Integration: Cardiovascular-Renal Axis

Systems Integration in Physiology

No organ system operates in isolation. This lecture synthesises the cardiovascular and renal systems to illustrate the principle of integrated physiological control. The central challenge is the regulation of arterial blood pressure (MAP) and body fluid volume — two variables that are intimately coupled because changes in one directly affect the other.

Blood Pressure Regulation: The Guyton Framework

Arterial blood pressure = Cardiac output (CO) × Total peripheral resistance (TPR). CO = heart rate × stroke volume. TPR is determined by arteriolar diameter (primarily regulated by sympathetic vasomotor tone, local metabolites, and circulating hormones). Arterial baroreceptors (carotid sinus and aortic arch) detect changes in MAP and provide moment-to-moment neural feedback via the nucleus of the solitary tract (NTS) in the medulla, adjusting sympathetic and parasympathetic outflow.

Cardiac Output and the Starling Mechanism

The Frank-Starling law states that stroke volume is proportional to end-diastolic volume (EDV) within physiological limits — the more the ventricle is filled, the more forcefully it contracts. EDV is determined by venous return, which depends on blood volume and venous tone. Therefore, if blood volume expands (e.g., excess Na⁺ and H₂O retention), venous return rises, EDV rises, CO rises, and MAP rises. Conversely, haemorrhage reduces blood volume, venous return, and CO.

Starling Forces and Fluid Exchange

Fluid exchange between capillaries and interstitium is governed by Starling forces: Net filtration = Kf [(P_c − P_i) − σ(π_c − π_i)], where P_c is capillary hydrostatic pressure, P_i is interstitial hydrostatic pressure, π_c is plasma oncotic pressure (primarily albumin, ~25 mmHg), and π_i is interstitial oncotic pressure. Net filtration occurs at the arteriolar end; net reabsorption at the venular end. Oedema occurs when net filtration exceeds lymphatic drainage: causes include ↑P_c (heart failure, venous obstruction), ↓π_c (hypoalbuminaemia), ↑capillary permeability (inflammation), or lymphatic obstruction.

The Renal-Body Fluid Feedback System

The renal-body fluid mechanism is the dominant long-term regulator of MAP. The concept of pressure natriuresis (Guyton): as MAP rises, the kidney excretes more Na⁺ and H₂O. This reduces blood volume, venous return, CO, and hence MAP — a powerful negative feedback loop. The equilibrium point where Na⁺ input = Na⁺ output determines the chronic MAP set point. The RAAS shifts the pressure-natriuresis curve: Ang II and aldosterone cause salt and water retention, resetting the equilibrium point at a higher MAP (essential hypertension mechanism).

Integrated Response to Haemorrhage

Following acute haemorrhage: (1) ↓ blood volume → ↓ venous return → ↓ CO → ↓ MAP; (2) baroreceptor unloading → ↑ sympathetic outflow → ↑ HR, ↑ contractility, ↑ TPR; (3) ↓ renal perfusion → ↑ RAAS → ↑ aldosterone → Na⁺ and H₂O retention; (4) ↑ plasma osmolality (haemoconcentration) + Ang II → ↑ ADH → ↑ aquaporin-2 → water reabsorption; (5) activation of thirst centres. Over hours to days, plasma volume is restored by fluid shifts from the interstitium and renal conservation.

Integrated Response to Exercise

During exercise: metabolic vasodilation in active skeletal muscle (↑CO₂, ↓O₂, ↑K⁺, ↑adenosine) → ↓ TPR in muscle beds; sympathetically mediated vasoconstriction in splanchnic and renal beds; ↑ HR and ↑ contractility (sympathetic); ↑ venous return (muscle pump, respiratory pump); net effect is ↑ CO (can reach 20–25 L/min in trained athletes) with modest ↑ MAP. Sweating → ↑ plasma osmolality → ↑ ADH → water conservation; ↓ renal blood flow → ↑ RAAS.

Pathophysiology: Hypertension and Heart Failure

Hypertension (MAP >100 mmHg chronically): results from rightward shift of the pressure-natriuresis curve. Primary hypertension (~95%): RAAS overactivity, increased sympathetic tone, reduced nephron number, or genetic variants in Na⁺ transporters. Treatment targets: ACE inhibitors/ARBs (↓ Ang II), thiazide diuretics (↓ Na⁺ reabsorption via NCC), calcium channel blockers (↓ TPR), β-blockers (↓ HR and CO).

Heart failure (HF): ↓ CO → compensatory RAAS and sympathetic activation → Na⁺ and H₂O retention → ↑ blood volume → ↑ venous pressures → pulmonary oedema (left HF) and peripheral oedema (right HF). A vicious cycle forms: fluid retention raises preload (Starling), but the failing heart cannot utilise this to increase CO; oedema worsens. Treatment: diuretics (reduce preload), ACE inhibitors (reduce afterload and preload), β-blockers (reduce HR and remodelling), SGLT2 inhibitors (promote natriuresis).