Renal Physiology

Reference: Human Physiology, by: Stuart Ira Fox; 11th Edition.

Fawzi A. BabikerDepartment of Physiology

E-mail: fawzi.b@hsc.edu.kw

Physiology of the kidneys

Structure and function of the kidney.

Glomerular filtration (GF).

GF regulation and reabsorption of salt and Water.

Renal plasma clearance.

Renal control of electrolyte and acid-base balance.

Step by step flow up of urine formation.

Short clinical ideas.

Contents:

Regulation of the blood plasma volume (Blood pressure).

Removal of metabolic wastes products from the blood (urea).

Electrolytes balance (Na+, Ca+2, K+, Mg+, SO4-2, HCO3

- and other ions).

Removal of toxic chemicals from the blood (drugs).

Acid-base balance (pH).

Hormone secretion.

Functions of the kidney

Hormone secretion

In addition to its excretory function the kidney acts as an endocrine gland as it secretes three hormones.

1- Erythropoieten which controls erythrocytes (blood cells) production.

2- Renin, which controls the formation of angiotensin which influences blood pressure and sodium balance.

3- 1.25-dihydroxyvitamin D3 which influences calcium balance.

Structure of the urinary system

Adrenal gland

Abdominal aortaInferior vena

cava

Urinary artery

Urinary vein

Renal circulation

Structure of the Kidney

The nephron

Types of nephrones:

Cortical nephrons:

Small nephrons with short tubules.

Juxtamedullary nephrons:

Big nephrons with long tubules.

Both types of nephrons originate in the cortex.

Nephron is the functional unit of the kidney.

Structure of a Nephron

The glomerulus

Glomerular capillaries

Glomerular Filtration

Capillaries of the glomerulus are fenestrated (have pores).

Blood pressure (hydrostatic pressure is the main driving force for filtration).

The amount of plasma that goes to the kidneys each minute ~ 600 ml/min (RPF= renal plasma flow).

Glomerular Filtration Rate (GFR) is the volume of filtrate/min ~ 125 ml/min, (15-20% of RPF).

Regulation of the glomerular filtration rate (GFR)

GFR is the volume of filtrate produced by the two kidneys per minute.

115 ml per min in females.125 ml per min in males.About 180 liters per day.

What happens if all this has to be excreted?What happens in reality?

PGC: hydrostatic pressure in the glomerular capillary.

πGC: oncotic pressure in the glomerular capillary.

PBS: hydrostatic pressure in Bowman's space.

πBS: oncotic pressure in Bowman's space.

PUF: net ultrafiltration pressure.

The negative signs for PBS and πGC indicate that these forces oppose formation of the glomerular filtrate.

Regulation of the glomerular filtration Factors that regulate filtration:

Regulation of the glomerular filtration rate

Glomerular filtration and renal blood flow is regulated by renal nerves.

The kidney is innervated by sympathetic nerve fibers (only constriction).

Sympathetic nerves secretes Noradrenalin (NE) which cause vasoconstriction.

Constricted afferent arterioles = GFR

Dilatation of afferent arterioles = GFR

1- Sympathetics VasoconstrictionReduction in GFR

2- Local effects: Autoregulation; effect of local chemicals

Blood flow Pressure x radius4

Viscosity x length

Effect of the sympathetic nerve

Regulation of the glomerular filtration rate cont…

1- Effect of the sympathetic nerves

2- Intrinsic (Autoregulation)

Systemic pressure

Systemic pressure

Afferent pressure

Afferent pressure

Renal blood flow

Renal blood flow

Net result is a constant renal blood flow

When blood pressure increases afferent arterioles constricts to prevent the increase of renal blood flow and GFR.

When blood pressure decreases the afferent arterioles dilates to prevent the decrease of renal blood flow and GFR.

Thus, this means that auto regulation happens in the afferent arterioles to keep renal blood flow and GFR constant.

Regulation of the glomerular filtration rate cont..

Regulation of the glomerular filtration rate (GFR)Fight/Flight

AutoregulationHigh Blood Pressure

Keep GFRconstant

The formation of the glomerular ultrafiltrate

Although the capillary pores are large enough to pass proteins but that never happens because of the negative charges of the proteins and that of the Glycoproteins in the basement membrane.

Due to the blood pressure some fluids enters the glomerular capsule (ultrafiltrate).