09/01/2024
Kidneys
colour :dark-red,
shape: bean shaped organs
length:12 cm
width: 6 cm
thickness :4 cm
weight:150 gms.
anatomical location: They are located just below the rib cage, one on each side of spine, between T12-L3 vertebrae.They are placed against the back wall of the abdominal cavity
The upper parts of the kidneys are partially protected by the eleventh and twelfth ribs. Their position is slightly asymmetrical, the right kidney being a little lower than the left one because of a liver lobe above it. The outer surface is convex and the inner surface is concave. Hilus: The inner surface has a deep notch called hilus. The renal artery and nerves enter the kidney, and the renal vein and ureter leave the kidney through hilus. The kidney is surrounded by tough membrane called renal peritonium.
internal structures: Internally, the kidney has three regions—an outer cortex, a medulla in the middle, and the renal pelvis .
renal cortex
The renal cortex is granular due to the presence of nephrons—the functional unit of the kidney.
The cortex contains renal corpuscles and convoluted tubules of nephrons. At intervals,
cortical tissue penetrates medulla in the form of columns,
which are called renal columns or columns of Bertini.
2. renal medulla
The medulla consists of multiple pyramidal tissue masses, called the renal pyramids. In between the pyramids are spaces called renal columns through which the blood vessels pass. The tips of the pyramids, called renal papillae, point toward the renal pelvis. There are eight renal pyramids in each kidney. The renal pyramids along with the adjoining cortical region are called the lobes of the kidney.
Renal pelvis
Upper expanded part of ureter called renal pelvis.
The renal pelvis leads to the ureter on the outside of the kidney. On the inside of the kidney, the renal pelvis branches out into two or three extensions called the major calyces, which further branch into the minor calyces.
Ureters
The ureters are urine passing muscular tubes that exit the kidney and empty into the urinary bladder.
length: 28 cm long.
location: They are a pair of narrow, muscular, tubular
structures which arise from the hilus of the kidney, run backward along the dorsal
body wall and open on the dorsal wall of the urinary bladder.
function:These pass urine from the kidneys to the urinary bladder.
15.5.3 Urinary Bladder
It is a pear shaped sac situated in the pelvic region of the abdominal cavity. It
has thick muscular distensible wall that allows its expansion.
shape:
The appearance of the bladder varies depending on the amount of urine stored. When full, it exhibits an oval shape, and when empty it is almost flat.
storage capecity :It can store about 0.5 to 1 litre of urine. It receives the ureters through the lower part of its back wall.
The lower part or neck of the bladder is guarded by 2 rings of muscle fibres called
sphincters. Both the sphincters must relax to let urine pass out from the bladder.
micturition:
The act of emptying the bladder is called micturition.
Anatomical Parts of Uninary bladder
is divided into four anatomical parts: the apex or dome, body, fundus, and neck.
functions:
Temporary storage of urine
Helps in the expulsion of urine
Urethra: The tube through which urine leaves the body. The urethra starts from the neck of the urinary bladder and leads to the exterior. In females it is about 2 - 3 cm long and carries only urine. It opens by the urethral or***ce or urinary aperture in the v***a in front of the vaginal aperture.
In male, urethra is about 20 cm long and carries urine as well as the spermatic
fluid. It passes through the p***s and opens out at the tip of the p***s by a urinogenital aperture.
Classification of male urethra
Prostatic urethra:
Membranous urethra
Pe**le (bulbous) urethra
NEPHRON
INTRODUCTION
Nephron is defined as the structural and functional unit of
kidney. Each kidney consists of 1 to 1.3 millions of nephrons.
parts of nephron
Each nephron is formed by two parts
1. renal corpuscle or Malpighian corpuscle
2. A tubular portion called renal tubule.
RENAL CORPUSCLE
Renal corpuscle is a spheroidal and slightly flattened structure.
location: situated in the cortex of the kidney.
Function :of the renal corpuscle is the filtration of blood which forms the first phase of urine formation
STRUCTURE OF RENAL CORPUSCLE
is formed by two portions:
1. Glomerulus
2. Bowman capsule.
Glomerulus
Glomerulus is a tuft of capillaries enclosed by Bowman
capsule.Glomerular capillaries arise from the afferent arteriole. After entering the Bowman capsule, the afferent arteriole on the other end. Thus, the vascular system in
the glomerulus is purely arterial (Fig. 49.3).
Glomerular capillaries arise from the afferent arteriole. After entering the Bowman capsule, the afferent arteriole divides into 4 or 5 large capillaries. Each large capillary subdivides into many small capillaries. All the smaller capillaries finally reunite to form the efferent arteriole, which leaves the Bowman capsule.
Bowman Capsule
Bowman capsule is a capsular structure, which encloses the glomerulus. The cells of Bowman's capsule in the kidneys that wrap around the capillaries of the glomerulus are called podocytes. The pores in the capillary endothelium and the gaps in between podocytes are quite large, and make it easy for any substance dissolved in the blood plasma to get through from the blood into the capsule. However, basement membrane of blood capillaries stops large protein molecules from getting through.
