Excretory System and Renal Function: Lecture Notes, Lecture notes of Anatomy

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Excretory System - Lecture notes

EXCRETORY SYSTEM

• Excretion is the process by which the unwanted substances and metabolic waste are eliminated from the body
• Various systems organs in the body are involved in performing the excretory function

1. DIGESTIVE SYSTEM excretes food residues in the form of faeces.
2. LUNGS remove carbon dioxide and water vapour
3. SKIN excrete water salts and some wastes
4. LIVER excretes many substances like bile pigments heavy metals drugs toxins bacteria etc.

• Renal system includes

1. a pair of kidneys
2. ureters
3. urinary bladder
4. urethra

FUNCTIONS OF KIDNEY

1) ROLE IN HOMEOSTASIS

• Primary function of kidney is homeostasis
• It is accomplished by formation of urine
• During formation of urine, kidneys regulate various activities in the body which are

a. Excretion of WASTE PRODUCTS

• kidneys excrete urea, uric acid, creatinine, Bilirubin, products of metabolism

FUNCTIONAL ANATOMY OF KIDNEY

• Kidney is a compound tubular gland covered by a CONNECTIVE TISSUE capsule
• There is a depression on the medial border of the kidney called HILUM, through which RENAL ARTERY, RENAL VEIN, NERVES, and URETERS pass.

DIFFERENT LAYERS OF KIDNEY

• Components of kidney are arranged in three layers

1. Outer cortex

• dark and granular in appearance
• contains renal corpuscles and convoluted tubules

2. Inner medulla

• medulla contains tubular and vascular structures arranged in parallel radial lines

3. Renal sinus

• renal pelvis and renal calyces

NEPHRON

• NEPHRON is defined as the STRUCTURAL AND FUNCTIONAL UNIT of kidney.
• Each kidney consists of 1 to 1. 3 million nephrons.

PARTS OF NEPHRON

• Each nephron is formed by two parts

1. A blind end is called RENAL CORPUSCLE or MALPHIGIAN CORPUSCLE
2. A tubular portion called RENAL TUBULE.

RENAL CORPUSCLE

• It is a spheroidal and slightly flattened structure with a diameter of about 200 microns

Situation of renal corpuscle and types of nephron

Renal corpuscle is situated in the cortex of the kidney either near the periphery or near medulla.

CLASSIFICATION OF NEPHRONS

1. Cortical nephrons or superficial nephrons
2. Juxtamedullary nephrons

Structure of renal corpuscle

Renal corpuscle is formed by two portions

1. GLOMERULUS

• It is a tuft of capillaries enclosed by Bowman's capsule. It consists of glomerular capillaries interposed between afferent arteriole on one end and efferent arteriole on the other end.
• After entering the bowman's capsule, the afferent arteriole divides into 4 or 5 large capillaries.

• Thick descending segment is the direct continuation of the PCT. It descends down into the medulla.
• Thin descending segment is continued as thin descending segment

2. Hair pin bend

• Continuation of thin descending segment
• Formed by flattened epithelial cells

3. Ascending limb

• It has two parts a) Thin ascending segment
• It is a continuation of hairpin bend
• It is lined by flattened epithelial cells without brush border
• Thin segment is continued as thick segment b) Thick ascending segment
• It is about 9 mm long lined by cuboidal epithelial cells
• Terminal portion which runs between afferent and efferent arterioles forms the MACULA DENSA

DISTAL CONVOLUTED TUBULE

• DCT is the continuation of thick ascending segment and occupies the cortex of kidney
• It is continued as collecting duct

COLLECTING DUCT

• DCT continues as the initial or arched collecting duct which is in the cortex
• Lower part of the collecting duct lies in the medulla.
• It is formed by two types of epithelial cells

1. Principal or P cells
2. Intercalated or I cell

Polar cushion or Polkissen

• JG cells form a thick cuff called Polar cushion or Polkissen around the afferent arteriole before it enters the Bowman’s capsule.

