Humans possess one pair of kidneys
one on the right and the other on the left side of the body the weight
of each of them is approx. 150gm each the location is retroperitoneal in
the abdominal cavity (that is behind the peritoneum)

The kidneys are bean shaped organs approx 10cm long, 5cm wide and 2.5 cm thick.

The
right kidney is slightly lower than the left one. The reason for this
is that liver is present on the right side which makes right kidney
slightly lower than the left one . Above each kidney is an adrenal gland
(also called the suprarenal gland).

Each kidney has two surfaces
anterior and posterior, two poles superior and inferior and two borders
one convex and the other one concave (medial one).there is a notch in
the centre of medial border known as hilum which contains the renal
blood vessels and nerves.

The upper parts of the kidneys are
partially protected by the eleventh and twelfth ribs, and the covering
of each kidney is made up of two layers of fat which are named as Para
renal and peri-renal fat the function of these layers is to give cushion
to each kidney.

Blood supply of the kidney

The
kidneys receive unfiltered blood directly from the heart through the
abdominal aorta which then branches to the left and right renal
arteries. Filtered blood then returns by the left and right renal veins
to the inferior vena cava and then the heart. Renal blood flow accounts
for 20-25% of the cardiac output


Structure of the kidney


 

Image taken from         www.sci.sdsu.edu


There are mainly two parts of kidney
1) Outer cortex
2)
Inner medulla it contains 10-15 pyramids which terminate medially in
the renal papillae .papillae projects into calyces; such 10-15 minor
calyces join to form two major calyces which come out through the pelvis
of kidney to the widened end of the ureter.

MICROSCOPIC STRUCTURE



 
Image taken from www.ivy-rose.co.uk


The
basic functional unit of the kidney is the nephron .there is approx 1to
1.3 million nephrons in each kidney which drain into the renal pelvis.
Total length of nephrons is 45 to 65 mm.

Different parts of nephron are

1 Bowmen capsule
2 Glomerules
3 The proximal convoluted tubule (PCT)
4 The loop of henle
5 The distal convoluted tubules (DCT)
6 The collecting tubules


BOWMEN CAPSULE  
It is the initial dilated part of the nephron. This is the site of
ultra filtration. Blood from the renal artery is forced into the
glomerulus under high pressure. Most of the liquid is forced out of the
glomerulus into the Bowmansinglequotes capsule which surrounds it.
Pressure gradient is needed for this to happen.

 


Image taken from zone.ni.com

   
GLOMERULES

To
form this tuft of capillaries invaginate into the Bowman’s capsule
.These capillaries are supplied by afferent arteriole and the blood
leaves from the tuft by efferent arteriole.

Bowmen capsule + Glomerules together constitute the Malpighian corpuscle.


THE PROXIMAL CONVOLUTED TUBULE (PCT)

Proximal means "near to" and convoluted means "coiled up" so this is the coiled up tube near to the Bowmansinglequotes capsule
Length is about 15 mm.

the
epithelium is brush bordered whose primarily concerned is to increase
the surface area for absorption ,it has numerous microvillus in it for
this purpose

This is the place where all that useful glucose is
re-absorbed from the ultra-filtrate and put back into the blood. In
diabetics the glucose is not reabsorbed and ultimately it comes in urine
,known as glycosuria

LOOP OF HENLE

It is a loop like structure consists of a descending and an ascending limb.
Water
is mainly reabsorbed from here , Osmosis, takes place here  water will
pass from a region of high water concentration (the ultra-filtrate and
urine) into a region of low water concentration (the medulla) through
cell membranes which are semi-permeable.

THE DISTAL CONVOLUTED TUBULES (DCT)

MACULA DENSA.

It
is characterized by cuboidal epithelium .this tubule comes very close
to its own glomerules and establishes a close proximity to the afferent
and efferent arteriole of the glomerulus.at this site the cells of DCT
get modified to become columnar and are closely crowded together ,so
this part is called MACULA DENSA .

This is where most of the salts in the ultra-filtrate are re-absorbed.
The
macula densa and the adjacent juxta glomerular part of the afferent
arteriolar wall are functionally associated forming JUXTA

GLOMERULAR APPARATUS.

