📚 Edexcel Biology 5.2 Excretion | 爱德思生物 5.2 排泄
Excretion is the removal of metabolic waste products from the body. These wastes are produced by chemical reactions inside cells, such as respiration, deamination of amino acids and breakdown of haemoglobin. Excretion is different from egestion, which is the removal of undigested food from the digestive tract.
排泄是指将代谢产生的废物从体内清除。这些废物由细胞内的化学反应产生，例如呼吸作用、氨基酸的脱氨作用和血红蛋白的分解。排泄不同于排遗；排遗是消化道中未消化食物的排出。
In humans the main excretory products are carbon dioxide from aerobic respiration, urea from the breakdown of excess amino acids in the liver, and water and salts that must be balanced. Carbon dioxide is transported in the blood to the lungs and removed in exhaled air. Urea is carried from the liver to the kidneys, where it is filtered out and eliminated in urine.
人体内主要的排泄物包括有氧呼吸产生的二氧化碳、肝脏中过多氨基酸分解产生的尿素，以及需要保持平衡的水和盐。二氧化碳经血液运输到肺部，随呼出气体排出。尿素从肝脏运送到肾脏，在那里被过滤并随尿液排出。
1. Defining Excretion | 排泄的定义
Excretion is the removal of the waste products of metabolism from the body. Metabolic wastes include carbon dioxide from respiration, urea from the breakdown of excess amino acids, and excess water and salts. The process is essential because accumulation of these substances can be toxic and disrupt homeostasis.
排泄是将代谢废物从体内清除。代谢废物包括呼吸作用产生的二氧化碳、过量氨基酸分解产生的尿素，以及过多的水和盐。这一过程必不可少，因为这些物质的积累可能具有毒性并破坏稳态。
It is important to distinguish excretion from egestion. Egestion is the elimination of undigested food material from the gut, whereas excretion removes substances that have been produced by metabolic activity inside cells and have entered the bloodstream or tissue fluid.
区分排泄和排遗很重要。排遗是将未消化的食物残渣从肠道排出，而排泄清除的是细胞内代谢活动产生并已进入血液或组织液的物质。
2. Excretory Organs and Their Roles | 排泄器官及其功能
The lungs excrete carbon dioxide and water vapour during breathing. The skin excretes water, salts and a small amount of urea through sweat glands. However, the kidneys are the main excretory organs because they remove urea, excess water, salts and other soluble waste products from the blood.
肺部在呼吸时排出二氧化碳和水蒸气。皮肤通过汗腺排出水、盐和少量尿素。然而，肾脏是主要的排泄器官，因为它们从血液中清除尿素、多余的水、盐和其他可溶性废物。
The liver is not an excretory organ, but it produces urea by deamination. In the liver, excess amino acids have their amino group removed, forming ammonia. Ammonia is highly toxic, so it is quickly converted to urea, which is much less toxic and can be transported safely in the blood to the kidneys.
肝脏不是排泄器官，但它通过脱氨作用产生尿素。在肝脏中，过多的氨基酸去除氨基，形成氨。氨有剧毒，因此很快转化为尿素；尿素毒性低得多，可以安全地通过血液运输到肾脏。
3. The Human Urinary System | 人体泌尿系统
The human urinary system consists of two kidneys, two ureters, the bladder and the urethra. Each kidney receives blood through the renal artery, which branches from the aorta. After blood is filtered, it leaves the kidney through the renal vein and returns to the vena cava.
人体泌尿系统由两个肾脏、两条输尿管、膀胱和尿道组成。每个肾脏通过从主动脉分支出来的肾动脉获得血液。血液过滤后经肾静脉离开肾脏，回到腔静脉。
Urine produced in the kidney drips into the renal pelvis and flows down the ureter by peristalsis. The ureter carries urine to the bladder, where it is stored. When the bladder is full, urine is expelled through the urethra. A sphincter muscle controls the release of urine.
肾脏产生的尿液滴入肾盂，并通过输尿管的蠕动向下流动。输尿管将尿液输送到膀胱储存。膀胱充盈时，尿液经尿道排出。括约肌控制尿液的排放。
4. Gross Structure of the Kidney | 肾脏的大体结构
A longitudinal section of the kidney shows three main regions: the outer cortex, the inner medulla and the central pelvis. The cortex is dark red and contains the Bowman's capsules and convoluted tubules of nephrons. The medulla is paler and contains the loops of Henle and collecting ducts.
肾脏纵切面显示三个主要区域：外层的皮质、内层的髓质和中央的肾盂。皮质深红色，包含肾小囊和肾单位的曲小管。髓质颜色较浅，包含髓袢和集合管。
The renal artery supplies oxygenated blood at high pressure, which is essential for ultrafiltration. The renal vein removes filtered blood. The pelvis collects urine before it enters the ureter.
肾动脉以高压提供含氧血，这对超滤至关重要。肾静脉排出过滤后的血液。肾盂在尿液进入输尿管前收集尿液。
5. The Nephron: The Functional Unit | 肾单位：功能单位
Each kidney contains about one million nephrons. A nephron is the structural and functional unit of the kidney. It is a long tubule with a cup-shaped beginning called the Bowman's capsule, which surrounds a knot of capillaries called the glomerulus.
每个肾脏大约含有一百万个肾单位。肾单位是肾脏的结构和功能单位。它是一条长管，杯状的起始端称为肾小囊，包绕着一团毛细血管，称为肾小球。
From the Bowman's capsule, the tubule continues as the proximal convoluted tubule, then the loop of Henle, then the distal convoluted tubule, and finally the collecting duct. The collecting duct receives urine from several nephrons and carries it through the medulla to the pelvis.
从肾小囊开始，小管依次为近曲小管、髓袢、远曲小管，最后是集合管。集合管接收多个肾单位的尿液，并穿过髓质将尿液运送到肾盂。
Blood flow through a nephron is unusual because it passes through two capillary beds: the glomerulus and the peritubular capillaries around the tubule. This arrangement allows filtration at the glomerulus and reabsorption along the tubule.
血液流经肾单位的方式很特殊，因为它经过两套毛细血管网：肾小球和肾小管周围的肾小管周围毛细血管。这种安排使肾小球处能够过滤，肾小管沿途能够重吸收。
6. Ultrafiltration in the Bowman's Capsule | 肾小囊内的超滤
Ultrafiltration occurs in the renal corpuscle, which is the glomerulus plus Bowman's capsule. The afferent arteriole is wider than the efferent arteriole, so blood in the glomerulus is under high hydrostatic pressure. This pressure forces water and small molecules out of the capillary into the Bowman's capsule.
超滤发生在肾小体中，肾小体包括肾小球和肾小囊。入球小动脉比出球小动脉宽，因此肾小球内的血液处于很高的流体静水压。这个压力迫使水和小分子从毛细血管进入肾小囊。
The capillary walls have fenestrations and the podocytes of the Bowman's capsule form filtration slits. The basement membrane acts as the main filter. Water, glucose, amino acids, urea, salts and ions pass through. Large molecules such as proteins and blood cells remain in the blood.
毛细血管壁有窗孔，肾小囊的足细胞形成滤过裂隙。基底膜是主要的过滤器。水、葡萄糖、氨基酸、尿素、盐和离子可以透过。蛋白质和血细胞等大分子则留在血液中。
The fluid entering the Bowman's capsule is called glomerular filtrate. Its composition is similar to blood plasma but without plasma proteins. Ultrafiltration is a passive process driven by blood pressure.
进入肾小囊的液体称为肾小球滤液。其成分与血浆相似，但不含血浆
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