FUNCTIONS OF JGA

• Primary function is SECRETION OF HORMONES
• It also regulates the glomerular blood flow and glomerular filtration rate.

Secretion of hormones

RENIN

• Renin is a peptide with 340 amino acids
• Along with angiotensins, renin forms the renin- angiotensin system, which is a hormone system that plays an important role in maintenance of blood pressure

RENIN ANGIOTENSIN SYSTEM

• When renin is released into the blood, it acts on the plasma protein ANGIOTENSINOGEN/ RENIN SUBSTRATE
• By activity of renin ANGIOTENSINOGEN is converted to ANGIOTENSIN I (DECAPEPTIDE)
• ANGIOTENSIN I is converted into ANGIOTENSIN II (OCTAPEPTIDE) by the activity of ACE (Angiotensin converting enzyme) secreted from lungs

• ANGIOTENSIN II is then rapidly degraded into ANGIOTENSIN III (HEPTAPEPTIDE) by ANGIOTENSINASES.
• ANGIOTENSIN III is converted into ANGIOTENSIN IV (HEXAPEPTIDE)

ACTIONS OF ANGIOTENSINS

• Angiotensin I is physiologically inactive. It serves as a precursor of Angiotensin II
• Angiotensin II is the most ACTIVE form
  • Increases arterial blood pressure
  • Stimulates the secretion of aldosterone
  • Regulate glomerular filtration
  • Inhibits baroreceptor reflex

Efferent arteriole

• It forms a second capillary network called peritubular capillaries which surround the tubular portions of nephrons

Peritubular capillaries

• They are found around the tubular portion of cortical nephrons only

Venous systems

• Peritubular capillaries and Vasa recta drain into the venous system

REGULATION OF RENAL BLOOD FLOW

AUTOREGULATION

• It is intrinsic ability of an organ to regulate its own blood flow
• Renal autoregulation
• It is important to maintain the glomerular filtration rate
• Two mechanism are involved

1. MYOGENIC RESPONSE

• Whenever the blood flow to kidneys increase, it stretches the elastic wall of afferent arteriole.

2. TUBULOGLOMERULAR FEEDBACK

• Macula Densa plays an important role in tubuloglomerular feedback

URINE FORMATION

Urine formation is a blood-cleansing function. Normally, about 1300ml of blood enters the kidneys. Kidneys excrete the unwanted substances along with water from the blood as urine.

Normal urine output – 1 to 1. 5L/day.

Process of Urine Formation

• GLOMERULAR FILTRATION is when blood passes through glomerular capillaries and plasma is filtered into the Bowman’s capsule.
• Filtrate from the Bowman’s capsule passes through the tubular portion of the nephron and undergoes changes in quality and quantity. Many unwanted substances like Glucose, Amino acids, water and electrolytes are reabsorbed from the tubules. This process is called TUBULAR REABSORPTION.
• Some unwanted substances are secreted into the tubule from the peritubular blood vessels. This is called TUBULAR SECRETION or EXCRETION.

Urine formation includes three processes :

A) Glomerular filtration B) Tubular reabsorption C) Tubular secretion

GLOMERULAR FILTRATION

It 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.

• MICROPUNCTURE TECHNIQUE that involves insertion of a

micropipette into the Bowman’s capsule and aspiration of filtrate is the method of collection of Glomerular Filtrate.

• GLOMERULAR FILTRATION RATE (GFR) is the total quantity of filtrate formed in all the nephrons of both the kidneys in the given unit of time. Normal GFR is 125mL/min or about 180L/day
• FILTRATION FRACTION is the fraction of the renal plasma, which becomes the filtrate. Filtration fraction = ( GFR / Renal Plasma Flow ) x 100 = 19. 2% Normal filtration fraction varies from 15 to 20%

PRESSURES DETERMINING FILTRATION

Pressures which determine the GFR are

1. Glomerular Capillary Pressure ( GCP )

• It is the pressure exerted by the blood in glomerular capillaries
• It varies between 45 and 70 mm Hg.
• It is highest capillary pressure in the body and favours glomerular filtration.