THE COLLECTING TUBULES

These
are lined with cuboidal epithelium and DCT join to form collecting
tubules .it passes through the renal cortex and the medulla to empty
into the pelvis of the kidney at the apices of medullary pyramids.

Collecting
ducts run through the medulla and loops of Henlé surrounds the
collecting ducts .when water and salts are removed from the  liquid in
the collecting ducts (ultra-filtrate) is turned into urine

Although our kidneys make about 160 liters of urine every 24 hours, we only produce about ½ litre of urine.

It is called a collecting duct because it collects the liquid produced by lots of nephrons.

TYPES OF NEPHRONS

There are two types of nephrons in the kidney

1) CORTICAL

2) JUXTA-MEDULLARY

CORTICAL
            
These
constitute the maximum number of nephrons about 85%. They are located
in renal cortex and are smaller in size. The rate of filtration is also
slow .These nephrons play a major role in eliminating waste products in
dissolved form in the urine

JUXTA-MEDULLARY

They
compromise only 15%of total nephrons. They have larger size glomeruli
located at the junction of the cortex and the medulla of the kidney. The
rate of filtration is high.

Juxta glomerular apparatus

It has three types of cells
1) Juxta-glomerular cells

2) Macula densa cells

3) Mesangial cells or lacis cells

Uricotelism

When
the main waste product is uric acid then this property is called
uricotelism. In this ammonia   produced by protein catabolism is
converted to uric acid in the liver, of uricoletic animal’s .Synthesis
of uric acid involves high expenditure of energy than the synthesis of
urea.
The advantage of this property is that with very limited access to water these animals can withstand the life.

Blood supply of the kidney:


 




Image taken from myhealth.ucsd.edu/

A
kidney receives the blood from the renal artery that arises from the
dorsal aorta. In the kidney, the renal artery divides and subdivides to
send an afferent arteriole into each Bowmansinglequotes capsule. Here
the afferent arteriole forms a bunch of about 50 parallel capillaries
called glomerulus. These rejoin to form the efferent arteriole. The
efferent arterioles forms a network in the cortex around the proximal
and distal convoluted tubules of the nephrons


Functions of kidney

Blood Filtering

This
is the main function of the kidney. The renal corpuscle is the site of
the nephron, where blood is "filtered”. All the waste products are
removed from the ultra filtrate 

Homeostasis

The
kidney is one of the major organs involved in whole-body homeostasis.
which involves regulation of body temperature ,blood pressure
,electrolyte balance and various other mechanisms

Acid-base balance

The
kidneys regulate the pH of blood by adjusting H+ ion levels, this
function is very important in making the body work normally because
every chemical reaction in the body requires a specific pH.

Blood pressure regulation

This
is the main function of kidneys this is done by regulating the
concentration of sodium ions in the body and by controlling the hormone
aldosterone.

Plasma volume

Antidiuretic hormone controls the plasma volume in response to command of hypothalamus.


A
rise in osmolality causes the gland to secrete antidiuretic hormone,
resulting in water reabsorption by the kidney and an increase in urine
concentration. The two factors work together to return the plasma
osmolality to its normal levels.

Hormone secretion

The kidneys secrete a variety of hormones. They are as follows

1  
Erythropoietin is released in response to low levels of O2 in the renal
circulation. It stimulates erythrocyte production in red bone marrow.

2 Renin ----   is involved in the regulation of aldosterone secretion by the renin-angiotensin-aldosterone system.

3  
Calcitriol, ---the activated form of vitamin D, promotes the absorption
of Ca2+ from the blood and the excretion of PO32-. They both help to
increase Ca2+ levels.

Study questions

Write briefly about

1) Juxta- glomerular apparatus
2) Renin angiotensin system
3) Functions of kidney
4) Difference between two types of nephrons of the kidney

Mechanism of urine formation

As
the blood passes through the kidneys, the nephrons retain some
substances which are useful for the body and let the toxins flow out
from the body.

Glomerular filtration

This is the
initial step in formation of urine The plasma that traverses the
glomerular capillaries is filtered by the highly permeable glomerular
membrane and the resultant fluid,the glomerular filtrate is passed into
the bowman’s capsule .glomerular filtrate contains no protein and no
cells.