RENAL TUBULES
classification:It is made up of three parts:
1. Proximal convoluted tubule
2. Loop of Henle
3. Distal convoluted tubule.
PROXIMAL CONVOLUTED TUBULE
Proximal convoluted tubule is the coiled portion arising from Bowman capsule. It is situated in the cortex. It is continued as descending limb of loop of Henle. Length of proximal convoluted tubule is 14 mm. Proximal convoluted tubule is continued as loop of Henle.
Functional histology
Proximal convoluted tubule is formed by single layer of cuboidal epithelial cells. Characteristic feature of these cells is the presence of hairlike projections directed towards the lumen of the tubule. Because of the presence of these projections, the epithelial cells are
called brush-bordered cells.it increases surface area for absorption.
LOOP OF HENLE
It is U shaped long tube of nephron and is more prominent in juxtamedullary nephron.
FUNCTION
Filtration of salts
consists of:
i. Descending limb
ii. Hairpin bend
iii. Ascending limb.
i. Descending Limb
Descending limb of loop of Henle is made up of two
segments:
a. Thick descending segment
b. Thin descending segment.
Thick descending segment
Thick descending segment is the direct continuation of the proximal convoluted tubule. It descends down into medulla.It is formed by brushbordered cuboidal epithelial cells.
Thin descending segment Thick descending segment is continued as thin descending segment (Fig. 49.5). It is formed by flattened epithelial cells without brush border and it is continued
as hairpin bend of the loop.
ii. Hairpin Bend
Hairpin bend formed by flattened epithelial cells without brush border and it is continued as the ascending limb of loop of Henle.
iii. Ascending Limb
Ascending limb or segment of Henle loop has two
Parts:
a. Thin ascending segment
b. Thick ascending segment.
Thin ascending segment
Thin ascending segment is the continuation of hairpin
bend. It is also lined by flattened epithelial cells without
brush border.
Thin ascending segment is continued as thick ascending segment.
Thick ascending segment
Thick ascending segment is lined by cuboidal epithelial cells without brush border.
The terminal portion of thick ascending segment, which runs between the afferent and efferent arterioles of the same nephrons forms the macula densa. Macula densa is the part of juxtaglomerular apparatus and continues as distal convoluted tubule.
Thick ascending segment ascends to the cortex and continues as distal convoluted tubule.
DISTAL CONVOLUTED TUBULE
Distal convoluted tubule is the continuation of thick ascending segment and occupies the cortex of kidney.
It is continued as collecting duct.
Functional histology
Distal convoluted tubule is lined by single layer of cuboidal epithelial cells without brush border. Epithelial cells in distal convoluted tubule are called intercalated cells (I cells).
COLLECTING DUCT
Distal convoluted tubule continues as the initial or arched collecting duct, which is in cortex. The lower part of the collecting duct lies in medulla. Seven to ten initial collecting ducts unite to form the straight collecting duct, which passes through medulla. finally at deliver urine to renal pelvis.
Functional histology
Collecting duct is formed by two types of epithelial cells:
1. Principal or P cells
2. Intercalated or I cells.
Types of Nephron
Two general classes of nephrons are cortical nephrons and juxtamedullary
nephrons, both of which are classified according to the location.
Cortical nephrons or superficial nephrons; are found in the cortex near the periphery. They have their renal corpuscle in the superficial renal cortex and have relatively short loops of Hente.
occurrence: 70 to 80% nephrons in human kidney are cortical.
Function:Under normal conditions of water availability the cortical nephrons deal
with the control of blood volume.
Juxtamedullary nephrons have their renal
corpuscle close to the junction of the cortex and medulla or in inner cortex. They have long loop of Henle which extends deep into the medulla.
HOW URINE FORMATION OCCURS IN NEPHRONS?
OR
Explain excretory function of nephron?
A. GLOMERULAR FILTRATION
Glomerular filtration is the process by which the blood is
filtered while passing through the glomerular capillaries
by filtration membrane. It is the first process of urine formation.
Ultrafiltration
It is filtration under pressure. The diameter of efferent arteriole is half as
compared to the afferent arteriole. It results in a high blood pressure in glomerulus. About 20% of the plasma is filtered into Bowman's capsule. This filtered fluid is called glomerular filtrate.
Filtration Membrane Filtration membrane is formed by three layers:
1. Glomerular capillary membrane
2. Basement membrane
3. endothelium of Bowman capsule.
This filtrate has to cross filtration membrane . It has chemical composition similar to that of blood plasma. It contains glucose, amino acids, Vitamins, ions, nitrogenous wastes, some hormones and water.
TUBULAR REABSORPTION
INTRODUCTION
Tubular reabsorption is the process by which water and
other substances are transported from renal tubules back to the blood. When the glomerular filtrate flows through the tubular portion of nephron. Large quantity of water (more than 99%), electrolytes and other substances are reabsorbed. The reabsorbed substances move into the blood Since the substances are taken back into the blood from the glomerular filtrate, the entire process is called tubular reabsorption.
