The kidney
Renal anatomy
Approximately 25% of blood from the heart flows through the kidneys. The primary functional units of the kidneys are called nephrons and there are approximately 1 million nephrons per kidney.
The glomerulus is the primary filtration unit of the kidney. Blood containing waste products enters the glomerulus via the afferent arteriole. It passes through the glomerulus within a capillary.
The capillary wall (glomerular membrane) allows certain molecules (for example, water and waste products) to pass through and enter the Bowman’s capsule. This fluid is called plasma filtrate. Large molecules such as protein and red blood cells cannot pass through the healthy glomerular membrane and remain in the blood.
The plasma filtrate passes down the tubules, where electrolytes and water are re-absorbed, and eventually enters the renal pelvis as urine. From here it travels down to the bladder and is eliminated from the body. The ‘cleaned’ blood leaves the glomerulus via the efferent arteriole.
Renal function
The kidney is responsible for:
- Filtering waste products from the blood
- Re-absorbing electrolytes and water within the kidney to maintain body fluid volume and composition
- Secreting certain hormones (renin, erythropoetin, calcitriol)
- Regulating the circulation via the renin-angiotensin-aldosterone system (RAAS)
The volume of blood filtered by all the kidney glomeruli per minute is called the glomerular filtration rate (GFR). This is the most accurate measure of kidney function, but cannot easily be directly measured in practice. It is usually assessed indirectly by measuring the quantity of waste products such as urea and creatinine in the blood.
The GFR varies according to the volume and pressure of blood passing through the glomeruli. In response to reduced renal blood flow, the juxtaglomerular apparatus releases renin. Just as in heart disease, this triggers the RAAS and angiotensin II is produced.
Angiotensin II causes vasoconstriction, which increases systemic blood pressure. In the kidney, angiotensin II also specifically acts on the efferent arteriole. Constriction of the efferent arteriole reduces outflow from the glomerulus, causing increased pressure within the glomerulus (glomerular hypertension). This increased pressure causes an increase in GFR.
The purpose of this response is to act as a protective mechanism whenever there is a drop in blood pressure. This response is appropriate in a life-threatening scenario, such as acute haemorrhage. The compensatory capacity of the kidneys is huge and the kidneys have great flexibility to adjust to varying demands. However, if these compensatory mechanisms are maintained over a period of time (as occurs in renal disease), they start to damage the kidneys.