A Brief Look at Renal Anatomy

8/4/2019 A Brief Look at Renal Anatomy

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A Brief Look at Renal Anatomy & PhysiologyStructures of the Renal System:

The renal system is comprised of the Kidneysand those structures including the ureters,bladder and urethra that form the urinary system. The primary role of the kidneys is to removemetabolic wastes and maintain fluid andelectrolyte balance. The kidneys also have arole in:

• Blood Pressure Control

• Red Blood Cell Synthesis

• Bone Metabolism

• Acid- Base Balance

Renal dysfunction can negatively impact on all

of these roles.11

Cross Section of the Kidney:



The kidneys are situated in the retroperitoneum located between T12 and L3 on each

side of the vertebral column.12

Two layers form them internally. The outer layer is the Cortex that contains:

• Glomeruli

• Proximal Tubules

• Cortical Portions of Loops of Henle

• Distal Tubules

• Cortical Collecting Ducts11,12

The inner layer or Medulla is comprised of Renal Pyramids. The pyramids contain:

• Medullary portions of Loops of Henle

• Medullary Portions of Collecting Ducts12

Multiple pyramids taper and join forming a minor calyx. Several combined make amajor calyx. The major calyces join and enter a funnel shaped renal pelvis that

directs urine into the ureter.11

Components of the Nephron:

Approximately one million nephrons comprise each kidney. The nephron consists of:



• Glomerulus

• Bowman Capsule

• Proximal Convoluted Tubule

• Loop of Henle

• Distal Convoluted Tubule

• Collecting Duct11,12

There are two types of nephron:

• Cortical Nephrons

• Juxtamedullary Nephrons11

Cortical Nephrons:

• Approximately 85 %

• Perform excretory and regulatory functions11

Juxtamedullary Nephrons:

• Approximately 15 %

• Responsible for concentration and dilution of urine11

Urine Formation: Three processes required for urine formation include:

• Glomerular Filtration

• Tubular Reabsorption

• Tubular Secretion11,12

Glomerulus

• Filters fluid and solutes from blood

Proximal Convoluted Tubule

• Reabsorbs Na+, K

+, Cl

-, HCO

3

-, urea, glucose & amino acids

• Filtrate Continues

Loop of Henle



• Reabsorbs Na+, K

+& Cl

-

• Blocks reabsorption of H2O

• Dilutes/Concentrates Urine


Distal Tubule

• Na+, K

+, Ca

++, PO

4selectively reabsorbed

• H2O reabsorbed in presence of Antidiuretic Hormone (ADH)


Collecting Duct

• Reabsorption similar to distal tubule

• HCO3

-& H

-reabsorbed/secreted to acidify urine

• Filtrate leaves hyperosmotic/hypoosmotic depending on the body’s requirements11,12



Composition of Urine:H

2O

Electrolytes- Na+, K

+, Cl

-, HCO

3

-

End products of protein metabolism- urea, creatinine, PO4, SO

4

End products of nucleic acid metabolism- uric acidBreakdown products of phosphoric and sulphuric acid

H+ions excreted bound to buffers such as PO

4and NH

3

11

Renal Anatomy & Physiology in Summary:

• Kidneys filter blood of waste products

• Functional units of the kidneys are called nephrons

• Nephrons consist of a glomerulus, tubule and collecting duct

• Urine is formed through glomerular filtration, tubular reabsorption and tubular

secretion

• Urine moves from the collecting duct via the renal pelvis and ureters into thebladder, where it is excreted from the body through the urethra

• Some substances are reabsorbed into the blood and others excreted into thefiltrate



Summary of Acute Renal FailureDefinition:Acute renal failure (ARF) is a clinical syndrome, characterised by an abruptdecline in glomerular filtration rate (GFR). There is a subsequent retention of metabolic waste products and an inability to maintain electrolyte and acid-base

homeostasis. Regulation of fluid volume is also affected.

1,7,8,16,18,23

ARF occurs rapidly resulting in fifty percent or more nephrons to lose function,and as this occurs quickly the body is unable to compensate. There are three

classifications of ARF based on the location of the cause.23

PrerenalRenal dysfunction is largely related to systemic factors that limit blood flow andreduce glomerular filtration rate. Examples include:

• Hypotension

• Hypovolaemic shock- dehydration, blood loss

• Cardiogenic shock – post MI

• Septic Shock

• Bilateral renal vascular obstruction- thrombosis1,8,9,16,18,23

IntrarenalRenal impairment occurs secondary to damage that is sustained at the site of thenephrons. This may be the result of a number of conditions or nephrotoxins:

• Acute Tubular Necrosis (ATN)

• Acute Glomerulonephritis

• Acute Pyelonephritis

• Acute Cortical Necrosis

• Malignant Hypertension

• Acute Vasculitis

• Rhabdomyolysis - drugs, trauma

• Nephrotoxins - IV contrast, aminoglycosides1,8,9,16,18,23

PostrenalRenal failure secondary to obstruction that prevents excretion of urine

• Prostatic Hypertrophy

• Renal Calculi

• Tumour

• Blocked Urinary Catheter1,8,9,16,18,23



Signs & Symptoms

• Fluid and electrolyte abnormalities

• Metabolic acidosis

• Anaemia

• Pruritis secondary to uremic frost

• Nausea & vomiting

• Confusion

• ↓LOC

• Congestive heart failure resulting in acute pulmonary oedema1,8,16,18,23



Renal Replacement TherapyRenal replacement therapy (RRT) is an extracorporeal technique of blood purification.Blood passes over a semipermeable membrane (filter) allowing solutes and water tocross over to a collection side. There are various modalities included under the

umbrella of RRT.19

Continuous Modalities (CRRT)

• Haemofiltration

• Haemodialysis

• Haemodiafiltration

• Ultrafiltration

Advantages:

