Urinary Physiology Lecture Notes

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Overview of kidney function

1. Maintain water balance and plasma osmolarity

2. Regulating plasma and ECF electrolytes and volume

3. Excreting waste products like urea and other foreign compounds like drugs

4. Regulating blood PH, so that, keeping it around 7.4

5. Produce hormone like substances like; rennin (which cause massive vasoconstriction after it convert angiotensinogen to angiotensin I and the later converted in lungs to angiotensin II which cause vasoconstriction)

Note: blood PH is very important for normal functions of body enzymes and organs that’s why its important to keep it with in narrow range around 7.4

Brief anatomy:

1- Kidneys are bean like  structure in shape

2- Functional unit in the kidney is the nephron (smallest part in the kidney can perform the function of the kidney completely)

3- nephron consists of: golmerulus or Bowman’s capsule (which is globe like that contain capillary tuft), proximal convoluted tubule, loop of Henle, distal convoluted tubule and collecting duct that drain urine to renal pelvis

4- kidney is consist of two parts: outer layer, the cortex which contain glumeruli and part of tubular system, and an inner layer, renal medulla, which contain most of tubular system especially loop of Henle and collecting duct

5- renal medulla converge to inside, so that, collecting duct empty the urine to a cavity called renal pelvis which is connected to ureter

6- golmerulus receive arteriole called afferent arteriole, which branches inside the bowman’s capsule to form glomerular capillary tuft, the tuft rejoin to form the efferent arteriole that emerge from golmerulus, then this arteriole will follow the course of tubular system and form a network around them, this network is called peritubular capillaries that drain to venules and then to renal vein

7- glomeruli in the cortex are two sets (types): (1) either lie in the outer 2/3 of the cortex and have their loop of Henle extending to the upper part of the medulla and these called cortical glomeruli, or (2) lie in inner 1/3 of cortex and have their loop of Henle extending most of medulla (deeper than the first type) and these called juxta medullary glomeruli. (Juxta= near)

8- Part of peritubular capillaries that is associated with loop of Henle is called Vasa recta

9- main function of cortical glomeruli, that are rich in blood vessels more than juxta medullary glomeruli, is reabsorption of important substances back to blood, while for juxta medullary glomeruli is to modulate the urine concentration (ability to produce concentrated or diluted urine is by these glomeruli)

10- Bowman’s capsule is made of two layers forming bowman’s space which contain fluid passing from glomerular capillary, this fluid is called the Filtrate, the bowman’s space is continuous with the tubular system

11- descending limb of loop of Henle has the filtrate passing through it from up to down, while the ascending limb has the filtrate passing from down to up (in reverse direction), also this applied for the direction of blood flow in Vasa recta associated with loop of Henle (descending part has flow from up to down and ascending part has flow from down to up), this type of reverse direction of flow is called counter current flow

12- when distal convoluted tubule ascend up toward the golmerulus it pass through the fork made by afferent and efferent arterioles when they attach to golmerulus, the cells of tubule and afferent and efferent arterioles cells form a important structure called juxta glomerular apparatus (which has a role in regulation of blood flow through kidney

Structure of golmerulus

1- glomerulus is composed of glomerular capillary tuft surrounded by bowman’s capsule, these capillaries are not true capillaries which provide gases and nutrient exchange with tissues but they are designed so that blood passing through them will filtrate a plasma-like fluid, free of proteins (i.e. all plasma constituents except proteins and large molecules) and this filtrate represent 20% of plasma passing through these capillaries

2- Fluid passing from glomerular capillary to Bowman’s capsule space should pass through three layers, these layers called glomerular membrane:

a) Endothelium cells of the capillary which is flat single layer of cells and has large pores between cells for passing of filtrate, making it 100 times more permeable than usual capillaries

b) Basement membrane of the capillary, which is fenestrated (مثقب)

c) Inner layer of Bowman’s capsule which contain special cells called Podocytes (foot processes cells) which encircle the glomerular tuft

Notes

* These Podocytes project their foot processes so that the processes of adjacent foots from different Podocytes will interdigitate with each other forming important structure called filtration slits

*This glomerular membrane is just like a sieve (منخل) that filters water and solutes, but retain cells and proteins

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Medullar osmotic gradient

* As mentioned before, that, there are two types of nephrones (cortical), and (juxta medullary) and the function of the first is for reabsorption and the second for modulating urine concentration

* Measuring osmolarity of medulla will show that, osmolarity of interstitial fluid of medulla will increase gradually whenever passing to deep parts of medulla starting from about 300 mosm up to 1200 – 1400 mosm, and this graduation is called medullary osmotic gradient

* The function of urine concentration is accomplished by (تتم بـ) مهمة:

1) Medullar osmotic gradient

2) Counter current flow of Vasa recta and loop of Henle

3) Specific characters (صفات) of tubular and capillaries cells passing through medulla

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Three basic renal processes:

Kidney basically has 3 processes; filtration, tubular reabsorption and tubular secretion:

Filtration: will be mentioned below

Tubular reabsorption: where the important substances that are needed, like glucose, amino acids and some electrolytes and water will return to blood from the lumen of the tubules. E.g. the filtrate is about 180 liter per day, about 178.5 liters will reabsorbed

Tubular secretion:

* About 20% of plasma passing through capillaries will be a filtrate in Bowman’s space, and remaining is flowing through efferent arteriole to go to peritubular capillaries

* Secretion is the process by which some substances are selectively transferred from peritubular capillary to tubular lumen, and by this, providing additional mechanism of elimination  in addition to glomerular filtration

Filtration:

1- Kidneys have large share of the cardiac out put, which is about 20%, which is large amount in comparison to kidney size and weight

2- Filtration through glomerulus occur when blood flow through it with a pressure that is enough to force fluid from capillaries to bowman’s space

3- Afferent arteriole is wider in diameter than efferent and that make blood pass through with pressure, favoring filtration through the sieves

Note: when blood flow is increased in kidney, this will increase filtration (because of increased glomerular capillary pressure)

4- Actually , filtration occurs as a result of the Net of three forces:

A) Glomerular capillary pressure

B) Plasma colloid osmotic pressure

C) Bowman’s capsule hydrostatic pressure

* First force is about 55 mm hg, which is with filtration, other two forces both act against filtration, colloid osmotic pressure is about 30 mm hg, Bowman’s capsule hydrostatic pressure is about 15 mm hg, so the net filtration force is = 55 –(30+15) = 10

* Plasma colloid osmotic pressure resulted from retaining of proteins in plasma and filtration of water to bowman’s capsule and by this, creating osmotic force that tend to move water from Bowman’s capsule to capillary, as to say, against filtration

* Bowman’s capsule hydrostatic pressure resulted from the filtrate present in Bowman’s capsule space

Notes:

1- About 125 ml of filtrate is formed by all glomeruli per minute or, about 180 liter per day

2- Amount of filtrate can be modulated or adjusted by amount of blood flow

3- Juxta glomerular apparatus can modulate blood flow that passes through glomerulus

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