Urine Formation Results from Glomerular Filtration, Tubular Reabsorption, and Tubular SecretionThe rates at which different substances are excreted in the urine represent the sum of three renal processes

1. Glomerular filtration

2. Reabsorption of substances from the renal tubules into the blood

3. Secretion of substances from the blood into the renal tubules. Expressed mathematically,

Urinary excretion rate = Filtration rate – Reabsorption rate + Secretion rate

1. Filtration Formation of urine begins with filtration, in

which fluids and small solute are forced under pressure to flow from the glomerulus to the capsular space

2. Reabsorption As the filtrate passes through the tubules,

specific substances are reabsorbed back into the blood of the peritubular capillaries

3. Secretion Some solute are removed from the blood of the

peritubular capillaries and secreted by the tubular cells into the filtrate

Urine Formation Results from Glomerular Filtration, Tubular Reabsorption, and Tubular SecretionMost substances in the plasma, except for proteins, are freely filtered, so that their concentration in the glomerular filtrate in Bowman's capsule is almost the same as in the plasma.

As filtered fluid leaves Bowman's capsule and passes through the tubules, it is modified by reabsorption of water and specific solutes back into the blood or by secretion of other substances from the peritubular capillaries into the tubules.

Filtration, Reabsorption, and Secretion of Different Substances

End products of metabolism such as urea, creatinine, and uric acid are poorly reabsorbed and are therefore excreted in large amounts in the urine.

Electrolytes, such as sodium ions, chloride ions, and bicarbonate ions, are highly reabsorbed, so that only small amounts appear in the urine.

In certain kidney diseases, some of the lower-molecular-weight proteins, especially albumin, are filtered and appear in the urine, a condition known as proteinuria or albuminuria.

Filtration, Reabsorption, and Secretion of Different Substances

Filtration, Reabsorption, and Secretion of Different Substances

Each of the processes-glomerular filtration, tubular reabsorption, and tubular secretion-is regulated according to the needs of the body.

When there is excess sodium in the body, the rate at which sodium is filtered increases and a smaller fraction of the filtered sodium is reabsorbed, resulting in increased urinary excretion of sodium.

Capillary Beds of the Nephron

Blood pressure in the glomerulus is high because:Afferent arterioles have larger diameters

than efferent arterioles

Fluids and solutes are forced out of the blood throughout the entire length of the glomerulus

Net Filtration Pressure (NFP)

The pressure responsible for filtrate formation

NFP equals the glomerular hydrostatic pressure (HPg) minus the colloid osmotic pressure of glomerular blood (OPg) combined with the capsular hydrostatic pressure (HPc)

NFP = HPg – (OPg + HPc)

Determination of GFRNet Filtration pressure:

(1) hydrostatic pressure inside the glomerular capillaries (glomerular hydrostatic pressure which promotes filtration

(2) the hydrostatic pressure in Bowman's capsule outside the capillaries, which opposes filtration

(3) the colloid osmotic pressure of the glomerular capillary plasma proteins, which opposes filtration The GFR can therefore be expressed as

High hydro-static pressure in the glomerular capillaries (about 60 mm Hg) causes rapid fluid filtration

Lower hydrostatic pressure in the peritubular capillaries (about 13 mm Hg) permits rapid fluid reabsorption

By adjusting the resistance of the afferent and efferent arterioles, the kidneys can regulate the hydrostatic pressure in both the glomerular and the peritubular capillaries, thereby changing the rate of glomerular filtration, tubular reabsorption, or both in response to body homeostatic demands

Approximately 1100 ml/min of systemic cardiac output flows through the kidneys

Glomerular Filtration Rate (GFR)

The total amount of filtrate formed per minute by the kidneys

The filtration coefficient (Kf) is calculated to be 12.5 ml/min/mm Hg of filtration pressure, this value depends on the permeability and the surface area of the filtration barrier

GFR = Kf x net filtration pressure

Factors determine the GFR

Total surface area available for filtration

Filtration membrane permeability

Net filtration pressure

Regulation of Glomerular Filtration

If the GFR is too high:Needed substances cannot be

reabsorbed quickly enough and are lost in the urine

If the GFR is too low:Everything is reabsorbed, including

wastes that are normally disposed out

Regulation of Glomerular Filtration

Three mechanisms control the GFR 1. Renal autoregulation 2. Sympathetic control3. Hormonal control

Renal Autoregulation

The ability of nephrons to adjust their own blood flow and GFR without external control

Autoregulation entails two types of controlMyogenic Mechanism – responds to changes in

pressure in the renal blood vesselsTubuloglomerular Feedback Flow-dependent ,

senses changes in the juxtaglomerular apparatus

Myogenic Mechanism

When arterial blood pressure rises, it stretches the afferent arteriole and the arteriole contracts and thus prevents blood flow into the glomerulus from changing very much

Glomerular blood flow and filtration remain stable

Tubuloglomerular Feedback

The juxtaglomerular apparatus monitors the fluid entering the distal tubule and adjust the GFR

High GFR

Rapid flow of filtrate in renal tubules (NaCl)

Sensed by macula densa

Paracrine secretion

Constriction of afferent arteriole

Reduced GFR

The Renin-Angiotensin Mechanism

In response to low flow of filtrate and therefore low NaCl as a result of low GFR caused by low blood pressure, the macula densa cells signal a JG cells to release renin into the blood stream

Renin triggers production of angiotensin II In the kidney increase Angiotensin II causes

constriction of the efferent arteriole Glomerular hydrostatic pressure to increase

and increase GFR Angiotensin II stimulate release of aldosterone