Glomerular filtration rate   (GFR)

Total nephrons (of both the kidneys) forms the glomerular filtrate in each minute is called GFR
Normal value = 170-180ml/day

Two major processes takes place here

1) Reabsorption
2)  Secretion in renal tubules

Secretion:  The solutes are transported from the peritubular capillaries’ into the tubular lumen this is called secretion 

Reabsorption
denotes the active transport of solutes and the passive movement of
water from the tubular lumen into the peritubular capillaries

Excretion rate is the amount of the substance that appears in the urine per unit of time which is different for each substance .

Transport of individual substances in different segments of the renal tubule

General

PCT reabsorbs
                               70%-80% --------------------of the filtered Na+, Cl-, HCO3-and water,
                 Almost 100% -------------------------------------of the filtered K+ amino acid and glucose .

Reabsorption of water is passive.

Glucose reabsorbption

it
depends upon plasma level of glucose .when there is low glucose in the
plasma then no glucose is excreted in urine ,when plasma glucose exceeds
180-200mg%the glucose reabsorption is not complete and it starts
appearing in urine (glycosuria).condition which occurs in diabetics

This plasma level at which glucose first appears in urine is called renal threshold for glucose.

Na Reabsorption sodium is reabsorbed approximately 100% of which 

80%by PCT
20%circulates between loop of Henlé and is passed toDCT from which 
2%is excreted in urine. 

K+ Reabsorption  
it is completely filtered at the glomerules and is the only plasma
electrolyte that is both reabsorbed and secreted into the renal tubules

K+ reabsorption occurs by active transport in the PCT, ascending loop of Henlé, DCT, and CT
HCO3- Reabsorption it occurs throughout the nephrons except in the descending loop of Henlé

H+ Secretion   like HCO3-the process of H+ secretion occurs throughout the nephron except in the descending limb of the loop of Henlé

Cl— transport it is inversely related to HCO3- reabsorption rate

Water reabsorption  
water is reabsorbed passively by diffusing along an osmotic gradient,
which primarily is established by the reabsorption of Na+ and Cl-

In PCT                            passive reabsorption of 75%-80% of water occurs

In loop of Henlé  
The descending loop of Henlé is highly permeable to water, but the
ascending limb is impermeable .so while passing through the loop of
Henlé, another 5%-10% of the filtered water is removed, so approx. 15%
of the filtered water enters the

DCT.

Reabsorption in DCT
and CT the early part of DCT is relatively impermeable to water and
continued removal of the solutes in excess of solvent further dilutes
the tubular fluid.

In the terminal DCT and CT under the influence of ADH another 10%-12% of filtered water is reabsorbed

Renal
clearance the clearance value of a substance is the volume of plasma
(in ml) that contains the amount of the substance which is excreted in
the urine in one minute.

Renal clearance of a substance is given by

                               Renal excretion rate of the substance
------------------------------------------------------------------------------------------------------
                     Concentration of the substance in the arterial plasma


Example: clearance value for urea

           Amount of urea excreted in urine in one minute
----------------------------------------------------------------------------
               Plasma concentration of urea

=U urea V/ /P urea

Normally

U urea: urinary concentration of urea =20mg/ml
V: urine flow per minute =1ml/min
P urea: plasma concentration of urea=30mg%
=20x100 /30 =67ml/min

Study questions

1) Write briefly about
2) Mechanism of glomerular filtration
3) Renal clearance.

Counter current system

A
counter current system is a system in which the inflow runs parallel
to, counter to and in close proximity to the outflow for some distance.

The
kidneys forms urine which varies in its solute concentration according
to the need of the body .the components of counter current system is

1) Descending limb of the loop of Henlé (DLH)
2) Thin and thick segments of ascending of ascending loop of Henlé (ALH)
3) Medularly interstitium
4) Distal convoluted tubule
5) Collecting duct,
6) Vasa recta
.
Renal
regulation of acid –base balance kidney perform two major functions of
stabilizing the HCO3_ ions in the body these are also called buffers of
the body.

