Renal clearance

Question 1

Filtration fraction

Answer 1

The % of plasma flow filtered through the glomerulus and into the nephron. Normally ~20%

Question 2

What is the distribution of total body water?

Answer 2

Total body fluid is 60% bodyweight.

40% is intracellular and 20% is extracellular.

Intracellular fluis is within cells and RBCs

Extra cellular fluid is in plasma, and intersitial fluid

Question 3

Renal blood flow

Answer 3

Rate at which blood flows through the kidneys

Question 4

How is renal blood flow calculated?

Answer 4

Using para-aminohippuric acid (PAH).

Substances which are filtred and reabsorbed but not secreted by the tubules take several passes to clear the plasma. Substances that are filtered and secreted are cleared in a single pass. PAH is filtered and secreted.

Renal clearance of PAH indicates renal blood flow

Question 5

Clinical features of hypovolemia

Answer 5

Symptoms: thirst, dizziness on standing, confusion

Signs: low JVP, postural hypotension, weight loss, dry mouth, reduced skin turgor, reduced urine output

Question 6

Clinical features of hypervolaemia

Answer 6

Symptoms: ankle swelling, breathlessness

Signs: raised JVP, oedema, weight gain, hypertension, pulmonary crepitations

Question 7

3 fundamental processes in the nephron

Answer 7

Glomerular filtration: produces ultrafiltrate which contains plasma, electrolytes and small organic solutes

Tubular secretion: regulates pH, K+, NH3, organic acids and bases and foreign substances

Tubular reabsorption: 99% of water, HCO3, 100% glucose, 50% urea, 99% Na+

Question 8

Describe the glomerular filtration barrier

Answer 8

Filtration of blood entering the glomerulus at the afferent arteriole relies on the permeability of the glomerular capillary walls.

The filtration bed is characterised by:

• Fenestrations in the endothelial cells
• Podocytes that interdigitate to close and strengthen the vessels
• Basment membrane consisting of a network of matrix proteins which have a negative chage and therefore repel negatively charged molecules.

Question 9

Causes of renal failure

Answer 9

Prerenal - due to depressed renal vascular perfusion so the hydrostatic forced required for filtration are reduced. Effective glomerular filtration cannot occur e.g. cardiac failure, hypotension

Renal - due to disease of the nephron, glomeruli, microvascular or the tubules e.g. diabetes, nephrotoxic, acute tubular necrosis

Postrenal - due to obstruction in the outflow of the kidney e.g. renal pelvis, ureters down to the bladder, recurrent ascending infections

Question 10

Forces regulating filtrate formation

Answer 10

Blood pressure (glomerular capillary)

Hydrostatic pressure in the lumen of the tubule

Oncotic pressure from plasma proteins

Damage that allows plasma proteins into the nephron decrease oncotic pressure and so increase GFR

Question 11

Glomerular filtration rate

Answer 11

Amount of filtrate produced from the blood flowing through the glomerulus per unit time

Directly related to the function of the nephrons and declines in all forms of progressive kidney diseases

Question 12

Factors that affect GFR

Answer 12

Surface area

Age (lower in elderly)

Sex (lower in females)

Muscle mass

Diet

Ethnicity

Question 13

Osmolarity

Answer 13

molar concentration of solute particles per litre of solution. (total solute concentration of a solution)

Question 14

Osmolality

Answer 14

Molar concentration of solute paricles per kg of solvent

Question 15

Factors that affect tissue fluid formation

Answer 15

At the artiole end of the capillary hydrostatic pressure is greater than colloid oncotic pressure and narrowing of the vessel produces resistance to flow which forces fluid out of the capillary

As fluid moves out of the capillaries via the permable wall, oncotic pressure incerases and the pressure draws fluid back into the capillaries

Question 16

Renal Clearance

Answer 16

Volume of plasma that is cleared of a substance in a unit time

Measure of the kidney’s ability to remove a substance form the plasma and excrete it.

Clearance of a substance will provide an accurate measurement of the GFR if the substance follows the filtrate and is not altered by the kidney (metabolised, secreted)

Question 17

How is renal clearance measured?

Answer 17

Using creatinine. Produced from the metabolism of creatine in muscle metabolism.

Plasma creatine levels remain constant if renal funtion, muscle mass and metabolism are stable.

Creatine is filtered freely and not largely affected by the kidney (some secreted).

Question 18

eGFR

Answer 18

Used because plsma creatinine levels do not accurately reflect renal function, and GFR is difficult to measure in clinical practice.

GFR estimated using MDRD equation - uses age, sex, race and creatinine level.

Used to classify CKD.

Question 19

Measurement of renal plasma flow

Answer 19

Amount of substance removed from the plasma per min is equal to the amount that appears in the urine per min.

using PAH - organisc acid that is filtered at the glomerulus and secreted by the proximal tubules on first pass throguh the kidneys. Amount of PAH equals amount filtered and secreted by the kidneys.

Question 20

Feedback control of GFR

Answer 20

1. Tuboglomerular feedback: Responds to changes in tubular fluid flow rate. Rise in pressure/flow are transmitted from the macula densa to the juxtaglomerular appartus which produces vasoconstrictor substances (angiotensin II, prostaglandins) to contract the smooth muscle and decrease renal plasma flow.
2. Myogenic mechanism: Responds to changes in arterial pressure. When pressure rises, the afferent arteriole stretches and the smooth muscle contracts. This prevents the rise in pressure being transmitted to the golmerular capillary so GFR is unchanged