• Better for haemodynamic instability

• Readily accessible

• Effective fluid removal and clearance of solutes

• Can be performed by ICU staff rather than specialised renal nurses2,4,5,9,17,20

Disadvantages:

• Patient mobilisation is limited

• Access complications

• Anticoagulation

• Reduced blood flow rates secondary to small filters when compared to IHD2,4,5,9,17,20

Intermittent Haemodialysis (IHD)Advantages:

• Quick and effective

• Large amounts of fluid and solutes can be removed over a short period5,9

Disadvantages:

• Access Complications- formal access such as A.V. Fistula is eventually required

• Requires specialised staff and is therefore not readily accessible

• May not be well tolerated by haemodynamically unstable patients

• Intermittent fluid removal with IHD can be associated with fluid overload and

increased electrolytes between treatments 5,9

Peritoneal Dialysis This form of dialysis utilises the peritoneum as the semipermeable membrane.Advantages:

• Comparatively Cheaper



• No anticoagulation required



• No haemodynamic instability 5,9,24

Disadvantages:

• High incidence of peritonitis

• Slow clearance

• Access – Formal access required (Tenkhoff catheter)

• Limitations on patient as it is required frequently 5,9,24



Continuous Renal Replacement Therapy (CRRT)Indications

• Fluid Overload, pulmonary oedema

• Worsening Metabolic Acidosis

• Hyperkalaemia

• Worsening Azotaemia

• Drug overdoses

• Removal of toxins9

Basic Principles The basic principles incorporated in the function of CRRT involve:

• Convection

• Diffusion

• Ultrafiltration

• Hydrostatic Pressure2,4,9,17,20

TerminologyDiffusion The movement of small and middle molecule solutes from an area of high

concentration to low concentration across a semipermeable-membrane.5,9

22

Osmosis The movement of water from an area of high water concentration to an area of lower

water concentration across a semi-permeable membrane.5,9

22



Ultrafiltration The movement of water and solutes across a semipermeable membrane by solvent

drag created by convection and hydrostatic pressure.5,9

22

Convection

Water flow across a semi-permeable membrane by hydrostatic pressure that dragssolutes with it (the way a waterfall moves pebbles and sand)

9

Hydrostatic Pressure The force that pushes fluid and solutes across the membrane. The mechanical blood

pump on the dialysis machine creates this.5,9

Oncotic Pressure Plasma proteins including albumin, globulin and fibrinogen create the pulling

pressure that favours fluid retention and opposes hydrostatic pressure.5,9

Counter Current The flow of two fluids in opposing directions. The direction of dialysis flows opposite tothat of blood flow maximising the concentration difference between blood and

dialysate.5

Dialysate

A synthetic solute free solution used to achieve diffusive solute clearance5

Effluent

Erroneous term used to indicate the solute and solvent discarded form the patient.9

Replacement Pre or post dilution fluidPre-dilution

Administration of the replacement fluid into the circuit prior to the filter5,9

Post-dilution

Administration of replacement fluid into the circuit after the filter5,9



Modalities

• Slow Continuous Ultrafiltration (SCUF)

• Continuous Arterio/Venovenous Haemofiltration (CAVH/CVVH)

• Continuous Arterio/Venovenous Haemodialysis (CAVHD/CVVHD)

• Continuous Arterio/Venovenous Haemodiafiltration (CAVHDF/CVVHDF)

SCUFSlow Continuous Ultrafiltration is the method used when fluid removal is the only objective. Dialysate and replacement fluids are not utilised. Maximum fluid removal is

2000ml/hr.2,17,20

= pump

SCUF System Setup 13

CAVH/CVVH

Continuous Venovenous Haemofiltration uses convective clearance to remove waterand solutes. Replacement is used to replace ultrafiltrate. Maximum fluid removal is

1000ml/hr.2,17,20

= pump

CVVH System Setup 13



CAVHD/CVVHDContinuous Venovenous Haemodialysis uses diffusion to remove fluid and solutes.Dialysate is pumped in a counter current to blood flow. Maximum fluid removal is

1000ml/hr.2,17,20

= pump

CVVHD System Setup 13

CAVHDF/CVVHDFContinuous Venovenous Haemodiafiltration utilises both convection and diffusion toremove fluid and solutes. Dialysate and replacement is used. Maximum fluid removal

is 1000ml/hr.2,17,20

= pump

CVVHDF System Setup 13



CRRT Modality Summary2,17,20

Mode FilteringProcess

Removes Indication

Slow Continuous Ultra Filtration(SCUF)

Convection-Ultrafiltration

Fluid, Minimalsolutes

Fluid OverloadHeart failure

Continuous Arterio/VenovenousHaemofiltration (CAVH/CVVH)

Convection-Ultrafiltration

Fluid removal.Moderate soluteremoval. Urea

clearanceapproximately 15-

17ml/min

Moderate electrolyteimbalances

Oliguria withparenteral nutrition

or bloodrequirementsSeptic Shock

Continuous Arterio/VenovenousHaemodialysis (CAVHD, CVVHD)

Diffusion Fluid removal. Moreaggressive solute

removal. Ureaclearance

approximately 17-21ml/min.

Fluid overload withhaemodynamic

instability AzotaemiaElectrolyte

disturbance andacidosis

Parenteral nutritionaccompanying fluid

overload

Continuous Arterio/VenovenousHaemodiafiltration (CAVHDF/CVVHDF

Convection &Diffusion

Maximum fluid andsolute removal. Urea

clearanceapproximately 25-

26ml/min.

Fluid Overload,pulmonary oedema

Worsening MetabolicAcidosis

HyperkalaemiaWorsening Azotaemia

Drug overdosesRemoval of toxins

A Brief Look at Renal Anatomy

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