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Kidneys: Structure, function, and diseases

514px-Physiology_of_Nephron
Source: Wikipedia
Anatomical and Histological Introduction
The kidneys are located behind the peritoneum, equal in size by normal state. Kidney length 10-12 cm, width about 6 cm, the thickness of 4-5 cm and weight of 150 g. The thickness of renal cortex is one-third of the thickness of the kidney (1.5-2 cm).

The basic things you should see on ultrasound in a natural kidney:
  • corticomedullary differentiation (when differentiating the resonance of renal cortex and renal medulla) and its absence indicates renal disease
  • Kidney length
  • The thickness of the renal cortex

Glomerulus
Kidney_Nephron
Source: Wikipedia
The glomerulus is not renewed when destroyed (unlike the tubule), Glomerulus is classified into:
Surface: 85%, smaller than the deep, its primary function is blood filtering.
Deep: 15%, larger than the surface, its main concentration function, and is more susceptible to Glomerulus diseases.
Glomerulus consists of two poles (angular pole and urinary pole), and the Glomerulus is with portal circulation.

The glomerular filtration barrier (GFB) consists of:

The Vascular endothelium:
500-angstrom holes (0.5 microns), passing the albumin.
When an increase in cytoplasmic thickness occurs in endothelial cells (as in acute kidney infection) endothelial cells become edematous, lumen the vessel closes, and the urine scarce occurs.

Baseline membrane: 
smaller holes, negative charge, does not pass protein.
When a basement membrane ruptures an "immunological rupture", this causes proteinuria.
The endothelial cells outside the basal membrane (negative charge)

The functions of the kidney
  • Blood Clearance (filtration, reabsorption, and excretion).
  • Regulation of arterial pressure.
  • Organizing the internal environment.
  • Maintain acid-alkaline balance.
  • Glandular function.

Blood Clearance

The pressure of the glomerular filtration = 15 mmHg, which is applied to the whole area of the basal membrane, which is equal to 1 square meter.
It is the result of several pressures (forces that stimulate filtration + forces that are inverse), and these pressures are:
  • The capillary pressure (static pressure) inside the glomerulus: (+60) mmHg, resulting from cardiac pumping and small diameter of the efferent arteriole relative to afferent arteriole diameter "the most positive and most important element by filtration".
  • Oncotic pressure of plasma proteins inside the glomerulus: (-35) mmHg.
  • Static water pressure inside Bowman's capsule: (-10) mmHg.
  • Capillary pressure inside Bowman's capsule: negligible value, except in special cases.

The effectiveness of the filtration = the capillary pressure inside the glomerulus - (Oncotic pressure + pressure inside Bowman's capsule)= 60 - (35 + 10) = 15 mmHg. glomerular filtration: 180 l /day, Reabsorption 99% → urine volume 1.5 liters per day

Reabsorption and excretion
2618_Nephron_Secretion_Reabsorption
Source: Wikipedia
70% in Proximal convoluted tubule (water + electrolytes), active for sodium, passive for water and chlorine

20% Loop of Henle (water + electrolytes):
  • Descending limb of the loop of Henle: absorption of water only and the concentration of urine, are more piece able to concentrate urine, Peak focus up to 1200 mosmol.
  • Ascending limb of the loop of Henle: Non-permeable to water, absorbs the electrolytes only leading to diluted urine, the absorption of chlorine active, while the absorption of sodium passive.

10% cortical and distal convoluted tubule: active absorption of sodium by aldosterone, passive absorption of chlorine and water.

Absorption of Sodium: This is offset by the release of potassium by the aldosterone (the potassium originates from the secretion, while the sodium is left from the filtration)

Absorption of bicarbonate: Re-absorbed completely leading to acidic urine (natural bicarbonate in the blood 22-26).
The above is for sodium, water, and chlorine, and for potassium, it is fully absorbed until the end of the Ascending limb of the loop of Henle actively. This explains the high blood potassium in renal insufficiency because its secretion needs energy

The role of the kidney in the organization of the internal environment

This is done by understanding the glomerulus and proximal tubule to preserve the internal environment. For example, if the filtered sodium is increased (because of an increase in the intake), the proximal tubule reduces the reabsorption of sodium and vice versa.

Endocrine gland function

1. Hormones that affect the kidney:
  • Aldosterone: excreted from the adrenal cortex, affecting distal convoluted tubule, resulting in increased Na absorption with excretion of K and H.
  • ADH: secreted by the hypothalamus, affects the distal convoluted tubule and cortical connecting tubule (increasing water reabsorption(
  • PTH: Regulate the reabsorption of P, Ca (calcium absorption and phosphorus excretion at the distal convoluted tubule level), Increases the synthesis of a1 Hydroxylase that responsible for the synthesis of the bioactive form of Vitamin D
  • ANP: excreted at dilated atria (natriuretic, vasodilator).

2. Hormones that are secreted from the kidney:
  • Vasodilator: Prostaglandin - Kallikrein enzyme, prostaglandin: excreted from renal interstitial tissue, topical effect: increased renal blood flow, increased Na excretion, ADH inhibition 
  • vasoconstrictor: renin-angiotensin, angiotensin II (systemic): strong vasoconstrictor, stimulates the adrenal synthesis of aldosterone, modulates the renal flow of kidneys, inhibits renin secretion, increases catecholamines secretion, has a central effect enhanced for the sympathetic effect, stimulant the thirst and the secretion of ADH.

Notes:
Renin: It is excreted from the renin-angiotensin-aldosterone system (RAAS) in the kidney, its function is that the renin constricts blood vessels.
Renin converts angiotensin in the blood to angiotensin 1.
Angiotensin 1 is divided into Angiotensin 2 by Angiotensin Converting Enzyme (ACE) in lung vessels.

Renin secretion is controlled by:
  • Pressure changes in the afferent arteriole
  • Sympathetic effect
  • Topical prostaglandin release.
  • The concentration of chlorine and osmotic pressure in distal convoluted tubule

Hormones made by the kidney and released in the blood circulation to effect out of kidneys
  • Erythropoietin: produced in the renal interstitial tissue, stimulates the bone marrow to produce erythrocytes. 
  • The bioactive form of vitamin D: is formed in the skin under the UV effect, initially being ineffective, then hydroxyl 25 radical is added in the liver and is relatively ineffective. Then add 1 hydroxyl or 24 hydroxyl radical in the kidney with the help of 1-a hydroxylase, and this yeast: 
  1. stimulated by: parathyroid hormone, and lack of blood phosphorus.
  2. Inhibited by: Calcitonin and increased blood phosphorus.

Note :
Most non-steroidal anti-inflammatory drugs such as indomethacin and aspirin inhibit the secretion of prostaglandins. If used in a patient with renal insufficiency, the patient may experience a severe reduction in the glomerular filtration rate (GFR), which may result in severe renal insufficiency.

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