The kidneys are solid, ‘bean-shaped’ retroperitoneal organs located at vertebral levels T12 to L3. From inside to outside, the kidney is surrounded by the renal capsule, perirenal fat, renal fascia and pararenal fascia. At the midpoint of the concave medial border of each kidney is the hilum, the point of entry of the nerves, vessels and lymphatics. In cross-section, the kidney contains.
The kidney has an array of functions that can be classified as:
Homeostasis of blood composition, including:
– Regulation of plasma volume and electrolyte concentration.
– Control of plasma osmolarity.
– Removal of waste products and drugs or their metabolites.
– Gluconeogenesis: the kidney is a major gluconeogenic organ.
– Control of the metabolic aspects of acid-base balance.
– Erythropoietin synthesis, which in turn controls erythrocyte production;
– Activation of vitamin D to 1,25-dihydroxycholecalciferol;
– Secretion of renin, the first hormone of the renin–angiotensin–aldosterone axis.
The kidneys are solid, ‘bean-shaped’ retroperitoneal organs located at vertebral levels T12 to L3. From inside to outside, the kidney is surrounded by the renal capsule, perirenal fat, renal fascia and pararenal fascia. At the midpoint of the concave medial border of each kidney is the hilum, the point of entry of the nerves, vessels and lymphatics. In cross-section, the kidney contains:
An outer renal cortex.
An inner renal medulla, interrupted by renal columns (extensions of the cortex) that penetrate deep into the renal medulla.
Towards the hilum, minor calyces coalesce to form major calyces, which merge to form the renal pelvis and finally the ureter.
The blood supply to the kidneys is carried by the renal arteries, paired arteries that arise directly from the aorta. The right renal artery is longer, as the aorta is positioned slightly to the left of the midline. Sometimes there are also additional accessory arteries. Venous drainage of the kidneys is through the renal veins, which drain directly into the inferior vena cava (IVC). The left renal vein is longer than the right owing to the position of the IVC to the right of the midline.
The functional unit of the kidney is the nephron (Figure 67.1), of which there are around 1,000,000 per kidney. A nephron consists of:
The glomerulus, a network of capillaries located in the renal cortex whose role is the filtration of plasma (see Chapter 68). The fluid is collected in Bowman’s capsule and passes along a series of tubes.
The loop of Henle (LOH) – the tubule straightens and then enters the medulla to become the thin descending limb. This undergoes a hairpin bend to continue as the thin and then the thick ascending limb of the LOH. The main role of the LOH is to generate a longitudinal osmotic gradient in the renal medulla, which allows controlled water reabsorption from the collecting ducts.
The distal convoluted tubule (DCT) – the thick ascending limb of the LOH returns to the renal cortex to form the DCT, the site of regulated reabsorption.
The collecting duct (CD) – the DCT becomes the CD, before forming a minor calyx. The CD is an important site of water reabsorption.
The arterial blood supply of the nephron is unique:
The renal arterial tree divides as usual to give afferent arterioles, which in turn divide to give rise to the glomerular capillaries. These capillaries then unite to form efferent arterioles.
By varying the relative resistances of the afferent and efferent arterioles, glomerular capillary hydrostatic pressure, which is the main driving force for glomerular filtration, can be modified. Glomerular filtration is therefore controllable (see Chapter 68).
The vasa recta are an additional set of arterioles that arise from the efferent arterioles, whose role is to supply blood to the renal medulla. The vasa recta also have an unusual feature: they descend with the ascending limb of the LOH and ascend with the descending limb, providing a countercurrent flow of blood. This countercurrent arrangement is required to generate the high solute concentration gradients of the renal medulla (see Chapter 69).
It is important to note that this anatomy results in a well-vascularised renal cortex, but a relatively poor blood supply to the renal medulla. This latter feature prevents washout of solutes from the medullary interstitium that are required for water reabsorption.
The DCT folds back to lie anatomically very close to its corresponding glomerulus. At this point are located a group of specialised cells that form the juxtaglomerular apparatus, consisting of three components:
Granular cells, located within the wall of the afferent arteriole, whose role is renin secretion.
Macula densa cells, located at the junction of the DCT and the thick ascending limb of the LOH. Macula densa cells sense tubular Na+ and Cl‾ concentration.
Extra-glomerular mesangial cells. These interact with the macula densa via a purinergic signalling mechanism to control the granular cells and the vascular smooth muscle of the afferent arterioles.
Despite only making up 1% of total body weight, the kidneys receive a blood flow of approximately 1000 mL/min, 20% of the cardiac output. Unlike tissues such as skeletal muscle, the kidneys receive far more blood than is required for their metabolic activity, reflecting their function of blood filtration. There is a greater proportion of blood flow to the glomerulus (500 mL/100 g of tissue/min) than the medulla (outer medulla: 100 mL/100 g/min; inner medulla: 20 mL/100 g/min).
RBF must be tightly controlled:
Too high a flow results in end-organ damage due to high pressure. In addition, there is insufficient time for reabsorption processes to occur in the tubules, resulting in a pressure diuresis.
Too low a flow results in ischaemia, particularly of the relatively poorly vascularised medulla and metabolically active PCTs, as well as a build-up of toxic metabolites in the blood due to reduced filtration.