ions (Na+), chloride ions (Cl- ) are actively reabsorbed in the PCT, DCT and collecting ducts. The kidney regulates the fluid balance by excreting more urine when a large amount of urine is taken and retains fluid when much has been lost.
Hormonal and metabolic functions.
The kidney produces many hormones which take part in various metabolic functions >Renin is produced in the “Juxta glomerular apparatus” and stimulates aldosterone secretion.
> Erythropoietin – stimulates red blood cells production
> Dihydroxy cholecalciferol helps in calcium metabolim
> Prostaglandins produced in the kidneys help in vasodilatation of blood vessels.

Processes Involved in urine formation

Filtration
Selective Re-absorption
Tubular Secretion

FILTRATION

This takes place through the semipermeable walls of the glomerulus and glomerular capsule. Water and other small molecules pass through, although some are reabsorbed later. Blood cells, plasma proteins and other large molecules are too large to filter through and therefore remain in the capillaries.

Filtration takes place because there is a difference between the blood pressure in the glomerulus and the pressure of the filtrate in the glomerular capsule. Because the efferent arteriole is narrower than the afferent arteriole, a capillary hydrostatic pressure builds up in the glomerulus. This pressure is opposed by the osmotic pressure of the blood, provided mainly by plasma proteins, and by filtrate hydrostatic pressure in the glomerular capsule,

The volume of filtrate formed by both kidneys each minute is called the glomerular filtration rate (GFR). In a healthy adult the GFR is about 125 ml/min, i.e. 180 liters of filtrate are formed each day by the two kidneys. Nearly all of the filtrate is later reabsorbed from the kidney tubules with less than 1%, i.e. 1 to 1.5 liters, excreted as urine. The differences in volume and concentration are due to selective reabsorption of some filtrate constituents and tubular secretion of others

SELECTIVE REABSORPTION

Most reabsorption from the filtrate back into the blood takes place in the proximal convoluted tubule, whose walls are lined with microvilli to increase surface area for absorption. Materials essential to the body are reabsorbed here, including some water, electrolytes and organic nutrients
such as glucose. Some reabsorption is passive, but some substances are transported actively. Only 60–70% of filtrate reaches the loop of the nephron. Much of this, especially water, sodium and chloride, is reabsorbed in the loop, so only 15–20% of the original filtrate reaches the distal convoluted tubule, and the composition of the filtrate is now very different from its starting values.
More electrolytes are reabsorbed here, especially sodium, so the filtrate entering the collecting ducts is actually quite dilute. The main function of the collecting ducts therefore is to reabsorb as much water as the body needs.

Tubular Secretion

Filtration occurs as the blood flows through the glomerulus. Substances not required and foreign materials, e.g. drugs including penicillin and aspirin, may not be cleared from the blood by filtration
because of the short time it remains in the glomerulus. Such substances are cleared by secretion from the peritubular capillaries into the convoluted tubules and excreted from the body in the urine.
Tubular secretion of hydrogen ions (H+) is important in maintaining normal blood pH.

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