1) The kidneys stabilizes the standard HCO3- pool
2)  The kidneys excrete metabolically produced non- carbonic acid, which represents an H+ excretion
 
The major buffers present in the kidneys are

1) Bicarbonate system
2) Dibasic phosphate system
3) Ammonia system

Bicarbonate system

                       H+ +   HCO₃    = H₂CO₃   +CO₂

The majority of secreted H+ in the PCT reacts with HCO₃ to form H₂CO₃ and is used to bring about HCO₃ reabsorption .The CO₂ formed in the lumen from secreted H+ returns to the tubular cells to form another H+ and no net H+ secretion occurs .

Ammonia system

PCT cells produce ammonia which is excreted in the urine

Formation of ammonium ions ammonia passively diffuses out of the tubular cells easily along its concentration gradient.
 
Composition of urine

Water 96%
Urea 2%

Other dissolved solids 2%

                   Uric acid -------derived from nucleic acid metabolism
                 Creatinine -----organic byproduct of muscle metabolism
               Inorganic salts     chlorides, phosphates, sulphates, and oxalates of Na +, K, Ca

Abnormal constituents of the urine

Protein (albumin) injury to renal tract it is called proteinuria
Bile salts –jaundice
Glucose   diabetes mellitus
Ketone bodies diabetic ketoacidosis

REGULATION OF BODY TEMPERATURE

Humans
are homeotherms (body temperature is maintained constant within a
narrow range inspite of wide variations in environmental temperature)

Comfortable zone or neutral temperature (25-27degree)

This
is the temperature at which there is no active heat loss or heat gain
mechanisms operated by the body .normally this is about 27degree.

Normal body temperature

1) Oral temperature average: 37deg centigrade
Range   36.3-37.1deg. Centigrade 

2) Core temperature it is an accurate index of the temperature of the blood to which the hypothalamic thermoregulatory receptors are exposed

It
is 0.5—1deg. Centigrade more than the oral temp. It varies least with
temp. Changes of environment, therefore gives poor reflection of rapid
changes in blood temp.

Sites for recording core temp. Are rectum, vagina, and tympanic membrane.

Lower lethal core temp. It is 26deg. Centigrade, this leads to death due to cardiac failure

Upper lethal core temperature This is 43.5 deg centigrade this leads to death due to heat stroke

Factors affecting body temperature 

Age infants show wide range of body temperature it is generally more by 0.5 deg above the established norms for adults.

In old age, temperature is sub-normal due to decrease activity, feeble circulation and weakness of thermoregulatory mechanisms

Sex females have slightly lower body temperature due to low BMR and more subcutaneous fat

Diurnal variation of 1.5 deg centigrade may occur in normal person

Lowest in the morning

Maximum in the evening (rhythm is reversed for night workers, which occurs after few days)

Diseases hyperthyroidism ? increase BMR ? body temperature is chronically elevated by 0.5 deg.

Hypothyroidism opposite phenomenon occurs

Exercise if severe it may increase the temperature up to 40-41 deg centigrade

Emotional factors can increase the body temperature up to 2 deg. Centigrade due to unconscious tensing of muscles.

Body heat production and loss

Heat
production or thermogenesis is mainly done by two routes firstly by
metabolic activities of the body and secondly via assimilation of food 

A)   By metabolic activities of the body in

1) Liver            2) heart                 3) Skelton muscle 

B) By assimilation of food

C)  From the environment from objects hotter than itself

D) By endocrine mechanisms by epinephrine and nor- epinephrine by thyroid hormones

E)  A special type of fat called brown fat, which is a source of considerable heat production.

Heat loss  

1)
These are the processes by which heat is lost from the body when the
environment temperature is below body temperature are as follows

By radiation from the body to cooler objects at a distance
By conduction and convection to the surrounding atmosphere
By vaporization of water by excretion in urine and faeces

2) By the way of vaporization of water from the skin and mucus membranes and respiratory passages

A) Through lungs 
B) Through skin

Insensible water loss

Sweating there are two kind of sweat glands in the body

Eccrine glands are densest on the palm and soles

Apocrine
glands are primarily present in hair follicles .found mainly in the
axilla and around the nipples and in female in labia majora and mons
pubis

Temperature regulating mechanisms

These mechanisms involve the reflex thermoregulatory responses.

