A-Level Biology: Regulation of Water Balance | A-Level 生物：水分平衡的调节机制

📚 A-Level Biology: Regulation of Water Balance | A-Level 生物：水分平衡的调节机制

Water is the solvent of life. In mammals, maintaining the correct water potential of blood plasma and tissue fluid is essential for normal enzyme activity, cell shape and transport processes. The regulation of water content and solute concentration in the body fluids is known as osmoregulation. In this article, we will examine how the kidneys filter blood, reabsorb useful substances, and how hormones such as ADH fine-tune water loss to keep the body in balance.

水是生命的溶剂。在哺乳动物中，维持血浆和组织液正确的渗透势（水势）对于正常的酶活性、细胞形态和运输过程至关重要。对体液中水分含量和溶质浓度的调节称为渗透调节。在本文中，我们将探讨肾脏如何过滤血液、重吸收有用物质，以及 ADH 等激素如何精细调节水分排出以维持身体平衡。

1. Why Osmoregulation Matters | 渗透调节为何重要

All cells are bathed in tissue fluid with a certain water potential. If the water potential of the plasma falls (becomes more negative), water moves out of cells by osmosis, causing them to shrink and disrupting metabolism. If it rises, cells absorb water and may burst. In humans, blood plasma normally has an osmotic concentration of about 300 mOsm dm⁻³, and even small deviations can be dangerous.

所有细胞都浸浴在具有特定水势的组织液中。如果血浆水势下降（变得更负），水会通过渗透作用从细胞流出，导致细胞收缩并扰乱代谢；如果水势升高，细胞会吸水甚至胀破。在人体中，血浆的正常渗透浓度约为 300 mOsm dm⁻³，即使是微小的偏差也可能是危险的。

Osmoregulation therefore involves controlling both the volume and the solute concentration of body fluids. The kidney is the main organ responsible, and it works together with the hypothalamus and the pituitary gland to adjust water excretion according to the body’s needs.

因此，渗透调节涉及控制体液的容积和溶质浓度两个方面。肾脏是主要负责的器官，它与下丘脑和垂体协同工作，根据身体需要调整水的排出量。

2. Overview of the Kidney | 肾脏概述

The kidneys are paired organs located in the abdomen. In longitudinal section, each kidney shows three distinct regions: the outer cortex, the inner medulla, and the renal pelvis which collects urine and leads to the ureter. The functional unit of the kidney is the nephron; each human kidney contains about one million nephrons.

肾脏是位于腹腔的一对器官。在纵切面上，每个肾脏显示出三个不同的区域：外层皮质、内部的髓质，以及收集尿液并通向输尿管的肾盂。肾脏的功能单位是肾单位；每个人肾大约含有约一百万个肾单位。

Each nephron consists of a Bowman’s capsule, a proximal convoluted tubule (PCT), the loop of Henle, a distal convoluted tubule (DCT) and a collecting duct. The Bowman’s capsule and the convoluted tubules lie in the cortex, while the loop of Henle and collecting duct extend into the medulla. Understanding this arrangement is essential for explaining the concentrating mechanism.

每个肾单位由鲍曼氏囊（肾小囊）、近端小管（PCT）、亨勒袢、远端小管（DCT）和集合管组成。鲍曼氏囊和蟠曲小管位于皮质，而亨勒袢和集合管延伸到髓质。理解这种排列对于解释浓缩机制至关重要。

3. Ultrafiltration at the Glomerulus | 肾小球的超滤作用

Blood enters the glomerulus through an afferent arteriole which is wider than the efferent arteriole. This difference in diameter creates a high hydrostatic pressure in the glomerular capillaries. The pressure forces water and small solutes through the capillary wall and into the Bowman’s capsule; this process is called ultrafiltration.

血液通过入球小动脉进入肾小球，入球小动脉比出球小动脉更宽。这种直径差异在肾小球毛细血管中产生较高的静水压。该压力迫使水和小的溶质通过毛细血管壁进入鲍曼氏囊；这一过程称为超滤作用。

The filtration barrier has three layers: the capillary endothelium with fenestrations (pores), a basement membrane made of glycoproteins, and the podocytes that wrap around the capillaries with filtration slits. The basement membrane acts as the main filter, preventing large proteins and blood cells from passing. Protein molecules with a relative molecular mass above about 68 000 are normally retained in the blood.

滤过屏障有三层：带有孔隙的内皮细胞、由糖蛋白构成的基膜，以及包裹毛细血管并具有滤过裂隙的足细胞。基膜是主要的滤过屏障，阻止大分子蛋白质和血细胞通过。相对分子质量大于约 68 000 的蛋白质分子通常被保留在血液中。

Glomerular filtrate ≈ plasma minus blood cells and plasma proteins

肾小球滤液 ≈ 血浆减去血细胞和血浆蛋白

4. Selective Reabsorption in the PCT | 近端小管的选择性重吸收

The proximal convoluted tubule is lined with cuboidal epithelial cells that have long microvilli on their apical surface. These microvilli greatly increase the surface area for reabsorption. About 85% of the filtrate volume is reabsorbed here, together with all glucose and amino acids.

近端小管由立方形上皮细胞组成，其顶端表面带有长微绒毛。这些微绒毛大大增加了重吸收的表面积。约 85% 的滤液在此被重吸收，同时所有葡萄糖和氨基酸

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