Archive for the ‘ L2-Physio Kidney ’ Category

L2-Physiology of Kidney

Kidneys–as waste removal

– Bean shaped organs, about the size of your fist

– Located near the middle of your back, just below the rib cage

– Sophisticated reprocessing machine

– Processes about 180 liters of blood to give about 1.5 liters of waste products and extra water every day that is essential for homeostasis

Picture1Cross section of a kidney

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– Every kidney has about a million nephrons

– Each nephron is formed of two  parts:

1-The Malpigian corpuscle, which consists of:

  1. The Bowman’s capsule
  2. Tuft of capillaries called the glomerulus

2-The renal tubule, which consists of 3 parts:

  1. The proximal convoluted tubule
  2. The loop of Henle
  3. The distal convoluted tubule

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Urine Formation

Mechanism of urine formation

-Renal blood flow is about 1300 ml /min from which 125 ml is filtered/min by the glomeruli GFR

-Urine formation occurs by 3 different  nephron mechanisms acting on the GF:

  1. filteration
  2. reabsorption
  3. secretion.

1)Renal Corpuscle (Glomerulus)

-Glomerular filteration is the first step of urine formation

-The glomeruli act as ultrafilter to the plasma (very fine sieve), that allow the passage of substances having small molecular weight and prevent those of colloidal size.

-The filteration membrane is a 3 layers barrier:

  1. The capillary endothelium
  2. Basement membrane
  3. Podocytes

Picture4Picture5Mechanism of glomerular filteration

-Glomerular filteration is an inactive process.

-The glomerular membrane acts as a passive membrane.

-No active transport of materials takes place in that membrane.

-The driving force for filteration is the high hydrostatic pressure in the glomerular capillaries.

Picture6Picture7Proteinuria

-It is the appearance of protein in urine, particularly albumin.

-Normally, no proteins appear in urine.

-Causes may be:

pre-renal, e.g. Bence-Jonesproteins

renal, e.g. damage of the glomerular membrane as in cases of nephritis.

post-renal e.g.cystitis, renal stones.

2)Renal Tubules

-The renal tubules receive about 180 liters  per day (125 ml/m x 60 x 24) by filteration.

-The renal tubules reabsorb about 178.5 liters daily and secrete other materials in urine

-The renal tubules consist of 3 portions:

  1. PCT (Proximal Convulated Tubules)
  2. Loop of Henle
  3. DCT (Distal Convulated Tubules)

-The tubular fluid pours in the collecting ducts.

-Functions

1-It is the only part of the renal tubule where absorption of the following nutrients occurs:

  • Glucose
  • Amino acids

Failure of absorption of any of these nutrients, leads to its excretion in urine.

2-About 70% of sodium ions are reabsorbed by an active process.

3-Almost all potassium ions are reabsorbed in this part

4-Passive reabsorption of the anions Cl-, HCO3- occurs in this part along with the reabsorbed Na+ and K+

5-Obligatory water reabsorption: About 70% of the filtered water

6-Back diffusion of urea in this part following water reabsorption.

7-Secretion of several metabolic waste products as uric acid and creatinine, foreign substances, and drugs e.g. penicillins.

1)Functions of the PCT

Picture82)Functions of Loop of Henle

-The loops of Henle are concerned with the production of concentrated or diluted urine.

-The U-shaped structure of these loops allows them to constitute a countercurrent system. Fluids passes inward and outward closely parallel to each other and in an opposite direction

-These loops are responsible for the development of high osmolality in the renal medullary interstitium. This is essential for urine concentration

Picture9Medullary Gradient

Picture10Picture113)Functions of DCT

In this part of the renal tubule, the final adjustment of urine occurs according to body needs:

-urine volume

-pH

-Electrolyte content

Regulation of urine volume

-The main regulator of water in urine is the antidiuretic hormone (ADH).

-The ADH is secreted when the crystalloid osmotic pressure increases, when plasma volume decreases, and in certain emotions, pain, and drug intake.

-Urine volume increases in hydration and decreases in dehydration.

Mechanism of action of ADH

-ADH increases the permeability of the DCT by opening of the aquaporins present in there walls.

-Now, water absorption occurs passively by the effect of renal medullary hypertonicity

Lack of ADH-Diabetes insipidus

The walls of the DCT become impermeable to water, so most of fluid delivered to DCT is lost in urine.

-Urine volume/day is 15-20 liters

-Polydepsia

-Urine is very diluted

Regulation of electrolyte excretion in urine

In the DCT, sodium ions reabsorption continues by an active ionic exchange mechanism i.e. in exchange with either K+ or H+.

The process is controlled by the Aldosterone hormone

Regulation of pH of urine

The pH of urine is changeable so that to keep the pH of blood constant. It varies from 4.5 in severe acidosis to 8 in severe alkalosis.

Mechanism of pH regulation

1- By getting rid of several acid metabolites e.g. lactic, carbonic, phosphoric and keto acids.

2-The renal tubules absorb the alkali reserve (NaHCO3) which helps to maintain the blood pH constant.

Picture12Kidney – Contd.

In addition to removing wastes, your kidneys release three important hormones:

Erythropoietin or EPO, which stimulates the bone marrow to make red blood cells

Renin (REE-nin), which regulates blood pressure

The active form of vitamin D, which helps maintain calcium for bones and for normal chemical balance in the body

Juxtaglomerular Apparatus

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