2. Colloidal Osmotic Pressure. ( COP )

• It is the pressure exerted by plasma proteins in the glomeruli.
• The plasma proteins are not filtered through the glomerular -capillaries and develop the colloidal osmotic pressure, which is about 25 mm Hg.
• It opposes glomerular filtration

3. Hydrostatic Pressure in Bowman ’ s Capsule

• It is the pressure exerted by the filtrate in Bowman’s capsule
• It is also called CAPSULAR PRESSURE and is about 15 mm Hg.
• It also opposes glomerular filtration.

NET FILTRATION PRESSURE is the balance between pressure favouring filtration and pressures opposing filtration.

It is known as EFFECTIVE FILTRATION PRESSURE or ESSENTIAL FILTRATION PRESSURE

NFP = GCP - ( COP + Hydrostatic Pressure in Bowman’s capsule )

= 60 - ( 25 + 15 ) = 20 mm Hg.

NFP varies between 15 and 20 mm Hg.

FILTRATION COEFFICIENT

It is the GFR in terms of net filtration pressure.

Filtration Coefficient = 125 mL / 20 mm Hg

= 6. 25 mL / mm Hg.

FACTORS REGULATING GFR.

1. Renal Blood Flow

• It is the most important factor that is necessary for glomerular filtration.
• GFR is directly proportional to Renal Blood Flow.
• Normal blood flow to both the kidneys is 1300 mL/min.

2. Tubuloglomerular Feedback

• It is the mechanism that regulates GFR through Renal tubule and Macula Densa.
• MACULA DENSA of juxtaglomerular apparatus is sensitive to NaCl in the tubular fluid.
• When the glomerular filtrate passes through the terminal portion of thick ascending segment, Macula Densa detects

4. Colloidal Osmotic pressure

• GFR is inversely proportional to COP.
• When COP increases, as in DEHYDRATION, GFR decreases
• When COP is low, as in HYPOPROTEINEMIA, GFR increases

5. Hydrostatic Pressure in Bowman ’ s capsule

• GFR is inversely proportional to this.
• HP in Bowman’s capsule increases in conditions like obstruction of urethra and oedema of kidney beneath renal capsule.

6. Constriction of Afferent Arteriole

• It reduces the blood flow to Glomerular capillaries, which in turn reduces the GFR.

7. Constriction of Efferent Arteriole

• If efferent arteriole is constricted, initially the GFR increases because of stagnation of blood in the capillaries.
• Later, further filtration does not occur.
• It is because, the efferent arteriolar constriction prevents outflow of blood from glomerulus and no fresh blood enters the glomerulus for filtration.

8. Systemic Arterial Pressure

• Renal blood flow and GFR are not affected as long as the mean arterial blood pressure is in between 60 and 180 mm Hg.
• Variation affects the Renal blood flow and GFR accordingly.

9. Sympathetic Stimulation

• Afferent and efferent arterioles are supplied by sympathetic nerves.
• Strong sympathetic stimulation causes severe constriction of the blood vessels by releasing NORADRENALINE.

• The effect is more on the efferent arterioles than on the afferent arterioles.
• So initially, there is increase in filtration, but later it decreases.
• If the stimulation is continued for more than 30 minutes, there is recovery of both Renal Blood Flow and GFR.

10. Surface area of Capillary Membrane

• GFR is directly proportional to the surface area of the capillary membrane.

11. Permeability of Capillary membrane

• GFR is directly proportional to the permeability of glomerular capillary membrane.
• In many conditions like hypoxia, lack of blood supply, presence of toxic agents, the permeability of capillary membrane increases.

12. Contraction of Glomerular Mesangial Cells

• They are situated in between the glomerular capillaries.
• Contraction of these cells decreases surface area of capillaries resulting in reduction in GFR.

13. Hormonal and Other Factors

Factors INCREASING GFR by VASODILATION

• Atrial Natriuretic Peptide
• Brain Natriuretic Peptide
• cAMP
• Dopamine
• Endothelium derived NO
• PGE