B. SELECTIVE REABSORPTION
Tubular reabsorption is known as selective reabsorption because the tubular cells reabsorb only the substances necessary for the body. Essential substances such as glucose, amino acids and vitamins are completely reabsorbed from renal tubule. Whereas the unwanted substances like metabolic waste products are not reabsorbed and excreted through urine.
MECHANISM OF REABSORPTION
Basic transport mechanisms involved in tubular reabsorption are of two types:
1. Active reabsorption
2. Passive reabsorption
SITE OF REABSORPTION
Reabsorption of the substances occurs in almost all the segments of tubular portion of nephron.
1. Substances Reabsorbed from Proximal Convoluted Tubule about 80% of glomerular filtrate is reabsorbed in proximal convoluted tubule. The brush border of epithelial cells in proximal convoluted tubule increases the surface area and facilitates the reabsorption. Substances reabsorbed from proximal convoluted tubule are glucose, amino acids, sodium, potassium, calcium, bicarbonates, chlorides, phosphates, urea, uric acid and water.
2. Substances Reabsorbed from Loop of Henle Function of loop of Henle is to conserve water. wall of ascending limb is impermeable to water however sodium chloride potassium and other ions are observed actively here the descending limb is highly permeable to water. the counter current multiplier system here result in reabsorption of a lot of water and solutes.
3. Substances Reabsorbed from Distal Convoluted Tubule DCT have osmoregulatory role and also control blood PH by secreting hydrogen ions.
Sodium, calcium, bicarbonate and water are reabsorbed from distal convoluted tubule.
4. COLLECTING DUCT
collecting ducts are impermeable to water normally In the presence of ADH, collecting duct become permeable to water resulting in water reabsorption and final concentration of urine. It is found that in the collecting duct, Principal (P) cells are responsible for ADH induced water reabsorption.
The urine becomes hypertonic to plasma.
C. TUBULAR SECRETION
Tubular secretion is the process by which the substances are transported from blood into renal tubules. It is also called tubular excretion. In addition to reabsorption from renal tubules, some substances are also secreted into the lumen from the peritubular capillaries.such as hydrogen ions, potassium ions and organic ions . many drugs are eliminated by tubular secretion. hydrogen ion secretion is important in acid base balance of the body.
FACTORS REGULATING (AFFECTING) GFR ( this topic is out of course but important for MCQs )
1. Renal Blood Flow
It is the most important factor that is necessary for
glomerular filtration. GFR is directly proportional to renal blood flow.
2. Glomerular Capillary Pressure Glomerular filtration rate is directly proportional to glomerular capillary pressure.
3.Colloidal Osmotic Pressure Glomerular filtration rate is inversely proportional
to colloidal osmotic pressure, which is exerted by plasma proteins in the glomerular capillary blood
4. Constriction of Afferent Arteriole
Constriction of afferent arteriole reduces the blood flow to the glomerular capillaries, which in turn reduces
GFR.
5. Constriction of Efferent Arteriole
If efferent arteriole is constricted, the GFR
increases because of stagnation of blood in the
capillaries.
6. Surface Area of Capillary Membrane
GFR is directly proportional to the surface area of the
capillary membrane.
7. Permeability of Capillary Membrane
GFR is directly proportional to the permeability of glomerular capillary membrane Kidney as Osmoregulatory Organ
Osmoregulation the maintenance of constant osmotic conditions in the body.
Control of water level
Body maintains the solute potential of blood at an approximately steady
state. It is done by balancing water uptake from the diet with water lost in
evaporation, sweating, egestion and urine
Role of ADH Hormone:
The solute potential is primarily achieved by the effect of antiduretic hormone. Anti diuretic hormone is secreted by the posterior lobe of pituitary gland. When body is deficient in water, hypothalamus detects a fall in blood solute potential and directs pituitary to release anti diuretic hormone. This hormone increases the permeability of the distal convoluted tubules and collecting ducts to water. More water is absorbed, reducing the volume of urine
and making it more concentrated. When there is a high intake of water anti diuretic hormone release is inhibited. Less water is absorbed and a large volume of dilute urine is excreted.
Control of blood sodium level
The maintenance of sodium level at a steady state is controlled by the steroid
hormone aldosterone. It is secreted by adrenal cortex. Aldosterone activates sodium-
potassium pumps in the distal convoluted tubules. Sodium is pumped back to blood from filtrate actively.
PH regulation:
Kidney plays an important role in maintenance of acid base balance by excreting hydrogen ions and retaining bicarbonate ions.
Normally, urine is acidic in nature with a pH of 4.5 to 6. Metabolic activities in the body produce large quantity
of acids (with lot of hydrogen ions), which threaten to push the body towards acidosis. However, kidneys prevent this by two ways:
1. Reabsorption of bicarbonate ions (HCO3–)
2. Secretion of hydrogen ions (H+