These include autonomic, somatic, endocrine, and behavioral changes

1) Thermoregulatory responses activated by exposure to cold (via posterior hypothalamus)

Increase heat production

Shivering   it is involuntary response of Skelton muscle contraction and can increase the muscle metabolism up to 3 times

Hunger increases the food intake and thereby in creases sympathetic discharge.

Increase TSH secretion from anterior pituitary

Increase catecholamine secretion from adrenals.

Decrease heat loss

Cutaneous vasoconstriction

Curling up (in a ball) decrease body surface exposed to the environment

Horripilation
(erection of the hair due to contraction of pilo-erector muscle
attached to the hair) results in goose pimples, which reduces heat loss.

Thermoregulatory responses activated by exposure to heat (via anterior hypothalamus)
 
Role of nervous system in regulation of heat

Hypothalamus is the site where thermoregulation takes place.

The
anterior hypothalamus is the centre for responses to rises temperature
and posterior hypothalamus the centre for responses to falling
temperature.

Role of endocrines

Adrenal medulla
exposure to cold reflexly stimulates epinephrine secretion that
stimulates metabolism and decreases heat loss the effect is rapid and of
short duration

Thyroid exposure to cold via hypothalamus
and pituitary cause’s stimulation of thyroid secretion .this increase
heat production mobilizes glycogen and stimulates gluconeogenesis.

Adrenal cortex

Exposure to external heat or cold stimulates secretion of adrenal corticoids.

Study questions

Write short notes on

1) Factors controlling body temperature
2) Heat production and heat loss
3) Core temperature
4) Endocrine role of temperature control

Artificial kidney

When
the kidneys are completely damaged and does not function, the patient
often receive hemodialysis. (Treatment with artificial kidney)

It
is the process of separation of certain substances from blood by use of
a semi permeable membrane .the pores in the membrane allows certain
substances to pass through, however prevents others .The patient is
connected to the machine by a tube attached to the artery often the
radial artery .blood from the artery is pumped into a tube that runs
through the dialyzer. The dialyzer is filled with the dialysis fluid
which contains the same quantities of electrolytes and nutrients as
normal plasma but contains no waste products
.It works on the principle of osmosis that is substances flow from higher concentration to lower concentrations

Molecules
of waste substances such as urea ammonia and waste diffuse into the
dialysis fluid but diffusion of other substances such as glucose, amino
acids is prevented by the presence of these substances in the dialysis
fluid in the same concentrations in normal plasma, now the blood is
returned to the patient body through the radial vein

Image taken from www.kidney.org/
 



                                            Kidney transplantation

Kidney transplantation or renal transplantation
is the organ transplant of a kidney in a patient with end-stage renal
disease. Kidney transplantation is typically classified as
deceased-donor (formerly known as cadaveric) or living-donor
transplantation depending on the source of the recipient

The
indication for kidney transplantation is end-stage renal disease (ESRD),
regardless of the primary cause. Clinically this is defined as a drop
in the glomerular filtration rate (GFR) to 20-25% of normal.

Common
diseases leading to ESRD include malignant hypertension, infections,
diabetes mellitus and glomerulonephritis; genetic causes include
polycystic kidney disease as well as a number of inborn errors of
metabolism as well as autoimmune conditions

Contraindications to kidney transplant includes   include both cardiac and pulmonary insufficiency, as well as hepatic disease


Diseases of urinary tract

Nephritis                 the infection is caused by bacteria which results in inflammation of the kidney
Pyelonephritis           inflammation of the tissue f kidney in the pelvic region
Cystitis                      inflammation of urinary bladder
Renal stone             formation of stones in the kidney
Polyurea                 amount of urine passed out is more generally the feature of diabetes mellitus
Uremia                     the concentration of urea is increased in blood
Pyuria                       presence of pus in the urine
Glycosuria               presence of glucose in the urine
Heamturia                 presence of blood in the urine.
Ureteric stone               kidney stone in the urine

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