Renal system

Dr. Zainab H.HDept. of Physiology

Lec.1,2

Objectives

• List the functions of kidney• Describe the morphology of the typical

nephron and its blood supply.• Describe the concept of clearance

What is excretion?Throwing out of waste product is known as excretion.The organs through which excretion occurs.1. Kidneys: Excrete water and water soluble waste products.2. Lungs: Excrete carbon dioxide, water vapour and other volatile substances such as acetone.3. Skin: Excretes water and salts mainly in the form of sweat.4. Gastrointestinal tract: Excretes undigested food.

The Role of the Kidneys in the Body Includes:

• Regulation of the volume and composition of the ECF, by

maintaining a balance between intake and output of water and electrolytes in the body.

• Excretion and elimination of waste products of metabolism, such as the excretion of urea, creatinine and uric acids; as well as the excretion of various toxins such as drugs and food additives.

• The kidneys act as endocrine glands producing hormones, such as “erythropoietin hormone” and renin.

• Playing a dominant role in the long-term and short-term regulation of arterial blood pressure.

• Kidneys along with the respiratory system contribute to acid-base regulation.

• Finally, kidneys synthesize glucose from amino acids and other precursors

Metabolic waste products excreted by kidneys are:

• Urea from protein.• Uric acid from nucleic acid.• Creatinine from muscle creatine.• End products of haemoglobin breakdown.

STRUCTURE OF NEPHRON

• Nephron is a structural and functional unit of the kidney.

• Each nephron is capable of forming urine. • There are two types of nephrons:• 1. Cortical nephrons. Glomeruli are present near

the surface of the kidneys. – These nephrons constitute about 86% of total

nephrons.– The main function of cortical nephrons is absorption of

sodium.

• 2. Juxtamedullary nephrons. Glomeruli lie at the junction of cortex and medulla of the kidney. – These constitute 14% of the nephrons.– The main role of juxtamedullary nephron is to

increase concentration of medullary interstitial fluid.

Two kidneys together have two millions nephrons.Nephron consists of two major parts:– Glomerulus.– A long renal tubule.

1. Glomerulus. It is made up of tuft of capillaries which connect afferent arteriole with an efferent arteriole.

• Capillaries have single layer of endothelial cells attached to a basement membrane.

• Bowman’s capsule encloses the glomerulus and is formed of two layers:– inner layer which covers the glomerular capillaries

is called visceral layer, – outer layer is called parietal layer. Space between visceral and parietal layers is

continued as the lumen of the tubular portion.

–fluid-filled space, Bowman’s space, is formed within the capsule. –Blood of capillary and fluid of Bowman’s space

are separated by the glomerular membrane. – From the Bowman’s capsule, tubule of the

nephron extends, the lumen of which is continuous with the Bowman’s space.

• 2. Renal tubule. It is mainly formed of three parts:

(a) Proximal convoluted tubule.(b) Loop of Henle consisting of:– Thin segment :walls of descending limb and lower

end of ascending limb are very thin. Therefore, they are termed thin segment.

– Hair pin bend.– Thick ascending limb or segment

• (c) Distal convoluted tubules. • open into initial arched collecting ducts called

cortical collecting ducts present in renal cortex. – Seven to ten such ducts form straight collecting

duct which passes into medulla forming medullary collecting ducts.

• In the inner zone of medulla they form papillary ducts or ducts of Bellini. – These open into papilla of minor calyces. – Three or four minor calyces unite to form one

major calyx.– The major calyces open into pelvis of ureter. – The pelvis is an expanded portion present in renal

sinus and it continues as ureter

BLOOD FLOW TO KIDNEYS

• Rate of blood flow to kidneys is 1200 ml/min. • This is quite high as compared to their size. • State peculiarities of renal circulation.

1.Very high blood supply, about 21% of cardiac output.

2.Two sets of capillaries. – The glomerular capillaries.

• These combine to form efferent arteriole which in turn breaks into peritubular capillary network around the tubules of cortical nephrons.

– vasa recta which are loop-shaped vessels in juxtamedullary nephrons• the efferent arterioles continue as these loops dip into the

medullary pyramids alongside the loops of Henle

3.Glomerular capillary bed has a high hydrostatic pressure because efferent arteriole is of a smaller diameter than afferent arteriole which offers considerable resistance to blood flow.

4. Peritubular capillary bed is a low pressure bed.

5. Only 1 to 2% of blood flows through vasa recta. The flow is very sluggish.

6. Renal blood flow shows remarkable constancy in face of blood pressure changes due to autoregulation

• The kidneys regulate the hydrostatic pressure in both capillary beds (glomerular & peitubular ) , by adjusting resistance of the afferent and efferent arterioles.

• High hydrostatic pressure in GC (60 mmHg) causes rapid fluid filtration; whereas a much lower pressure in the peritubular capillaries (13 mmHg) permits rapid fluid reabsorption.

• The afferent arteriole is a short, straight branch of the interlobular artery.

• The efferent arteriole, that drains the GC, has a relatively high resistance than the afferent arteriole.

The Process of Urine Formation:• Urine formation begins with the • Filtration of plasma through the GC into the Bowman’s

space.

• As the filtered fluid flows through the remaining portions of the tubule, its composition is altered as a result of two main processes:

1. Tubular reabsorption.

2. Tubular secretion. and both processes will produce the final product, urine.

Concept of Clearance:

• The renal clearance of a substance is the volume of plasma that is completely cleared or cleaned of that substance by the kidney per unit of time. (usually expressed as mL/ minute).

• However, renal clearance provides a useful way of quantifying renal excretory functions.

• It can be used to quantify the rate at which blood flows through the kidneys, as well as to measure the basic kidney functions such as GFR.

Concept of Clearance (continues):• Renal clearance of a substance (S) is calculated by

dividing the urinary excretion rate of (S) (US × V•) by its plasma concentration (PS), as expressed below:

US × V•

CS = PS

• Where: US = urine concentration of S.

V• = urine flow rate/ minute = (0.9 mL/ minute).

PS = plasma concentration of S.

• Clearance of any substance depends on the behavior of the tubular cell towards that substance:

• If a substance like inulin, which is freely filtered at the glomerulus level and is neither reabsorbed nor secreted by the renal tubule, then its clearance equals to GFR.

• On the other hand, if a substance is reabsorbed by the renal tubule, its clearance is lower than the GFR.

• Finally, if a substance that is in addition to filtration is secreted by the renal tubule, then its clearance is higher than the GFR.

THANK YOU